Research Items (111)
Visual detection of small black objects surrounded by a light background depends on background luminance, pupil size, optical blur, and object size. Holding pupil and optics fixed, we measured the minimum background luminance needed for foveal detection of small black targets as a function of target size. For all three observers, absolute threshold varied inversely with target area when disk diameter subtended less than 10′ of visual angle. For target diameter ≥10′, threshold remained constant at about 0.3 Td, which was also the absolute threshold for detecting light spots 10′ or larger in diameter on a black background. These results are consistent with Ricco’s law of spatial summation: a “black hole” is just detectable when the background luminance is sufficiently high for its absence inside the Ricco area to reduce 555 nm photon flux by 7500 photons/s, which is the same change needed to detect light spots on a black surround. These results can be accounted for by a differential pair of Ricco detectors, each about the size of the receptive field center of magocellular retinal ganglion cells when projected into object space through the eye’s weakly aberrated optical system. Statistical analysis of the model suggests the quantum fluctuations due to internal, biological noise (i.e., “scotons”) are a greater handicap than the photon fluctuations inherent in the light stimulus at absolute foveal threshold.
- Mar 2019
Experimental visual acuity (VA) of eight subjects was measured using the Freiburg vision test in a custom-made adaptive optics system. Measurements were conducted under one control and five defocus-induced conditions. In the defocus-induced conditions, 1 diopter of myopic defocus was added to the system using the Badal stage, and defocus vibrations with five different levels of amplitude were generated by a deformable mirror at 50 Hz. Computational simulations of the visual Strehl ratio (VSOTF) were performed using average aberrations of each subject recorded in the control condition. For the first time, to the best of our knowledge, it has been shown experimentally that both the simulated VSOTF and experimentally measured VA improve when defocus vibrations are added to a defocused eye.
It is well known that depth-of-focus (DOF) is influenced by optical factors (such as pupil size and monochromatic aberrations). However, neural factors such as blur sensitivity and defocus adaptation may play an important role on the extent of DOF. A series of experiments were conducted to study if optical or neural factors are most pertinent in explaining the variability of DOF across subjects. An adaptive optics system with a black and white target, a 3.8-mm artificial pupil, and a subjective criterion (based on objectionable blur) were used to measure depth of field ([DOFi]; DOF computed in the object space) in 11 participants, after at least 6 min of adaptation. This was done under three conditions: (a) with their own higher order aberrations (HOA); (b) after correction of their monochromatic HOA; and (c) after altering the HOA pattern for some participants to reflect the HOA pattern measured for a different participant. Natural DOFi and DOFi after HOA correction were positively correlated (R2 = 0.461), but a significant decrease in DOFi (21% on average) was found after HOA correction (p = 0.042). Effect of HOA on the intersubject variability of DOFi was 3.9 times smaller than the effect of the image neural processing. This study shows that DOFi depends on both optical and neural factors, but the latter seems to play a more important role than the former.
Purpose: To test the hypothesis that accommodation responds to the convergence or divergence of light rays that form out-of-focus images and not simply to blur.
Children 8 and under on average spend 2.3h with screen media daily. The average time spent daily with mobile devices has tripled since 2013. Modern mobile devices are fully suited to measure illumination and near work distance in natural conditions at hight acquisition rates. We have developed a new mobile application, called VisionApp, capable of measuring the near work distance and face illumination in real time, with high accuracy. The application works in the background so that the user can use the device in their habitual manner. A data sample (near work distance and face illumination) is stored in a cloud database every 20 seconds (180 samples every hour). The new app can serve as a research tool to track the progression of myopia in children and adolescents, and also as a myopia control tool. In the latter case the app can be configured to darken the screen when the near work distance is too close. The poster details the results of the calibration of both distance and light level measurements.
- May 2018
Trolands are a widely used measure of retinal illuminance in vision science and visual optics, but disagreements exist for the definition and interpretation of this photometric unit. The purpose of this communication is to resolve the confusion by providing a sound conceptual basis for interpreting trolands as a measure of angular flux density incident upon the retina. Using a simplified optical analysis, we show that the troland value of an extended source is the intensity in micro-candelas of an equivalent point source located at the eye’s posterior nodal point that produces the same illuminance in the retinal image as does the extended source. This optical interpretation of trolands reveals that total light flux in the image of an extended object is the product of the troland value of the source and the solid angle subtended by the source at the first nodal point, independent of eye size.
Purpose: To develop a flexible model of the average eye that incorporates changes with age and accommodation in all optical parameters, including entrance pupil diameter, under photopic, natural, environmental conditions. Methods: We collated retrospective in vivo measurements of all optical parameters, including entrance pupil diameter. Ray-tracing was used to calculate the wavefront aberrations of the eye model as a function of age, stimulus vergence and pupil diameter. These aberrations were used to calculate objective refraction using paraxial curvature matching. This was also done for several stimulus positions to calculate the accommodation response/stimulus curve. Results: The model predicts a hyperopic change in distance refraction as the eye ages (+0.22D every 10 years) between 20 and 65 years. The slope of the accommodation response/stimulus curve was 0.72 for a 25 years-old subject, with little change between 20 and 45 years. A trend to a more negative value of primary spherical aberration as the eye accommodates is predicted for all ages (20-50 years). When accommodation is relaxed, a slight increase in primary spherical aberration (0.008μm every 10 years) between 20 and 65 years is predicted, for an age-dependent entrance pupil diameter ranging between 3.58mm (20 years) and 3.05mm (65 years). Results match reasonably well with studies performed in real eyes, except that spherical aberration is systematically slightly negative as compared with the practical data. Conclusions: The proposed eye model is able to predict changes in objective refraction and accommodation response. It has the potential to be a useful design and testing tool for devices (e.g. intraocular lenses or contact lenses) designed to correct the eye's optical errors.
Purpose: We tested the hypothesis that changes in accommodation after instillation of Phenylephrine Hydrochloride (PHCl) observed in some studies could be caused by changes in optics. Methods: We performed two experiments to test the effects of PHCl on static and on dynamic accommodation in 8 and 6 subjects, respectively. Objective wavefront measurements were recorded of the static accommodation response to a stimulus at different distances or dynamic accommodation response to a sinusoidally moving stimulus (between 1 and 3 D of accommodative demand at 0.2Hz). The responses were characterized using two methods: one that takes into account the mydriatic optical effects on the accommodation produced by higher-order aberrations of the eye and another that takes into account only power changes paraxially due to the action of the ciliary muscle and regardless of the pupil size. Results: When mydriatic optical effects were taken into account, differences in responses before and after PHCl instillation were 0.51±0.53 D, and 0.12±0.15, for static and dynamic accommodation, respectively, and were statistically significant (p<0.039). When mydriatic optical effects were not taken into account, the differences in responses before and after PHCl instillation were -0.20±0.51 D, and -0.05±0.14, for static and dynamic accommodation, respectively, and were not statistically significant (p>0.313). Conclusions: The mydriatic effect of the PHCl causes optical changes in the eye that can reduce the objective and subjective measurement of accommodation.
- Jan 2018
Purpose: To test the hypothesis that marginal ray deviations determine perceived starburst sizes, and to explore different strategies for decreasing starburst size in highly aberrated eyes. Methods: Perceived size of starburst images and visual acuities were measured psychophysically for eyes with varying levels of spherical aberration, pupil sizes, and defocus. Computationally, we use a polychromatic eye model including the typical levels of higher order aberrations (HOAs) for keratoconic and post-LASIK eyes to quantify the image quality (the visually weighted Strehl ratio derived from the optical transfer function, VSOTF) with different pupil sizes at both photopic and mesopic light levels. Results: For distance corrected post-LASIK and keratoconic eyes with a night-time pupil (e.g., 7 mm), the starburst diameter is about 1.5 degrees (1 degree for normal presbyopic eyes), which can be reduced to ≤0.25 degrees with pupil sizes ≤3 mm. Starburst size is predicted from the magnitude of the longitudinal spherical aberration. Refracting the eye to focus the pupil margin also removed starbursts, but, unlike small pupils, significantly degraded visual acuity. Reducing pupil diameter to 3 mm improved image quality for these highly aberrated eyes by about 2.7 × to 1.7 × relative to the natural pupils when light levels were varied from 0.1 to 1000 cd m-2 , respectively. Conclusion: Subjects with highly aberrated eyes observed larger starbursts around bright lights at night predictable by the deviated marginal rays. These were effectively attenuated by reducing pupil diameters to ≤3 mm, which did not cause a drop in visual acuity or modelled image quality even at mesopic light levels.
Purpose: The aim of this work was to examine the impact of Seidel spherical aberration (SA) on optimum refractive state for detecting and discriminating small bright lights on a dark background. Methods: An adaptive-optics system was used to correct ocular aberrations of cyclopleged eyes and then systematically introduce five levels of Seidel SA for a 7-mm diameter pupil: 0,±0.18, and±0.36diopters (D)mm-2. For each level of SA, subjects were required to detect one or resolve two points of light (0.54 arc min diameter) on a dark background. Refractive error was measured by adjusting stimulus vergence to minimize detection and resolution thresholds. Two other novel focusing tasks for single points of light required maximizing the perceived intensity of a bright point's core and minimizing its overall perceived size (i.e. minimize starburst artifacts). Except for the detection task, luminance of the point of light was 1000cdm-2 on a black background lower than 0.5cdm-2. Results: Positive SA introduced myopic shifts relative to the best subjective focus for dark letters on a bright background when there was no SA, whereas negative SA introduced hyperopic shifts in optimal focus. The changes in optimal focus were -1.7, -2.4, -2.0, and -9.2D of focus per Dmm-2 of SA for the detection task, resolution task, and maximization of core's intensity and minimization of size, respectively. Conclusion: Ocular SA can be a significant contributor to changes in refractive state when viewing high-contrast point sources typically encountered in nighttime environments.
Eyes of children and young adults change their optical power to focus nearby objects at the retina. But does accommodation function by trial and error to minimize blur and maximize contrast as is generally accepted? Three experiments in monocular and monochromatic vision were performed under two conditions while aberrations were being corrected. In the first condition, feedback was available to the eye from both optical vergence and optical blur. In the second, feedback was only available from target blur. Accommodation was less precise for the second condition, suggesting that it is more than a trial-and-error function. Optical vergence itself seems to be an important cue for accommodation.
Purpose To develop an objective refraction formula based on the ocular wavefront error (WFE) expressed in terms of Zernike coefficients and pupil radius, which would be an accurate predictor of subjective spherical equivalent (SE) for different pupil sizes. Methods A sphere is fitted to the ocular wavefront at the center and at a variable distance, t. The optimal fitting distance, topt, is obtained empirically from a dataset of 308 eyes as a function of objective refraction pupil radius, r0, and used to define the formula of a new wavefront refraction metric (MTR). The metric is tested in another, independent dataset of 200 eyes. Results For pupil radii r0 ≤ 2 mm, the new metric predicts the equivalent sphere with similar accuracy (<0.1D), however, for r0 > 2 mm, the mean error of traditional metrics can increase beyond 0.25D, and the MTR remains accurate. The proposed metric allows clinicians to obtain an accurate clinical spherical equivalent value without rescaling/refitting of the wavefront coefficients. It has the potential to be developed into a metric which will be able to predict full spherocylindrical refraction for the desired illumination conditions and corresponding pupil size.
Purpose: Part of the large increase of myopia prevalence in children is due to prolonged use of electronic screens (smartphones and tablets) at near-work distances. Myopia progression is correlated with near vision. However, there are no commercially available solutions to measure work distance in natural conditions at high acquisition rates. We developed an application (VisionApp, www.vision-app.eu) for the Android OS, capable of measuring the distance between the user and the device in real time. We test its accuracy and precision. Methods: We used the front camera of two smartphones of different brands: Xiaomi Mi5s, and Samsung Galaxy S6+ to acquire images of two subjects at varying distances, at a rate of 30 FPS. The head was fixed using a chin rest. The device was mounted in a holder affixed to a stepper motor, which allowed the distance to the subject to be precisely controlled. The distance between the subject and the device was measured within the range from 190 to 420mm in 10mm steps. Three consecutive measurements were obtained at each distance in each case. Both devices were calibrated by means of applying a calibration factor CF, which accounted for differences in proprietary device parameters, such as the ratio between total and active camera sensor area. The CF was measured at 5 distances from the subject (200, 250, 300, 350 and 400mm), and found to be the same for the same device. Results: Linear fit to the smartphone-measured distance in function of real, stepper motor-controlled distance showed a slope of 1.110 (R2=0.99) and 0.978 (R2=0.99) for Xiaomi and Samsung, respectively. The mean and SD was 11±10 mm and 5±10 mm for Xiaomi and Samsung, respectively. The calibration factor CF obtained for Xiaomi was 0.87±0.03, and 1.15±0.03 for Samsung. The mean maximum CF variation for different distances was 6.5%. Conclusions: We have developed and evaluated a new app that can be used to measure the work distance between the user and the device. The main benefit is that the app runs on the very device, eliminating the need for any extra equipment. The accuracy of the distance measurement was ~1 cm. The accuracy did not depend on the model of smartphone used, nor the calibration distance (calibration can be performed at any distance and assure the same, high accuracy). The app was designed to store the measurement data in the device and can be used in parallel with other running applications. It can be a suitable tool for research or clinical studies that aim to evaluate the distance between a user and an electronic device under natural conditions.
Purpose: To determine if human accommodation uses the eye’s own monochromatic aberrations to track dynamic accommodative stimuli. Methods: Wavefront aberrations were measured while subjects monocularly viewed a monochromatic Maltese cross moving sinusoidally around 2D of accommodative demand with 1D amplitude at 0.2 Hz. The amplitude and phase (delay) of the accommodation response were compared to the actual vergence of the stimulus to obtain gain and temporal phase, calculated from wavefront aberrations recorded over time during experimental trials. The tested conditions were as follows: Correction of all the subject’s aberrations except defocus (C); Correction of all the subject’s aberrations except defocus and habitual second-order astigmatism (AS); Correction of all the subject’s aberrations except defocus and odd higher-order aberrations (HOAs); Correction of all the subject’s aberrations except defocus and even HOAs (E); Natural aberrations of the subject’s eye, i.e., the adaptive-optics system only corrected the optical system’s aberrations (N); Correction of all the subject’s aberrations except defocus and fourth-order spherical aberration (SA). The correction was performed at 20 Hz and each condition was repeated six times in randomised order. Results: Average gain (+/-2 standard errors of the mean) varied little across conditions; between 0.55 +/- 0.06 (SA), and 0.62 +/- 0.06 (AS). Average phase (+/-2 standard errors of the mean) also varied little; between 0.41 +/- 0.02 s (E), and 0.47 +/- 0.02 s (O). After Bonferroni correction, no statistically significant differences in gain or phase were found in the presence of specific monochromatic aberrations or in their absence. Conclusions: These results show that the eye’s monochromatic aberrations are not necessary for accommodation to track dynamic accommodative stimuli.
The aim of this study was to determine whether dynamic accommodation responds to isolated blur cues without feedback, and without changes in the distance of the object. Nine healthy subjects aged 21 to 40 years were recruited. Four different aberration patterns were used as stimuli to induce blur: (1) the eye's natural, uncorrected, optical aberrations, (2) all aberrations corrected, (3) spherical aberration only, and (4) astigmatism only. The stimulus was a video animation based on computer-generated images of a monochromatic Maltese cross. Each individual video was generated for each subject off-line, after measuring individual aberrations at different accommodation levels. The video simulated sinusoidal changes in defocus at 0.2 Hz. Dynamic images were observed through a 0.8 mm pinhole placed at a plane conjugated with the eye's pupil, thus effectively removing potential feedback stemming from accommodation changes. Accommodation responses were measured with a Hartmann-Shack aberrometer for the four different aberration patterns. The results showed that seven out of nine subjects did not respond to any stimuli, whereas the response of the other two subjects was erratic and they seemed to be searching rather than following the stimulus. Therefore, a significant reduction in average accommodative gain (from 0.52 to 0.11) was obtained when the dioptric demand cue was removed. No statistically significant differences were found between experimental conditions used. We conclude that aberration related blur does not drive the accommodation response in the absence of feedback from accommodation.
Purpose : To obtain fast and accurate measurements of the equivalent sphere (M) and amplitude of accommodation (AA; subjective SAA, objective OAA), taking into account the potential changes in refraction due to the changes of pupil size. Methods : An empirical wavefront refraction metric (MTR) was formulated using objective (OR) and subjective refraction (SR) measurements, compiled from 3 datasets: Murcia (174 eyes), Houston (40), and Alabama (94). It was tested using an independent dataset, Indiana (200 fully corrected eyes). OR were calculated for all eyes, for pupil radii (rescaled) from 1.00 to 3.75mm. The results were compared with ones obtained with traditional metrics; Paraxial and minRMS. The MTR metric was then used to compute the OAA in 140 eyes (ages 17-55), for which OR measurements spanning the full range of accommodation had been obtained using the irx3 aberrometer (Imagine Eyes, France), in 0.5D steps of stimulus vergence. OAA was calculated as the maximum minus minimum value of OR. The results were compared to SAA, obtained with minus lenses, and custom Badal optometer. Results : The MTR metric predicted the subjective M with a mean, inter-subject error ≤0.12D for all pupil radii, both in the 3 datasets used to formulate it (308 eyes), and the independent dataset, Indiana (200 eyes). Equivalent sphere values calculated by the MTR metric lie between these of the minRMS and Paraxial metrics (Fig. 1), which for large pupils usually give values too myopic or too hyperopic, respectively. The OAA estimated by the MTR metric was on average 0.91±0.10D smaller than SAA obtained with the Badal method, and 0.75±0.09D smaller for the minus lenses method. Conclusions : The new MTR metric gives accurate prediction of the subjective M for the whole range of pupil radii, even though the SR may change due to spherical aberration (SA) for large pupils. On average, it was found to underestimate the SAA by 0.85±0.10D, which is expected, as the effects of the depth of field, present in subjective measurements, are not taken into account in objective measurements of far, and especially near points (see Bernal et al. OVS, 2014). The new metric can be used to estimate OR and OAA for pupil radii different than the measurement radius, and of interest to studies of children, and low illumination, where the pupil radius and SA can be large.
- May 2017
In visual experiments that require real-time partial correction of wavefront aberrations, small errors occur that accumulate over time and lead to drifts in Zernike coefficients of the uncorrected aberrations. A simple algorithm that does not require the inclusion of an additional optical path to obtain independent measurements of the eye's aberrations is described here, and its effectiveness in preventing these drifts is demonstrated.
We tested the hypothesis that accommodation operates as a contrast-maximizing negative feedback system.
- Apr 2017
- JINTE 2017
El objetivo de este estudio es averiguar cómo afectan a la función visual los cambios típicos, básicamente cambios en la aberración esférica (AE), que se producen en la óptica ocular tras una cirugía para corregir la miopía o hipermetropía (cirugía refractiva). Para ello se diseñó y montó un sistema de óptica adaptativa acondicionado para las características especiales del ojo humano consistente en un aberrómetro de Shack-Hartmann, un espejo deformable, un microdisplay para generar estímulos visuales y un sistema para monitorizar la posición de la pupila del ojo. Se realizaron medidas de la función visual en 5 sujetos, a quienes por medio del espejo deformable se les corrigió de su propia óptica ocular y se les indujo las condiciones ópticas propias tras diferentes grados de cirugías refractivas de miopía e hipermetropía: AE desde -0.75 hasta +0.75 micras en pasos de 0.25. La agudeza visual no mostró variación en el rango de AE estudiado. De igual modo, los resultados sugieren que para pequeños valores de AE entre -0.25 y +0.25 micras la función visual no se ve afectada de manera medible. Para mayores valores de AE se encontró una correlación entre la cantidad de AE y una disminución de la sensibilidad al contraste. Asimismo, para altos valores de AE se halló un cambio en la graduación cuando se variaba el tamaño de la pupila, lo que puede ser un factor contribuyente al fenómeno de la miopía nocturna.
PURPOSE. To determine whether changes in wavefront spherical curvature (optical vergence) are a directional cue for accommodation. METHODS. Nine subjects participated in this experiment. The accommodation response to a monochromatic target was measured continuously with a custom-made adaptive optics system while astigmatism and higher-order aberrations were corrected in real time. There were two experimental open-loop conditions: vergence-driven condition, where the deformable mirror provided sinusoidal changes in defocus at the retina between +1 and -1 diopters (D) at 0.2 Hz; and blur-driven condition, in which the level of defocus at the retina was always 0 D, but a sinusoidal defocus blur between +1 and -1 D at 0.2 Hz was simulated in the target. Right before the beginning of each trial, the target was moved to an accommodative demand of 2 D. RESULTS. Eight out of nine subjects showed sinusoidal responses for the vergence-driven condition but not for the blur-driven condition. Their average (6SD) gain for the vergence- driven condition was 0.50 (60.28). For the blur-driven condition, average gain was much smaller at 0.07 (60.03). The ninth subject showed little to no response for both conditions, with average gain <0.08. Vergence-driven condition gain was significantly different from blur- driven condition gain (P 1⁄4 0.004). CONCLUSIONS. Accommodation responds to optical vergence, even without feedback, and not to changes in defocus blur alone. These results suggest the presence of a retinal mechanism that provides a directional cue for accommodation from optical vergence.
A psychophysical approach has been designed to measure straylight from intraocular lenses (IOLs) in vitro. This approach uses a clinical straylight meter (C-Quant) and an observer’s eye as optical detector. Based on this, we introduced a method for study of straylight-wavelength dependency for IOLs. This dependency can be used to distinguish between 2 types of scattering particles (small and large) as defined by Mie theory. Validation was performed using a turbidity standard and scattering filters. Several IOLs were analyzed to identify potential scattering sources. Large particles were found to predominate in scattering from the studied lenses. This was confirmed by straylight-angular dependency found in these IOLs.
Human crystalline lens has a layered, shell-like structure with the refractive index increasing from cortex to nucleus (gradient index or GRIN structure). Moreover, every iso-indicial layer has a certain curvature which also varies from cortex to nucleus, with a gradient of curvature (G). In the present manuscript, the role of G on the lens power is investigated along with its implications regarding the lens paradox (change of lens power with age) and intra-capsular accommodation mechanism (larger than expected changes of lens power during accommodation compared to a homogenous lens). To this end, a simplified formulation of paraxial lens power based on thin lens approximation is developed and applied to the anterior and posterior parts of the lens. The main theoretical result is that the power of both anterior and posterior lens is given by the sum of the power of a lens with a homogeneous refractive index equal to that of the nucleus and power associated with the contribution of the internal GRIN structure, which depends on G. This general result suggests that the sign of G is fundamental in increasing or decreasing the lens power. We found that the curvature gradient has a strong impact on lens power, helping to explain both the lens paradox and intra-capsular accommodation mechanism.
- Dec 2016
Purpose: Multifocal contact lenses have been growing in popularity as a modality to correct presbyopic eyes, although visual side effects such as disability glare have been reported. The objective of this study was to investigate the effect of multifocal contact lenses on disability glare by means of ocular straylight. Methods: A prospective randomized, comparative study was performed that included 16 subjects free of ocular pathology. Straylight was measured using a commercial straylight meter with the natural and dilated pupil. Participants were fitted with Proclear Multifocal (Distance/Near), ACUVUE Oasys for Presbyopia, and Air Optix Aqua Multifocal randomized to the left or right eye. Straylight measurements were repeated with the contact lens in situ after the pupil dilation. Results obtained with the dilated pupil without contact lens acted as a control. Results: Diameter of the natural and dilated pupil was 2.87 ± 0.40 mm and 7.45 ± 0.86 mm, respectively (P < .001). After pupil dilation, straylight increased from 0.92 ± 0.13 log(s) to 1.04 ± 0.11 log(s) (P < .001). Of the four studied lenses, a significant difference was only found between Air Optix and the control group (P = .006). The latter showed also slightly increased light scatter. Conclusions: A difference in measured straylight was found between the studied multifocal lenses. The observed variability and the straylight-pupil size dependency should be taken into account to avoid elevated straylight in multifocal contact lens wearers. The reason for the observed differences in straylight must be the subject of future studies.
Accommodation is controlled by the action of the ciliary muscle and mediated primarily by parasympathetic input through postganglionic fibers that originate from neurons in the ciliary and pterygopalatine ganglia. During accommodation the pupil constricts to increase the depth of focus of the eye and improve retinal image quality. Researchers have traditionally faced the challenge of measuring the accommodative properties of the eye through a small pupil and thus have relied on pharmacological agents to dilate the pupil. Achieving pupil dilation (mydriasis) without affecting the accommodative ability of the eye (cycloplegia) could be useful in many clinical and research contexts. Phenylephrine hydrochloride (PHCl) is a sympathomimetic agent that is used clinically to dilate the pupil. Nevertheless, first investigations suggested some loss of functional accommodation in the human eye after PHCl instillation. Subsequent studies, based on different measurement procedures, obtained contradictory conclusions, causing therefore an unexpected controversy that has been spread almost to the present days. This manuscript reviews and summarizes the main research studies that have been performed to analyze the effect of PHCl on the accommodative system and provides clear conclusions that could help clinicians know the real effects of PHCl on the accommodative system of the human eye.
Purpose: Longitudinal chromatic aberration is present in all states of accommodation and may play a role in the accommodation response and the emmetropisation process. We study the change of the depth of field (DOFi) with the state of accommodation, taking into account the longitudinal chromatic aberration. Methods: Subjective DOFi was defined as the range of defocus beyond which the blur of the target (one line of optotypes of 0.1 logMAR shown on a black-and-white microdisplay, seen through different colour filters) was perceived as objectionable. The subject's eye was paralysed and different, previously-measured accommodative states (corresponding to the accommodative demands of 0D, 2D and 4D) were simulated with a deformable mirror. Different colour conditions (monochromatic red, green and blue and polychromatic (white) were tested. The DOFi was measured subjectively, using a motorised Badal system. Results: Taking as reference the average accommodative response for the white stimulus, the blue response exhibits on average a lead of 0.45 ± 0.09D, the green a negligible lead of 0.07 ± 0.02D and red a lag of 0.49 ± 0.10D. The monochromatic DOFi, calculated by averaging DOFi over the red, green and blue colour conditions for each accommodative demand was 1.10 ± 0.10D for 0D, 1.20 ± 0.08D for 2D, and 1.26 ± 0.40D for 4D. The polychromatic white DOFi were greater than the average monochromatic DOFi by 19%, 9% and 14% for 0D, 2D, and 4D of accommodative demand, respectively. Conclusion: The longitudinal chromatic aberration causes a dioptric shift of the monochromatic accommodation response. The study did not reveal this shift to depend on the accommodative demand or to have an effect on the DOFi.
We present and test a methodology for generating simultaneous vision with a deformable mirror that changed shape at 50 Hz between two vergences: 0 D (far vision) and −2.5 D (near vision). Different bifocal designs, including toric and combinations of spherical aberration, were simulated and assessed objectively. We found that typical corneal aberrations of a 60-year-old subject changes the shape of objective through-focus curves of a perfect bifocal lens. This methodology can be used to investigate subjective visual performance for different multifocal contact or intraocular lens designs.
The purpose of this work was to find plausible predictors among optical parameters that may explain the inter-individual differences in subjective amplitude of accommodation not explained by age. An exploratory multivariable regression analysis was carried out retrospectively on a dataset with 180 eyes from 97 subjects (ages ranged from 20 to 58years). Subjective amplitudes of accommodation were recorded with the use of a custom-made Badal system. A commercial aberrometer was used to obtain each eye's wavefront during the full range of accommodation. The plausible predictors under study were pupil diameter in the unaccommodated eye, its reduction with accommodation; fourth- and six-order Zernike spherical aberration, their reduction with accommodation, and subjective refraction. At a significance level of 0.05, only fourth- and sixth-order Zernike spherical aberration were found to be predictors of subjective amplitude of accommodation not explained by age, each explaining on their own less than 5% of the variance, and about 9% together. All other optical parameters explained less than 2%. Spherical aberration did not explain the greater variability for younger eyes than for older eyes. The remainder variability in amplitude of accommodation not explained by age or spherical aberration was about ±2.6D for 20year-old subjects, ±1.5D for 40year-old subjects and, about ±0.6D for 55year-old subjects. Optical factors do not seem to account for much of the inter-individual differences in subjective amplitude of accommodation. Most of the variability not explained by age must be due to anatomical differences and physiological, psychological, or other factors.
Purpose : We propose a new strategy that may permit the visual system to get objective information of the sign of defocus under monocular viewing conditions. It is based on the differences between the retinal images focused in front or behind the peripheral retina produced by the presence of a blood vessel network of arteries and veins in front of the photoreceptor layer. Methods : We performed two types of studies: geometrical and diffractional retinal image simulations using ray tracing software (Optics Studio, Zemax®); and experimental measurements, using a camera with a fine printed irregular reticle of 25 microns, placed a few tenths of a millimeter in front a high resolution CCD. Two types of images were compared: with the image out of focus in front (myopic) or behind (hyperopic) the ideal image plane (photoreceptors or CCD layer). Results : Myopic and hyperopic retinal images of extended objects are different when the simulated blood vessel network or a real reticle is placed in front of the ideal image plane. These differences are not found in the absence of the vessels/reticle. Ray-tracing simulations and experimental images show that myopic images of extended objects formed close to the vessels/reticle plane present larger values of contrast at high spatial frequencies than the hyperopic or in focus images (figure). This effect is more evident when the spatial frequency of objects and vessels are similar. In particular, Fourier analysis of the experimental myopic and hyperopic images defocused approximately by the same amount, shows a different phase transfer function as well as a larger power spectrum magnitude (between 2.4 and 9 times larger at 12 c/deg) in the myopic image compared to the hyperopic one. This phenomenon can be explained by geometrical optics as it is due to different shadow and penumbra shapes originated by the vessels/reticle depending on the sign of defocus. Conclusions : The retinal image formed through a blood vessel network contains information about the sign of defocus that may be used by the visual system as an objective cue for accommodation under monocular vision. The increase of higher spatial frequency content in the myopic image in the retinal periphery (where blood vessels are present) may play a role in the emmetropization process and inhibit myopia progression.
Purpose : To objectively evaluate the optical performance and simulate retinal images of an extended object in eyes wearing MPlus multifocal IOL's (Oculentis®), that offer an expanded depth of focus. To discover the reasons why some patients wearing those IOL's express discontent regarding their vision quality by using a novel computational method. Methods : A custom simulator program has been developed using MATLAB®, that allows the computation of polychromatic smulated retinal images from experimental aberrometric data. The software uses a novel implementation of Fourier and geometrical optics and the Indiana Eye Model to calculate polychromatic PSF's (point-spread functions) and simulated retinal images. It can perform through-Zernike simulations and image quality (IQ) estimation using metrics such as the Visual Strehl Ratio. It has been used to perform through-focus IQ analyses of 5 presbyopic eyes (mean age 59 years) with implanted IOL's (MPlus, 3.0D of addition). The optical design of this family of lenses, where the power varies continuously along the surface, allows to perform aberrometric measurements using a Hartmann-Shack sensor. The subjects' aberrations were measured using irx-3 (Imagine Eyes) and OPD-Scan III (NIDEK) aberrometers. Only the higher-order aberrations were taken into account. Results : In our through-focus simulations of the IQ with the MPlus bifocal IOL's (Fig. 1), the dioptric range between the two points of maximum IQ was 2.2 ± 0.3D. In the "content" group of eyes the nuclei of the best IQ PSF's subtended 2.5 ± 0.5 arcmin (visual acuity (VA) of 0.4 - 0.5 logMAR). The IQ in the "discontent" group expressed as the Strehl Ratio was 2 to 3 times lower and the simulated images either exhibited double "ghost" images (Fig. 2) or no bi-focality at all. Conclusions : Through-focus analysis performed using the software confirmed that most of the subjects have two zones of distances with better image quality. The dioptric distance between these two zones were 2.2 ± 0.3D which is in good agreement with the nominal addition of the IOL at the spectacle plane. The simulations have revealed two possible causes of discontent: lack of bi-focality or double "ghost" images yielded by secondary nuclei of the PSF's. It is possible to find the required change of Zernike coefficients in order to optimize the IQ for each particular eye.
The goal of this work was to objectively characterize the external morphology, topography, and optics of the cornea after orthokeratology (ortho-k). A number of 24 patients between the ages of 17 and 30 years (median=24years) were fitted with Corneal Refractive Therapy ® contact lenses to correct myopia between-2.00 and-5.00 diopters (D) (median=-3.41D). A classification algorithm was applied to conduct an automatic segmentation based on the mean local curvature. As a result, three zones (optical zone, transition zone, and peripheral zone) were delimited. Topographical analysis was provided through global and zonal fit to a general ellipsoid. Ray trace on partially customized eye models provided wave aberrations and retinal image quality. Monozone topographic description of the ortho-k cornea loses accuracy when compared with zonal description. Primary (C40) and secondary (C60) spherical aberration (SA) coefficients for a 5-mm pupil increased 3.68 and 19 times, respectively, after the treatments. The OZ area showed a strong correlation with C40 (r=-0.49, p<0.05) and a very strong correlation with C60 (r=0.78, p<0.01). The OZ, as well as the TZ, areas did not correlate with baseline refraction. The increase in the eye's positive SA after ortho-k is the major factor responsible for the decreased retinal optical quality of the unaccommodated eye. © 2016 Society of Photo-Optical Instrumentation Engineers (SPIE).
https://www.osapublishing.org/boe/abstract.cfm?uri=boe-7-7-2538 We propose an alternative solution to improve visual quality by spatially modulating the amplitude of light passing into the eye (related to the eye's transmittance), in contrast to traditional correction of the wavefront phase (related to the local refractive power). Numerical simulations show that masking the aberrated areas at the pupil plane should enhance visual function, especially in highly aberrated eyes. This correction could be implemented in practice using customized contact or intraocular lenses.
Purpose: Amplitude of accommodation (AA) is reportedly greater for myopic eyes than for hyperopic eyes. We investigated potential explanations for this difference. Methods: Analytical analysis and computer ray tracing were performed on two schematic eye models of axial ametropia. Using paraxial and nonparaxial approaches, AA was specified for the naked and the corrected eye using the anterior corneal surface as the reference plane. Results: Assuming that axial myopia is due entirely to an increase in vitreous chamber depth, AA increases with the amount of myopia for two reasons that have not always been taken into account. First is the choice of reference location for specifying refractive error and AA in diopters. When specified relative to the cornea, AA increases with the degree of myopia more than when specified relative to the eye's first Gaussian principal plane. The second factor is movement of the eye's second Gaussian principal plane toward the retina during accommodation, which has a larger dioptric effect in shorter eyes. Conclusions: Using the corneal plane (placed at the corneal vertex) as the reference plane for specifying accommodation, AA depends slightly on the axial length of the eye's vitreous chamber. This dependency can be reduced significantly by using a reference plane located 4 mm posterior to the corneal plane. A simple formula is provided to help clinicians and researchers obtain a value of AA that closely reflects power changes of the crystalline lens, independent of axial ametropia and its correction with lenses.
- Apr 2016
Purpose: To test and validate a method for measuring the optical quality and optical power of monofocal intraocular lenses (IOLs) in the presence of a tilt or a decentration as well as its optical power. Methods: The experimental system consists of an artificial eye (wet-cell) and a commercial aberrometer with a Shack-Hartmann sensor. Optical image quality parameters such as point spread function (PSF), modulation transfer function (MTF), and the simulations of the retinal image of an extended object are computed from the in vitro wavefront data of an IOL. Repeatability and reproducibility of the system are tested. Mathematical and ray-tracing simulations are used to ascertain the precision and accuracy of the method. Comparison with a standard single-pass method of measurement is also made. Optical properties of four commercial monofocal IOLs are measured under tilts of 0°, 2°, and 4°, and vertical decentrations of 0, 0.2, and 0.4 mm; the results of such measurements are compared with a ray-tracing simulation. Results: Precision and accuracy of the system are in good agreement with theoretical calculations. Reproducibility and repeatability are within standard ISO norms. MTFs obtained with this method and with the standard method are very similar. The precision of the estimation of the IOL's optical power is higher than 98.6%. Commercially tested IOLs show a decrease of optical quality in the presence of decentrations and tilts, and their susceptibility to tilts and decentrations depends on the asphericity. These results are in concordance with the results obtained by ray-tracing simulation. Conclusions: A simple wavefront-based method for in vitro measurements of the wavefront aberrations and power of an IOL is proposed and tested showing accurate and precise results.
- Jan 2016
- ARVO 2016
Purpose:We test an alternative solution to improve visual quality by spatially modulating the amplitude of light passing into the eye (modulating transmittance), in contrast to traditional correction of the wavefront phase (modulating local refractive power) achieved by ophthalmic, contact or intraocular lenses. Methods:A custom software programmed in MATLAB® based in a region-growing algorithm was developed to find the pupil shape that maximizes retinal image quality by maximizing the modulation transfer function (MTF) weighted by the neural contrast sensitivity function without normalization (VSMTF_abs). The algorithm was applied to 10 wavefronts from 10 eyes:1 emmetrope; 2 myopic; 2 astigmatic; 3 keratoconic (Kc); and 2 that underwent a penetrating keratoplasty (PK). The MTF, the point spread function (PSF) and its convolution with an extended object were obtained and compared with circular pupils of the same area, corresponding to a circular pupil of a diameter of 4 mm . Robustness to decentration and rotation of the irregular pupils was analyzed. Results:Customized pupil shapes, usually non circular, resulted in improvements in the image quality when compared with circular or apodized pupils for all the cases analyzed. On average, optimized pupils produced a range of 4 to 52% increase in the VSMTF_abs with respect a circular pupil of the same area. Larger improvements of the metric were obtained in the eyes with low aberrations due to the sensitivity of high MTFs to small corrections. However, the convoluted images show the largest benefit of this technique is obtained in the high aberrated eyes such as Kc (see figure) and PK. Robustness to decentration and rotation of the irregular pupil depends on the aberration level and type, but in the worst case > 0.3 mm or 20 degrees displacement and rotation were required before reaching the VSMTF_abs values obtained by the circular pupil. Conclusions:Numerical simulations show that masking the aberrated areas at the pupil plane should enhance visual function, especially in highly aberrated eyes. This correction could be implemented in practice using customized contact, intraocular lenses or new aniridia implants.
Ocular straylight has been measured by means of psychophysical methods over the years. This approach gives a functional parameter yielding a straight comparison with optically defined light scattering, and the point-spread-function. This is of particular importance when the effect of intraocular lenses (IOLs) on postoperative straylight is sought. An optical system for straylight measurements of IOLs was adapted to a commercial device (C-Quant, Oculus), which employs such psychophysical method. The proposed modifications were validated using light-scattering filters and some sample IOLs. The measurements were performed by 3 observers to prove that results are independent from straylight of the eye. Other applications will be discussed.
- Oct 2015
PURPOSE: To evaluate visual quality when changing the intraocular orientation of the Lentis Mplus LS-312MF nonrotational symmetric +3.00 diopters aspheric multifocal intraocular lens ([IOL] Oculentis GmbH, Berlin, Germany) in normal eyes. METHODS: An artificial eye was used to measure the in vitro wavefront of the IOL. The corneal topography of 20 healthy patients was obtained. For each eye, a computational analysis simulated the implantation of the IOL. The modulation transfer function (MTF) and an image quality parameter (visually modulated transfer function [VSMTF] metric) were calculated for a 5.0-mm pupil and for three conditions: distance, intermediate, and near vision. The procedure was repeated for each eye after a rotation of the IOL with respect to the cornea from 0° to 360° in 1° steps. RESULTS: Statistical analysis showed significant differences in mean VSMTF values between orientations for distance vision. Optimal orientation of the IOL (different for each eye) showed a mean improvement of 58% ± 19% (range: 20% to 121%) in VSMTF values with respect to the worst possible orientation. For these orientations, intermediate and near vision quality were statistically indistinguishable. The MTFs were different between orientations, showing a mean difference of approximately 5 cycles per degree in the maximum spatial frequencies that can be transferred between the best and the worst orientations for distance vision. CONCLUSIONS: The results suggest that implantation of this nonrotational symmetric IOL should improve visual outcomes if it is oriented to coincide with a customized meridian. A simple, practical method is proposed to find an approximation to the angle that an Mplus IOL should be inserted.
The purpose of this study was to describe the design and characterization of a new opto-mechanical artificial eye (OMAE) with accommodative ability. The OMAE design is based on a second-pass configuration where a small source of light is used at the artificial retina plane. A lens whose focal length can be changed electronically was used to add the accommodation capability. The changes in the OMAE's aberrations with the lens focal length, which effectively changes the accommodative state of the OMAE, were measured with a commercial aberrometer. Changes in power and aberrations with room temperature were also measured. The OMAE's higher-order aberrations (HOAs) were similar to the ones of the human eye, including the rate at which fourth-order spherical aberration decreased with accommodation. The OMAE design proposed here is simple, and it can be implemented in an optical system to mimic the optics of the human eye.
We performed a theoretical and computational analysis of the through-focus axial irradiance in a system with a Gaussian amplitude pupil function and fourth- and sixth-order spherical aberration (SA). Two cases are analyzed: low aberrated systems, and the human eye containing significant levels of SA and a natural apodization produced by the Stiles-Crawford effect. Results show that apodization only produces a refraction change of the plane that maximized the Strehl ratio for eyes containing significant levels of negative SA.
Purpose: Recently it has been proven that spherical aberration has influence over the depth of field (DOFi) and the accommodation lag (Bernal-Molina et al., OVS, 2014). We study if the longitudinal chromatic aberration (LCA) also plays a role in increasing the DOFi during accommodation, thus providing an effectively larger range of accommodation. Methods: Wavefront aberration maps for three accommodative demands (AD) 0D, 2D, 4D were measured in 4 subjects using a custom-made adaptive optics system equipped with a Shack-Hartmann sensor, a deformable mirror (Mirao 52-e, Imagine Eyes), and a Badal system controlled by the subject. The accommodation was paralyzed with 2 drops of cyclopentolate (1%). The deformable mirror was then set to cancel out the paralyzed aberrations and add the ones obtained for each AD, to the end that the system simulated the optics of the accommodated eye. Subjects looked through the system including the mirror and a 5.7 mm artificial pupil at a target on an OLED microdisplay. The target consisted of 5 Sloan letters (0.1 logMAR) shown in sequence in B&W, red (R, 650nm), green (G, 550nm) and blue (B, 468nm). The luminance was in all cases constant at 12 cd/m2. By means of the Badal system, subjects indicated near and far borders of the perceived DOFi based upon the objectionable blur criterion. The DOFi of the cycloplegic eyes with all the aberrations corrected (VA < -0.3 logMAR) was also obtained in all color conditions. Results: Intersubject mean values of DOFi were similar between different color conditions: for the AD of 0D we obtained 0.90±0.15D for B&W, 1.10±0.14D for R, 0.95±0.25D for G, and 0.91±0.12D for B. The mean DOFi was also stable between different simulated accommodation conditions, for example the B&W DOFi was 0.9±15D for 0D, 1.0±15D for 2D and 0.8±0.28D for 4D, and 1.0±0.10D for the fully corrected eye. The DOFi due to the chromatic aberration (the dioptric span between the outer limits of the R and B DOFi) was on average 1.9 times larger than the DOFi found for the B&W condition, for each of the AD's. Conclusions: The values of the B&W and monochromatic DOFi are similar and practically do not change for any of the simulated AD's. However the DOFi due to the full range of LCA is nearly twice as large as the one obtained for the B&W. These results indicate that the presence of an accommodative lag should not be influenced by the presence of chromatic aberration.
- Apr 2015
- I Congreso de Jóvenes Investigadores de Canarias
Purpose To test and validate a simple and low-cost method for measure the wavefront of intraocular lenses (IOLs) Methods: The experimental system consist of an artificial eye (wet-cell) and a commercial aberrometer with a Shack-Hartmann sensor. Optical image quality parameters such as Point Spread Function (PSF), Modulation Transfer Function (MTF) and the simulations of the retinal image of an extended object, are computed from the in vitro wavefront data of an IOL. The optical performance of the system was evaluated with different measurements and computerized simulations. Results Precision and accuracy of the system revealed in good agreement with theoretical calculations. Reproducibility and repeatability were in line with standard ISO normative. The precision of the estimation of the IOL’s’ optical power is higher than 98.6%. Conclusions A simple wavefront-based method for in-vitro measurements of the wavefront aberrations and power of an IOL is proposed and tested showing accurate and precise results.
- Jan 2015
PurposeTo examine the role of Zernike secondary spherical aberration and its component terms on refraction, image quality and depth of focus. Methods Computational methods were used to define wavefronts with controlled levels of r(6), r(4) and r(2) terms, and image quality associated with these terms for a range of target vergences. Target vergences that generated maximum image quality were used as an objective measures of refractive error. ResultsUnlike primary Zernike spherical aberration, which generates peak image quality with a near paraxial focus, in the absence of other higher order aberrations, peak image quality with secondary spherical aberration is achieved with a near marginal focus. When alone, positive primary and secondary spherical aberration induce small hyperopic shifts in refraction, but in the presence of other higher order aberrations, secondary spherical aberration can induce significant myopic shifts in refractive error, as predicted by the combined lower order r(4) & r(2) component of Z(6)(0). The predicted expansion in depth of focus associated with increased primary or secondary spherical aberration is mostly absent if a strict image quality criterion is applied. The expansion of depth of focus observed with a low image quality criterion when opposite sign Z(4)(0) and Z(6)(0) are combined is primarily due to the elevated r(4) term. Conclusions Secondary Zernike spherical aberration can have a significant impact on refractive error, image quality and depth of focus, but mostly due to the lower order components within this polynomial. Our analysis shows that the r(6) term that defines secondary spherical aberration actually narrows rather than expands depth of focus, when in the presence of the r(4) term within Z(6)(0). Therefore, a multifocal lens generated with exclusively primary spherical aberration is likely to be more effective than one that includes opposite sign of primary and secondary spherical aberration.
- Sep 2014
Purpose: We evaluate the effect of pupil size on objectively measured amplitude of accommodation (AA). Methods: Pupil diameter and wavefront aberrometry were obtained in 15 eyes when stimulus swept across the range of clear vision in steps of 0.5 diopters. Wavefront refraction techniques were used to compute objective AA as the maximum refractive change. Measurements were obtained monocularly under low and high ambient room lighting conditions with a fixed luminance of the fixation target. Amplitude of accommodation computations were performed taking into account just paraxial rays (paraxial AA) or including the effects of the change of spherical aberration during accommodation (minRMS AA). Results: Mean pupil size values at low light level were 6.26 mm (relaxed) and 4.15 mm (maximum accommodation), whereas at the high light level, those values became 4.74 and 3.04 mm, respectively. The effects of both light level on accommodation were significant (p < 0.001), and accommodative miosis was slightly larger at low light levels. Mean minRMS and paraxial AA were always greater by more than 1 diopter in high than in low ambient lighting conditions (p < 0.01), indicating a significant impact of pupil size on AA. Conclusions: The influence of the ambient lighting on the objective AA is not only attributed to the increased effects of spherical aberration as the pupil dilates but mostly attributed to a decrease in the paraxial accommodation as pupil dilates.
- Aug 2014
Purpose: To obtain experimental values of the depth-of-field (DOFi) of the human eye for different accommodative states. Methods: First, the monochromatic ocular wavefront of seven eyes from young subjects (mean [±SD] age, 29.7 [±7.7] years) was measured at eight different accommodative demands (ADs) (from -1 to 6 diopters [D] in steps of 1 D). Then, in a second part, accommodation was paralyzed and an adaptive optics system was used to correct the aberrations of the paralyzed eye and to simulate, with the aid of an artificial pupil, the wavefront of the accommodated eye. The simulation was performed for each AD measured in the first part of the experiment. A Badal system was used to modify the stimulus vergence so as to obtain three repeated measurements of the subjective DOFi, based on the criterion of an objectionable blur. Results: When increasing AD from 0 to 6 D, the mean intersubject pupil diameter and DOFi changed from 5.70 to 4.62 mm and from 0.85 ± 0.26 D to 1.07 ± 0.19 D, respectively. All subjects presented a similar DOFi for all AD (intrasubject SD never exceeded 0.23 D). Paraxial accommodation response showed a lag that increased with the AD. For the lowest (0 D) and the highest (6 D) values of AD, the refractive state of the eye was close to the nearest and furthermost ends of the DOFi, respectively. Conclusions: The visual system takes advantage of the DOFi to change the refractive state less than necessary to form the paraxial image at the retina when it comes to focusing a near target (5 to 6 D of AD). This indicates that the main purpose of accommodation is not to maximize retinal image quality but to form one that is good enough.
To evaluate the error in the estimation of axial length (AL) with the IOLMaster partial coherence interferometry (PCI) biometer and obtain a correction factor that varies as a function of AL and crystalline lens thickness (LT). Optical simulations were produced for theoretical eyes using Zemax-EE software. Thirty-three combinations including eleven different AL (from 20mm to 30mm in 1mm steps) and three different LT (3.6mm, 4.2mm and 4.8mm) were used. Errors were obtained comparing the AL measured for a constant equivalent refractive index of 1.3549 and for the actual combinations of indices and intra-ocular dimensions of LT and AL in each model eye. In the range from 20mm to 30mm AL and 3.6-4.8mm LT, the instrument measurements yielded an error between -0.043mm and +0.089mm. Regression analyses for the three LT condition were combined in order to derive a correction factor as a function of the instrument measured AL for each combination of AL and LT in the theoretical eye. The assumption of a single "average" refractive index in the estimation of AL by the IOLMaster PCI biometer only induces very small errors in a wide range of combinations of ocular dimensions. Even so, the accurate estimation of those errors may help to improve accuracy of intra-ocular lens calculations through exact ray tracing, particularly in longer eyes and eyes with thicker or thinner crystalline lenses.
- Mar 2014
To describe a new methodology that derives horizontal posterior retinal contours from partial coherence interferometry (PCI) and ray tracing using the corneal topography. Corneal topography and PCI for seven horizontal visual field eccentricities correspondent to the central 60 degrees of the posterior pole were obtained in 55 myopic eyes. A semicustomized eye model based on the subject's corneal topography and the Navarro eye model was generated using Zemax-EE software. The model was used to compute the optical path length in the seven directions where PCI measurements were obtained. Vitreous chamber depth was computed using the PCI values obtained at each of those directions. Matlab software was developed to fit the best conic curve to the set of points previously obtained. We tested the limit in the accuracy of the methodology when the actual cornea of the subject is not used and for two different lens geometries. A standard eye model can induce an error in the retina sagitta estimation of the order of hundreds of micrometers in comparison with the semicustomized eye model. However, the use of a different lens model leads to an error of the order of tens of micrometers. The apical radius and conic constant of the average fit were -11.91 mm and -0.15, respectively. In general, a nasal-temporal asymmetry in the retina contour was found, showing mean larger values of vitreous chamber depth in the nasal side of the eye. The use of a semicustomized eye model, together with optical path length measured by PCI for different angles, can be used to predict the retinal contour within tenths of micrometers. This methodology can be useful in studies trying to understand the effect of peripheral retinal location on myopia progression as well as modeling the optics of the human eye for a wide field.
- Feb 2014
In order to work in a consistent way with Zernike aberration coefficients estimated in different pupils, it is necessary to refer them to a common pupil size. Two standard approaches can be used to that end: to rescale algebraically the coefficients estimated in the original pupil or to refit them anew using the wavefront slope measurements available within the new one. These procedures are not equivalent; they are affected by different estimation errors that we address in this work. Our results for normal eye populations show that in case of reducing the pupil size it is better to rescale the original coefficients than to refit them using the measurements contained within the smaller pupil. In case of enlarging the pupil size, as it can a priori be expected, the opposite holds true. We provide explicit expressions to quantify the errors arising in both cases, including the expected error incurred when extrapolating the Zernike estimation beyond the radius where the measurements were made.
- Nov 2013
In some eyes, aberrometric measurements of the refractive error can differ by more than 1 diopter from standard subjective refraction. We aim to understand the reasons for these discrepancies and to study the role of both amplitude (irregular or inhomogeneous pupil transmission) and phase (aberrations) of the wavefront. The spherical equivalent was measured by different subjective, objective, and aberrometric methods in a population of 177 eyes. We first analyzed the degree of correlation between the outcomes of the different methods. Then we analyzed cases showing the highest discrepancies (>1 diopter) between subjective and aberrometric spherical equivalent. The refractive error sensing method was generalized and applied here to include the effect of inhomogeneous pupil transmittance (Stiles-Crawford effect) and irregular pupil shapes on refractive error. Objective and aberrometric methods showed a strong correlation with subjective methods (R > 0.89 in all cases). However, individual data points may show large discrepancies. Several eyes with discrepancies of 1 D or even 2 D usually presented higher values of higher-order aberration (mainly coma and/or spherical aberration) than average, which may cause these eyes to have a natural bifocal (or even multifocal) optical performance. Refractive error sensing analysis suggests that this multifocal performance could explain the high objective-subjective discrepancies found in these eyes. Nevertheless, the Stiles-Crawford effect (or irregular pupils) can substantially modify the energy distribution, tending to minimize multifocal effects, thus minimizing discrepancies between aberrometric and subjective refraction. Discrepancies between aberrometric and subjective refraction may appear in aberrated eyes when aberrometric methods ignore the impact of the wavefront amplitude (inhomogeneous or irregular pupil transmittance.) The generalized refractive error sensing proposed here seems promising and able to provide a reliable clinical refraction in monofocal and multifocal eyes.
Visual sensing of the sign of defocus is important not only for the rapid control of accommodation but also for regulating the slower long-term growth of the eye. We examined the possibility that ocular spherical aberration (SA) elicits a sign of defocus by optical modeling of image formation using the principles of physical optics. Retinal image contrast, as measured by the area under the visually weighted modulation-transfer function, depends on both the magnitude and the sign of defocus relative to the sign of the SA. Image contrast is greater for hyperopic blur than for myopic blur when SA is positive and vice versa when SA is negative. When coupled with Wallman's hypothesis that retinal activity caused by image contrast inhibits eye growth, these results provide a testable hypothesis to account for myopia progression. For example, we suggest that hyperopic blur is a risk factor for myopia progression only when the eye has a negative SA because that is the combination leading to relatively low contrast in the defocused retinal image. Because the likelihood of a negative SA increases with accommodation, avoiding long hours of near work in the presence of accommodative lag may help prevent the onset and progression of myopia.
- Sep 2013
To compare visual and optical quality of the Crystalens HD intraocular lens (IOL) with that of a monofocal IOL. The wavefront aberration patterns of the monocular Akreos Adapt AO IOL and the single-optic accommodating Crystalens HD IOL were measured in a model eye. The Crystalens IOL was measured in its nonaccommodative state and then, after flexing the haptic to produce 1.4 mm of movement, in its accommodative state. Using an adaptive optics system, subjects' aberrations were removed and replaced with those of pseudophakes viewing with either lens. Monocular distance visual acuity (DVA) at high (100%), medium (50%), and low (10%) contrast and contrast sensitivity (CS) were measured for both IOL optics. Near VA (NVA) and CS were measured for the Crystalens HD IOL in its accommodative state. Depth of focus around the distance and near focus was also evaluated for the Crystalens HD IOL. Modulation transfer function (MTF), point spread function (PSF), and Strehl ratio were also calculated. All measures were taken for 3- and 5-mm pupils. The MTF, PSF, and Strehl ratio showed comparable values between IOLs (p > 0.05). There were no significant differences in DVA and CS between IOLs for all contrasts and pupils (p > 0.05). When spherically focused, mean DVA and NVA with the Crystalens HD IOL were ≥20/20 at 100 and 50% contrasts for both pupils. Monocular DVA, NVA, and CS were slightly better with 3- than 5-mm pupils, but without statistically significant differences. The Crystalens HD IOL showed about 0.75 and 0.50 D of depth of focus in its accommodative state and nonaccommodative state, respectively. The optical and visual quality with the nonaccommodatied Crystalens HD IOL was comparable to that of a monofocal IOL. If this lens can move 1.4 mm in the eye, it will provide high-quality optics for near vision as well.
- Jul 2013
We asked if retinal image quality is maximum during accommodation, or sub-optimal due to accommodative error, when subjects perform an acuity task. Subjects viewed a monochromatic (552 nm), high-contrast letter target placed at various viewing distances. Wavefront aberrations of the accommodating eye were measured near the endpoint of an acuity staircase paradigm. Refractive state, defined as the optimum target vergence for maximising retinal image quality, was computed by through-focus wavefront analysis to find the power of the virtual correcting lens that maximizes visual Strehl ratio. Despite changes in ocular aberrations and pupil size during binocular viewing, retinal image quality and visual acuity typically remain high for all target vergences. When accommodative errors lead to sub-optimal retinal image quality, acuity and measured image quality both decline. However, the effect of accommodation errors of on visual acuity are mitigated by pupillary constriction associated with accommodation and binocular convergence and also to binocular summation of dissimilar retinal image blur. Under monocular viewing conditions some subjects displayed significant accommodative lag that reduced visual performance, an effect that was exacerbated by pharmacological dilation of the pupil. Spurious measurement of accommodative error can be avoided when the image quality metric used to determine refractive state is compatible with the focusing criteria used by the visual system to control accommodation. Real focusing errors of the accommodating eye do not necessarily produce a reliably measurable loss of image quality or clinically significant loss of visual performance, probably because of increased depth-of-focus due to pupil constriction. When retinal image quality is close to maximum achievable (given the eye's higher-order aberrations), acuity is also near maximum. A combination of accommodative lag, reduced image quality, and reduced visual function may be a useful sign for diagnosing functionally-significant accommodative errors indicating the need for therapeutic intervention.
- Mar 2013
To understand how primary and secondary spherical aberrations affect focusing of the retinal image and the measurement of refractive state in the accommodating eye. A computational eye model was constructed from published anatomical dimensions of the eye's refractive elements for a range of accommodative states. Two strategies for controlling accommodation were implemented, one in which paraxial rays are always perfectly focused and the other in which paraxial accommodative lag grew larger as target vergence increased. Multiple configurations of the model were achieved by selecting various combinations of pupil size and aberration structure. Refractive state was defined as optimum target vergence for maximizing retinal image quality according to several scalar metrics. When accommodation optimally focuses paraxial rays, retinal image quality is sub-optimal for metrics of image quality sensitive to non-paraxial rays. This loss of image quality can be recovered by optimizing target vergence computationally, which indicates the presence of real accommodative error according to the non-paraxial metric even though the eye is accurately focused paraxially. However, such errors are spurious if non-paraxial refractive state is misinterpreted as paraxial refractive state. Accommodative errors may indicate lag or lead, but in general the slope of the stimulus-response function is less than 1 for non-paraxial measures of image quality. These results depend strongly on pupil size and its variation due to accommodative miosis. spurious accommodative errors can appear when the eye focuses the retinal image optimally according to one metric of image quality (e.g. paraxial) while ocular refractive state is measured by another (e.g. non-paraxial). Spurious errors are small compared to real accommodative lag for small, photopic pupils but can be of the same order of magnitude as real lag for large, mesopic pupils.
To evaluate the optical quality of Acrysof Restor SN6AD1 refractive-diffractive +3.00 D and Lentis Mplus LS-312 nonrotational symmetric +3.00 D aspheric multifocal intraocular lenses (IOLs) and analyze the effect of tilt and decentration. University of Valencia, Valencia, and University of Murcia, Murcia, Spain. Experimental study. An artificial model eye was used to measure wavefront aberrations of both IOL models. The modulation transfer function (MTF) and point-spread function (PSF) were calculated for a 5.0 mm pupil and under 5 situations: centered, 0.2 mm and 0.4 mm decentered, and 2 degrees and 4 degrees tilted. The refractive-diffractive IOL had the highest MTF values at all spatial frequencies. Similarly, the PSF was worse for the refractive-diffractive IOL. The MTFs for the refractive-diffractive IOL decreased when the IOL was decentered or tilted, being more robust for tilting than decentration. The MTFs for the nonrotational symmetric IOL were similar under all testing conditions. Cutoff frequencies for the refractive-diffractive IOL were stable over the range of tilting studied and comparable with the cutoff when the IOL was centered (approximately 50 cycles per degree [cpd]). When the IOL was decentered, the cutoff frequency decreased by approximately 40 cpd. The nonrotational symmetric IOL cutoff frequencies for misalignment and tilt were low (approximately 30 cpd) and similar under all conditions. The refractive-diffractive IOL provided better optical quality than the nonrotational symmetric IOL. Tilt and decentration had a significant impact on optical quality with both IOLs, being more severe with the nonrotational symmetric IOL.
To analyze visual quality differences between intraocular lenses (IOLs) and assess the impact of IOL decentration and tilt on visual quality. University of Valencia, Valencia, Spain. Cohort study. The crx1 adaptive optics visual simulator was used to simulate the wavefront aberration pattern of 2 commercially available aspheric aberration-correcting IOLs (Acrysof IQ SN60WF and Tecnis ZA9003) and 2 spherical IOLs (Akreos Adapt and Triplato) in 5 situations: centered, decentered 0.2 mm and 0.4 mm, and tilted 2 degrees and 4 degrees. Monocular distance visual acuity at 100%, 50%, and 10% contrast and the depth of focus were measured. Ten eyes of 10 patients were evaluated. When the IOLs were centered, there were no differences in visual acuity between the 4 IOLs at any contrast. The aberration-correcting IOLs were more sensitive to tilt and decentration than the spherical IOLs; Tecnis ZA9003 IOL was the most sensitive to decentration and the Acrysof IQ SN60WF IOL was the most sensitive to tilt. Higher residual spherical aberration slightly improved depth of focus and the tolerance to defocus. The results in this study suggest that the aspheric aberration-correcting and spherical IOLs provided comparable visual quality when centered in eyes in which the corneal higher-order aberrations are those of the average of the human cornea. Tilt and decentration of the IOLs had an impact on visual quality, with aberration-correcting IOLs having a greater effect than the spherical IOLs.
To determine the theoretical effect of an increase in the negative Q-factor (hyperprolateness) on the refractive state and its effect on presbyopia treatment. Ray tracing software was used to simulate the Navarro eye model. To simulate refractive therapy, corneal Q-factor ranged from 0 to -1.50. Refractive state of the model eye was defined as the target vergence required to maximize retinal image quality according to several image quality metrics. The change of refractive state produced by pupil constriction from 6 to 2 mm was estimated. For 0 ≥ Q ≥ -0.6, spherical aberration of the model eye is positive, therefore, pupil miosis causes a hypermetropic shift in refractive state. Over the range -0.6 ≥ Q ≥ -1.5, spherical aberration becomes negative and pupil constriction causes the eye's far point to approach the eye, inducing a myopic shift. A maximum 0.60-diopter myopic shift in refractive state due to pupil constriction occurred for Q=-1.25. Optical modeling predicts that surgically induced negative corneal Q-value will produce a change in the refractive state of the eye after pupillary miosis.
- May 2012
First described during the 18th century, the cause of night myopia remains a controversial topic. Whereas several explanations have been suggested in the literature, particularly related with accommodation or chromatic shift in scotopic light conditions, no definitive explanation for its aetiology has been provided. We describe an experiment in which ocular refractive state was objectively and subjectively measured while viewing two kind of stimulus: letters on a bright background and a punctual source of light in a dark background. We found that under photopic conditions the optimum refractive state of the accommodating eye is significantly more myopic when maximizing perceived quality of a point source on a dark background compared to a conventional letter chart with black letters on a white background. Optical modeling suggested this difference in refractive state is due to spherical aberration. Since isolated point sources are more likely encountered at night, whereas extended objects are more likely encountered in the daytime, our results suggest that a significant part of the night myopia phenomenon is determined by the nature of the visual stimulus and the visual task used to assess ocular refractive state.
To evaluate visual quality differences among intraocular lenses (IOLs) in patients with previous hyperopic laser ablations and to assess the impact of decentration and tilt of IOLs on visual quality. An adaptive optics visual simulator was used to simulate the wavefront aberration pattern of one aberration-correcting IOL (AcrySof IQ SN60WF, Alcon Laboratories Inc) and two spherical IOLs with different amounts of positive spherical aberration (Akreos Adapt [Bausch & Lomb] and Triplato [AJL Ophthalmic]) in five situations-centered, 0.2 mm and 0.4 mm of decentration, and 2° and 4° of tilt-in two groups: simulated low hyperopic laser corneal ablation (low hyperopia group) and high hyperopic laser corneal ablation (high hyperopia group). Monocular distance visual acuity at 100%, 50%, and 10% contrast were measured. Ten eyes were evaluated. When the IOLs were centered, all IOLs obtained comparable results for the low hyperopia group, whereas for the high hyperopia group, the Akreos Adapt and AcrySof IQ SN60WF showed better visual acuity than the Triplato. When the IOLs were misaligned, for the low hyperopia group, the best visual acuity results were obtained with the Akreos Adapt and the most critical situation was at 0.4 mm of decentration. For the high hyperopia group, misalignments decreased visual acuity in a higher amount than for the low hyperopia group. Our results suggest that the IOLs studied offer good visual quality when they are centered for both groups. However, tilt and decentration of monofocal IOLs have an impact on visual function in patients with hyperopic ablations. For these patients, the Akreos Adapt is the most robust to misalignments.
To compare the effect of primary spherical aberration and vertical coma on depth of focus measured with 2 methods. Laboratoire Aimé Cotton, Centre National de la Recherche Scientifique, and Université Paris-Sud, Orsay, France. Evaluation of technology. The subjective depth of focus, defined as the interval of vision for which the target was still perceived acceptable, was evaluated using 2 methods. In the first method, the subject changed the defocus term by reshaping the mirror, which also corrected the subject's aberrations and induced a certain value of coma or primary spherical aberration. In the second procedure, the subject changed the displayed images, which were calculated for various defocuses and with the desired aberration using a numerical eye model. Depth of focus was measured using a 0.18 diopter (D) step in 4 nonpresbyopic subjects corrected for the entire eye aberrations with a 6.0 mm and 3.0 mm pupil and with the addition of 0.3 μm and 0.6 μm of positive primary spherical aberration or vertical coma. There was good concordance between the depth of focus measured with both methods (differences within 1/3 D, r(2) = 0.88). Image-quality metrics failed to predict the subjective depth of focus (r(2) < 0.41). These data confirm that defocus in the retinal image can be generated by optical or computational methods and that both can be used to assess the effect of higher-order aberrations on depth of focus. No author has a financial or proprietary interest in any material or method mentioned.
- Jan 2012
- ESCRS 2012
Purpose: To evaluate the optical quality of the AcrySof ReSTOR and Mplus intraocular lenses (IOLs) and analyze the effect of tilt and decentration Methods: An artificial model eye was used to measure the wavefront aberrations pattern of the refractive-diffractive aspheric AcrySof ReSTOR +3D and the non-rotationally symmetric refractive aspheric Mplus +3D IOLs. The Modulation Transfer Function (MTF) and Point Spread Function (PSF) were calculated. All measurements were taken for 5-mm pupil diameter and under five situations: centered, 0.2 mm and 0.4 mm of decentration, 2 and 4 degrees of tilt Results: The AcrySof ReSTOR +3D had the highest MTF values at all spatial frequencies. MTF values were always lower for the MPlus +3D IOL. Similarly, the PSF found for the MPlus +3D IOL was worse compared to the ReSTOR +3D IOL. MTFs for the ReSTOR +3D IOL were reduced when the lens was decentered and tilted, being more robust for tilting than decentration. MTFs for the MPlus +3D IOL were similar for all conditions of testing, centered, tilted and decentered. Cut-off frequencies for the ReSTOR +3D IOL is stable over the range of tilting studied and comparable to the cut-off when the IOL is centered (about 50 c/deg). When the lens was decentered the cut-off frequency is reduced about 40 c/deg. The MPlus +3D IOL cut-off frequencies for misalignment and tilt situations were low (about 30 c/deg) and similar for all conditions studied. Conclusions: The AcrySof ReSTOR +3D IOL showed better optical quality than the MPlus +3D IOL. Tilt and decentration has an important impact in optical quality for both IOLs, being severe for the MPlus +3D IOL.
- Dec 2011
We optimize the subjective depth of focus (DoF) with combinations of spherical aberration (SA4) and secondary spherical aberration (SA6) in various levels. Subjective DoF was defined as the visual interval for which three 20/50 high-contrast letters was perceived acceptable (objectionable blur limits). We used an adaptive optics system to dynamically correct the observer's aberrations and control their accommodation. DoF was measured with a 0.18-D step on three non-presbyopic subjects. The target seen by the subjects was modified to include 25 combinations of SA4 and SA6 (i.e. 0, ± 0.15 and ± 0.30 μm) for 3, 4.5 and 6mm of pupil diameter. We found a mean DoF of 1.97D with a 3mm pupil size, which decreased by 28% with a 4.5mm pupil and by 34% with a 6mm pupil. For 6mm pupil we found an increase of subjective DoF of 45% and 64% with the addition of 0.3 and 0.6 μm of SA4, and of 52% and 117% with the addition of 0.15 and 0.3 μm of SA6. The largest DoF measured (4.78D) increased 3.6 times that of the naked eye and was found for a combination of opposite signs of SA4 and SA6 of 0.6 and 0.3 μm respectively. Reducing the pupil size minimized the effect of aberrations on subjective DoF. Combination of SA4 and SA6 of opposite sign could increase DoF more than three times for pupils larger than 4.5mm. Subjective DoF is well predicted by measuring the induced variation of vergence arising in the pupil size.
- Jan 2011
- Invest. Ophthalmol. Vis. Sci.
PurposeAccommodative miosis is a natural decrease of pupil size during accommodation. We investigated the effect of miosis on the measurement of refractive state and accommodation. MethodsAn optical bench fitted with a modified aberrometer (COAS, Wavefront Sciences Inc.) was used to obtain wavefront aberration measurements in the right eyes of 5 non-presbyopes during accommodation. Vergence of the visual stimulus (Landolt C) ranged from +0.5 to -4.5 D relative to the eye's far-point. To ensure accurate accommodation during aberrometry, measurements were obtained when the subject was correctly reading letters near their own personal resolution limit. The experiment was performed monocularly with and without pharmacological dilation of the pupil. Pupil dilation without cycloplegia was achieved by instilling one or two drops of phenylephrine. Refractive state of the eye was measured 2 ways: (1) by the Zernike coefficient C20 for defocus (which specifies the target vergence required for optimal focusing of t
- Dec 2010
To study the impact on the subjective depth of field of 4th-order spherical aberration and its combination with 6th-order spherical aberration and analyze the accuracy of image-quality metrics in predicting the impact. Laboratoire Aimé Cotton, Centre National de la Recherche Scientifique, Université Paris-Sud, Orsay, France. Case series. Subjective depth of field was defined as the range of defocus at which the target (3 high-contrast letters at 20/50) was perceived acceptable. Depth of field was measured using 0.18 diopter (D) steps in young subjects with the addition of the following spherical aberration values: ±0.3 μm and ±0.6 μm 4th-order spherical aberration with 3.0 mm and 6.0 mm pupils and ±0.3 μm 4th-order spherical aberration with ±0.1 μm 6th-order spherical aberration for 6.0 mm pupils. The addition of ±0.3 and ±0.6 μm 4th-order spherical aberration increased depth of field by 30% and 45%, respectively. The combination of 4th-order spherical aberration and 6th-order spherical aberration of opposite signs increased depth of field more than 4th-order spherical aberration alone (ie, 63%), while the combination of 4th-order spherical aberration and 6th-order spherical aberration of the same signs did not (ie, 24%). Whereas the midpoint of the depth of field could be predicted by image-quality metrics, none was found a good predictor of objectionable depth of field. Subjective depth of field increased when 4th-order spherical aberration and 6th-order spherical aberration of opposite signs were added but could not be predicted with image-quality metrics.
- Nov 2010
Theoretical and ray-tracing calculations on an accommodative eye model based on published anatomical data, together with wave-front experimental results on 15 eyes, are computed to study the change of spherical aberration during accommodation and its influence on the accommodation response. The three methodologies show that primary spherical aberration should decrease during accommodation, while secondary spherical aberration should increase. The hyperbolic shape of the lens' surfaces is the main factor responsible for the change of those aberrations during accommodation. Assuming that the eye accommodated to optimize image quality by minimizing the RMS of the wave front, it is shown that primary spherical aberration decreases the accommodation response, while secondary spherical aberration slightly increases it. The total effect of the spherical aberration is a reduction of around 1/7 D per diopter of stimulus approximation, although that value depends on the pupil size and its reduction during accommodation. The apparent accommodation error (lead and lag), typically present in the accommodation/response curve, could then be explained as a consequence of the strategy used by the visual system, and the apparatus of measurement, to select the best image plane that can be affected by the change of the spherical aberration during accommodation.
Purpose: To evaluate the possible change in the optics of the human eye after iris constriction. Methods: Ocular aberrations were measured under natural viewing conditions in 26 eyes. The measured eyes fixated on a dim target while the contralateral eye was either occluded (so the measured eye had a large pupil) or highly illuminated (so the measured eye had a small pupil). The measured eyes fixated to a dim target placed 0.5 D beyond the subject’s far point. Zernike values obtained in both situations were compared within the same pupil diameter corresponding to the one obtained under the high illumination condition. Results: Significant variation in some aberration coefficients were found between the two illumination conditions. Specially, spherical aberration (SA) increased significantly after pupil miosis (P = .0017). The mean increase of SA measured was 0.018 microns, for a 3-mm pupil. Mean values of other ocular aberrations also vary significantly after pupil miosis (changes were larger than the standard deviation of the repeated measurements). A mean paraxial hyperopic shift of one third of diopter was found after iris constriction. Conclusion: Iris constriction slightly modifies the optics of the eye. The small hyperopic shift of the best image plane after iris constriction may be explained by a change in the lens shape and/or position.
Purpose: We studied the accuracy and precision of 32 objective wavefront methods for finding the amplitude of accommodation obtained in 180 eyes. Methods: Ocular accommodation was stimulated with 0.5 D steps in target vergence spanning the full range of accommodation for each subject. Subjective monocular amplitude of accommodation was measured using two clinical methods, using negative lenses and with a custom Badal optometer. Results: Both subjective methods gave similar results. Results obtained from the Badal optometer where used to test the accuracy of the objective methods. All objective methods showed lower amplitude of accommodation that the subjective ones by an amount that varied from 0.2 to 1.1 D depending on the method. The precision in this prediction also varied between subjects, with an average standard error of the mean of 0.1 D that decreased with age. Conclusions: Depth of field increases subjective of amplitude of accommodation overestimating the objective amplitude obtained with all the metrics used. The change in the negative direction of spherical aberration during accommodation increases the amplitude of accommodation by an amount that varies with age.
To study in a standard eye model the changes in modulation transfer function (MTF) of a monofocal intraocular lens (IOL) when a phakic IOL (pIOL) is placed in the anterior chamber, compare the MTFs of the rigid Artisan pIOL and foldable Artiflex pIOL, and evaluate the temporal evolution of the MTF of the foldable pIOL after the mechanical stress the pIOL undergoes when injected. Fundación Oftalmológica del Mediterráneo, Valencia, Spain. The MTF values of the IOLs were calculated from the cross-line spread function recorded with the Opal Vector System. The measurements were taken using an eye model following the British and EN-ISO standards with 2.0 mm, 3.0 mm, 4.0 mm, and 5.0 mm pupils. A 28.00 diopter (D) Ophtec monofocal IOL was used as the crystalline lens. The 2 pIOLs were -9.00 D. The MTF of the rigid pIOL was slightly better than the MTF of the foldable pIOL with all pupil sizes. Both pIOLs provided good optics quality when compared with the monofocal IOL. The injection effect of the foldable IOL disappeared after 2 hours. The MTF of the monofocal IOL was slightly reduced with implantation of a negative pIOL in the anterior chamber. The rigid pIOL provided better optical performance than the foldable pIOL with all pupil sizes, as shown by the MTF values. The decrease in MTF caused by the mechanical stress on the foldable pIOL was nullified after 2 hours with no effect on optical quality.
We analyze theoretically, by means of both computer simulations and laboratory experiments, the limitations of correcting aberrations with ideal customized contact lenses. Four experiments are presented: In the first one, we have analyzed the limitations of a static correction on the dynamic wavefront. In the second one, we studied the rotations of a contact lens on the eye using an optical method. The third one researched the limitations of the wavefront correction, focusing on a group of normal and highly aberrated eyes, when the correction suffers from a permanent rotation or translation. The fourth one estimates, under a simple approximation, the error made when applying on the corneal plane the correction corresponding to the wavefront measured at the entrance-pupil plane. Results show that a static correction of the wavefront leaves a residual aberration of 0.15-0.25 microm for a 5 mm pupil. Rotation of the contact lens (up to +/-4 degrees) diminishes the effectiveness of the correction. Horizontal or vertical translations of 0.5mm could generate a high-order-aberration RMS that is higher than the remaining one after a standard low-order correction. In particular, the group of eyes having normal values of high-order aberrations are more sensitive to translations than the one having higher values. Most of the results could be applied to other methods of aberration correction, such as refractive surgery or correction by means of intraocular lenses.
- Feb 2009
To assess the performance of a partial coherence interferometry (PCI)-based device for the determination of anterior segment biometry. Clinica Centrofama, Cartagena, Murcia, Spain. Central corneal thickness (CCT), anterior chamber depth (ACD), and lens thickness (LT) were measured with the ACMaster PCI anterior segment biometer and an Echoscan US-1800 ultrasound (US) biometer/pachymeter with and without cycloplegia. To determine the precision of the instruments, the same examiner took 30 consecutive CCT, ACD, and LT measurements in a single subject under the same conditions and with and without cycloplegia. The same measurements were performed in additional subjects. Twenty-one eyes (16 subjects) were evaluated. Repeated measurements of the single subject yielded a standard deviation of 4.0 microm for CCT, 106.0 microm for ACD, and 323.0 microm for LT; there were many peaks, mainly in the last 10 readings. There was a high correlation between CCT measurements with both systems with and without cycloplegia (r(2)>0.93), with the US system giving higher values. Differences were significant (P< .001), but not consistent, throughout the range of corneal thicknesses and were greater for thicker corneas. Differences in ACD and LT measurements were similar. Agreement between systems in ACD and LT measurements was considerably lower than for CCT measurements. The PCI biometer provided precise CCT measurements. The ACD and LT measurements had a higher variance. Differences in CCT measurements between the 2 systems were greater for thicker corneas, with higher values with the US system.
- Sep 2008
Purpose To study the effects of phenylephrine 5% topical administration on accommodative response and optical aberrations variation.Methods The research followed the tenets of the Declaration of Helsinki . 28 eyes from 14 volunteers with spherical equivalent defocus error between –2D and +1D, no eye disease history and between 20 to 25 years of age, underwent wavefront measurements with a Shack–Hartmann wavefront aberrometer which included a movable accommodative target. Wavefront data was acquired while applying 6 different increasing accommodative stimuli, from 0 D to 5 D by steps of 1 D, before and after pupil dilation with Phenylephrine 5%.Results The dilation using Phenylephrine 5% was found to larger lag errors in the accommodative response of about half of the subjects. The total RMS amount of aberrations above defocus remained stable during accommodation with and without dilation. Spherical aberration was positive in average in the non accommodated eye and changed toward negative values with increasing accommodation (p
- Aug 2008
To investigate the effect of 3rd-order aberrations on human vision. Grupo de Ciencias de la Visión, University of Murcia, Murcia, Spain. The 3rd-order aberrations coma and trefoil were induced with purpose-designed soft contact lenses, 3 inducing coma (low [0.05 microm], medium [0.13 microm], high [1.03 microm]) and 3 inducing trefoil (low [0.07 microm], medium [0.17 microm], high [0.96 microm]). Monocular high-contrast (HCVA) and low-contrast (LCVA) visual acuities and contrast sensitivity were measured in 11 subjects wearing contact lenses with a 5.0 mm artificial pupil. The reduction in HCVA and LCVA was statistically significant only for the highest coma and trefoil values (P<.0001). For coma, the mean change in HCVA was 0.193 logMAR +/- 0.100 (SD) and in LCVA, 0.386 +/- 0.136 logMAR. For trefoil, it was 0.204 +/- 0.128 logMAR and 0.395 +/- 0.141 logMAR, respectively. No differences were found for the lower degrees (P>.2). Contrast sensitivity was significantly reduced with the highest coma and trefoil values (P<.0001) (mean change 0.390 +/- 0.157 and 0.404 +/- 0.135, respectively). Lower degrees did not cause significant changes in contrast sensitivity (P>.1). The effect of induced coma and trefoil on HCVA, LCVA, and contrast sensitivity was similar at each level of induced aberration (P>.01). Large values of coma and trefoil (approximately 1 mum) significantly reduced visual performance. Only patients with high 3rd-order aberrations, such as those that occur in refractive surgery or in cases of distorted optics, would benefit from this correction.
To investigate the relationship between accommodation and the optical aberrations of the whole human eye, as a function of age. Sixty healthy subjects with spherical ametropia in the range +/-3 D, astigmatism less than 1 D, corrected visual acuity of 20/18 or better, and normal findings in an ophthalmic examination were enrolled. Subjects were divided into four groups, with age ranges of 19 to 29, 30 to 39, 40 to 49, and 50 to 60 years. Monochromatic optical aberrations and pupil size were measured with a Hartmann-Shack wavefront sensor under monocular viewing conditions, without pharmacological dilation or cycloplegia. Stimulus vergences were in the range of 0 to 5 D, with an increment of 0.5 D. The change in aberration during accommodation for different groups and different pupil conditions (natural and fixed 4-mm pupil) was compared. Fourth-order spherical aberration (SA) became more negative with accommodation, and the rate of this change was greater in older individuals. For natural pupil conditions, there were no significant differences between age groups in the changes of the higher-order aberrations, coma, and trefoil with accommodation. However, for a 4-mm pupil, the youngest and oldest group showed significant differences in higher order RMS (root mean square) and spherical aberration compared with the other groups. High-order RMS showed a lower increase during accommodation when the pupil accommodative miosis was taken into account (natural pupil condition) than when a fixed 4-mm pupil was used. Aberrations change with accommodation and with age. SA changes more with accommodation do than other higher-order aberrations. SA becomes more negative with accommodation, and this change is larger in older individuals. Accommodative miosis is useful for ameliorating the increase in higher-order aberrations with accommodation.
- Jan 2008
- Investigative Ophtalmology and Visual Science
Purpose:Intraocular lenses (IOLs) are theoretically designed to produce retinal images with a high quality for object placed at one or more distances. However, oftenly, visual performances after surgery (specially IOLs with aspheric designs) are limited by descentrations and/or tilts. We have studied, in vitro, the optical quality of several IOLs with a new device. Methods:A comercial aberrometer (Imagine Eyes irx3) together with a new device which consist on an artificial eye was used to obtain the in vitro wavefront in monofocal and multifocal IOLs after subtrating the aberrations of the artificial eye. The artificial eye has the possibiliy to produce a descentration, rotation and/or a tilt in relation to the artificial cornea. From the wavefront outcomes we obtained several optical quality metrics such us the PSF or the MTF. The aberrometer used also allows obtaining wavefront measurement for different object possitons and different pupil sizes simulating near vision performance for multifocal IOLs. Repeatability and exactitude of the system was tested comparing its results with computer simulations on a model eye. Results:Repited measurements on an IOL for a 5 mm pupil show a standard deviation in all Zernikes coefficients (up to 8th-order) always less that 0.01 microns except in the case of defocus for which standard deviation was 0.06 microns (about 0.07 D). In vitro measurement of the IOL differs in less than 0.01 D respect the optical simulations generated by the model eye. The figure represents a typical in vitro wavefront of a multifocal IOLs with a vertical decentration of 0.3 mm. Astigmatism and higher-order aberrations, specially coma, was generated after tilting and decentering the IOL. Conclusions:A new device was tested and used to measure in vitro optical performance of IOLs. The system may be used to predict tolerances of diferent IOLs under possible rotations and translations before the surgery.
- Jan 2008
- The accommodation club. 6th meeting.
Introduction: we present the main results obtained in the last two years by our group of research, concerning the objective measure of the amplitude of accommodation (AA) Methods: We measured the change of aberrations in 100 eyes using two subjective methods (Push-up and Badal Optometer) and an objective one (using an aberrometer). It has also being study the possible influence of pupil diameter and monocular convergence on the objective measurement. Results: For young subjects there is in general a good agreement between subjective and objective results, but in presbyopic subjects, the objective measurementes underestimates (about 10%) the AA, probably due to the effect of a larger depth of field in the presbyopic group. Monocular convergence increased the AA between 5 and 8% (not significant) depending on the degree of convergence. Conclusions: AA could be measured objectively by aberrometry although usually an small underestimation is found. The method could be applied to test objectively new techniques to restore accommodation.
- Sep 2007
Purpose: In this paper, we study the limitations of correcting aberrations with soft contact lenses, which are custom-designed for each subject aberrations with special interest on the effect of measuring and correcting wavefront in different planes (pupil and cornea plane). Methods: We analyze, by experiments, theoretical methods and computer simulations, issues such as aberration dynamics, lens movement and errors in pupil plane localization. The study is carried out for three types of eyes: normal, keratoconus, and eyes that have gone through penetrating queratoplasty. Results: Main factor limiting this type of aberration correction is the static horizontal and vertical translation as well as the rotation of the contact lenses on the eye. Typical rotations (3º, 5º) and translations (0.5 mm) of the soft contact lenses could null the benefit of the high-order corrections in normal eyes. The correction of the wavefront in a different plane were it has been measured is not negligible when the aberrations (low and high order) are high. Conclusions: We found that customized correction for normal eyes leads to a loss of optical quality, compared to standard correction (sphere and cylinder), when displacements of the contact lenses appear. In spite of the mentioned effects, there are, however, pathologic cases in which it is possible to reduce the RMS and increase visual quality, especially in the keratoconus eyes. In those high aberrated eyes special care should be taken due to the changes of the wavefront when propagating from the cornea plane to the pupil plane.
- Aug 2007
To describe the design of a new intraocular lens (IOL) capable of correcting spherical and chromatic aberrations when implanted in the human eye. University of Murcia, Murcia, and University of Valencia, Valencia, Spain. A hybrid singlet achromatic IOL was designed. The IOL has a combination of a refractive and a diffractive surface, with 1 of the surfaces being aspherical. Optical simulations were used to model the polychromatic modulation transfer function (MTF) in pseudophakic eyes to explain the differences in optical quality afforded by the achromatic IOL. Parameters such as focus shift, optical path difference, through-focus, and robustness to tilt and decentering of achromatic IOLs were obtained. The polychromatic MTF in an eye with a centered, not tilted achromatic IOL was near the diffraction-limited MTF. The focus shift change for the achromatic IOL through the visible spectrum was approximately 0.1 diopter. The polychromatic MTF in an eye with the achromatic IOL would be similar to that in an eye with a spherically centered IOL if the achromatic IOL were decentered 1.0 mm or tilted 4 degrees. The range of centration and tilt error for the achromatic IOL to obtain an optical benefit in the polychromatic MTF was larger than the typical postoperative IOL decentration and tilt errors. The hybrid singlet achromatic IOL design resolved the chromatic aberration problem, improving the overall optical quality in the human eye.
We investigate the potential for the third-order aberrations coma and trefoil to provide a signed cue to accommodation. It is first demonstrated theoretically (with some assumptions) that the point spread function is insensitive to the sign of spherical defocus in the presence of odd-order aberrations. In an experimental investigation, the accommodation response to a sinusoidal change in vergence (1-3D, 0.2Hz) of a monochromatic stimulus was obtained with a dynamic infrared optometer. Measurements were obtained in 10 young visually normal individuals with and without custom contact lenses that induced low and high values of r.m.s. trefoil (0.25, 1.03 microm) and coma (0.34, 0.94 microm). Despite variation between subjects, we did not find any statistically significant increase or decrease in the accommodative gain for low levels of trefoil and coma, although effects approached or reached significance for the high levels of trefoil and coma. Theoretical and experimental results indicate that the presence of Zernike third-order aberrations on the eye does not seem to play a crucial role in the dynamics of the accommodation response.
- Jan 2007
Purpose: It is known that in some animal species, such us avians, pupil miosis increases the curvature of the first surface of the lens increasing its power and consequently providing certain accommodation. With a simple experiment we analyzed if this fact happens also in the human eye. Methods: We measured the ocular aberrations in 26 eyes (subjects ranging in age from 20 to 40 years) using an Irx3 aberrometer (Imagine Eyes, France) under natural viewing conditions. The measured eyes fixated to a dim target (2 cd/m2) while the counterpart eye was either occluded (so the measured eye showed a large pupil) or highly illuminated with a white lamp (so the measured eye showed a small pupil). Then, Zernike values obtained in both situations were compared within the same pupil diameter corresponding to the one obtained under the high illumination condition. The experiment was performed fist with the unaccommodated eye and then with the eye accommodating to a stimulus of 3 D closer to its far point. Results: Although the changes of aberrations depend on the subject and were small due to the small pupil diameter, we have noted significant variation in some aberration coefficients for the two accommodation states between both illumination conditions. Specially, spherical aberration (SA) increased significantly after pupil miosis (p = 0.0017 and 0.0021 for 0 and 3D of accommodation, respectively). The mean increase of SA measured was 0.018 and 0.027 microns for 0 and 3D of accommodation, respectively, for a 3-mm pupil (which would represent 0.14 and 0.21 microns for a 5-mm pupil). Mean values of other ocular aberrations also vary significantly after pupil miosis (changes were larger than the standard deviation of the repeated measurements) Conclusions: Besides the optical change produced by a decrease in the entrance pupil, pupil miosis slightly modifies the optics of the lens altering its SA. These optical changes could slightly modify the accommodation response
- Sep 2003
Outstanding improvements in vision can theoretically be expected using contact lenses that correct monochromatic aberrations of the eye. Imperfections in such correction inherent to contact lenses are lens flexure, translation, rotation, and tear layer effects. The effects of pupil size and accommodation on ocular aberration may cause further difficulties. The purpose of this study was to evaluate whether nonaxisymmetric soft contact lenses could efficiently compensate for higher-order aberrations induced by keratoconus and to what extent rotation and translation of the lens would degrade this perfect correction. Height topography data of nine moderate to severe keratoconus corneas were obtained using the Maastricht Shape Topographer. Three-dimensional ray tracing was applied to each elevation topography to calculate aberrations in the form of a phase error mapping. The effect of a nonaxisymmetric soft contact lens tailored to the corneal aberrations was simulated by adding an opposite phase error mapping that would theoretically compensate all corneal-induced optical aberrations of the keratoconus eyes. Translation (0.25, 0.5, 0.75, and 1.0 mm) and rotation (2.5 degrees, 5.0 degrees, 7.5 degrees, and 10 degrees ) mismatches were introduced. The modulation transfer function (MTF) of each eye with each displaced correction and with various pupil sizes (3, 5, and 7 mm) was deduced from the residual phase error mapping. A single performance criterion (mtfA) was calculated as the area under the MTF over a limited spatial frequency range (5 to 15 periods per degree). Finally, the ratio (RmtfA) of corrected mtfA over uncorrected mtfA provided an estimate of the global enhancement in contrast sensitivity with the customized lens. The contrast improvement ratios RmtfA with perfectly located lenses were for an average pupil size of 4.5 mm between 6.5 and 200. For small translation errors (0.25 mm), RmtfA ranged between 2 and 7. The largest lens translation tested (1 mm) often resulted in poorer performance than without correction (RmtfA <1). More than threefold improvements were achieved with any of the angular errors experimented. RmtfA values showed significant variations for pupil diameters between 3 and 7 mm. Three-dimensional aberration-customized soft contact lenses may drastically improve visual performance in patients with keratoconus. However, such lenses should be well positioned on the cornea. In particular, translation errors should not exceed 0.5 mm. Angular errors appeared to be less critical. It is further questioned whether the visual system is able to adapt to variations in optical performance of the correction in situ due to lens positioning and pupil size.
To gain more insight into the relationship between foveal and peripheral refractive errors in humans, spheres, cylinders, and their axes were binocularly measured across the visual field in myopic, emmetropic, and hyperopic groups of young subjects. Both automated infrared photorefraction (the "PowerRefractor"; www. plusoptix.de) and a double-pass technique were used because the PowerRefractor provided extensive data from the central 44 deg of the visual field in a very convenient and fast way. Two-dimensional maps for the average cross cylinders and spherical equivalents, as well as for the axes of the power meridians of the cylinders, were created. A small amount of lower-field myopia was detected with a significant vertical gradient in spherical equivalents. In the central visual field there was little difference among the three refractive groups. The established double-pass technique provided complementary data also from the far periphery. At 45 deg eccentricity the double-pass technique revealed relatively more hyperopic spherical equivalents in myopic subjects than in emmetropic subjects [+/-2.73 +/- 2.85 D relative to the fovea, p < 0.01 (+/- standard deviation)] and more myopic spherical equivalents in hyperopic subjects (-3.84 +/- 2.86 D relative to the fovea, p < 0.01). Owing to the pronounced peripheral astigmatism, spherical equivalents (refractions with respect to the plane of the circle of least confusion) became myopic relative to the fovea in all three groups. The finding of general peripheral myopia was unexpected. Its possible roles in foveal refractive development are discussed.
- Sep 2002
We explored the potential of aberration correction in the human eye by using a new generation of soft contact lenses with aspheric and asymmetric surfaces. Soft contact lens samples were designed with one asymmetrical surface (front) and one spherical (back) to produce predetermined amounts of desired pure defocus, astigmatism, trefoil, coma, and spherical aberration. Contact lens wavefront aberrations were measured ex vivo using a Fizeau-Tolanski interferometer and compared with the in vivo wavefronts obtained by subtracting the aberrations of the eye with and without the contact lenses. These second set of measurements were obtained using a Shack-Hartmann sensor. We found that an aberration-free contact lens sample induced in the eye a small amount of residual aberration. We obtained a good match between the ex vivo and in vivo wavefront measurements for most of the samples of the contact lenses. The aberrations generated by soft contact lenses on the eye were predictable. Rotations and translations of the contact lenses with respect to correct position on the eye were, however, the main limitation for precise correction of the ocular aberrations.
- Jul 2002
Monochromatic ocular aberrations in 108 eyes of a normal young population (n=59) were studied. The wave-front aberration were obtained under natural conditions using a near-infrared Shack-Hartmann wave-front sensor. For this population and a 5 mm pupil, more than 99% of the root-mean square wave-front error is contained in the first four orders of a Zernike expansion and about 91% corresponds only to the second order. Comparison of wave-fronts aberrations from right and left eye in 35 subjects, showed a good correlation between most of the second- and third-order terms and a slight (but not clear) tendency for mirror symmetry between eyes.
The wave aberration of the human eye has been measured by means of a Hartmann-Shack wave-front sensor in a population of normal subjects. The set of data has been used to compute the phase distribution, the power spectrum, and the structure function for the average eye to analyze the statistics of the ocular aberration considered as a phase screen. The observed statistics fits the classical Kolmogorov model of a statistically homogeneous medium. These results can be of use in understanding the average effect of aberrations on the retinal image and can serve as a tool to analyze the consequences of ocular-aberration compensation by adaptive optics, customized ophtalmic elements, or refractive surgery.
Fincham (The accommodation reflex and its stimulus. Br. J. Ophthalmol. 35, 381-393) was the first to suggest that the Stiles-Crawford effect (Type I) might provide a stimulus for accommodation, but the possibility has not been investigated experimentally. The present paper outlines a theoretical basis for such a mechanism, and includes a case study on a subject with a nasally decentred Stiles-Crawford (S-C) function. Accommodation to a monochromatic sine grating was monitored continuously with the natural S-C function intact, or with apodising filters imaged in the subject's pupil to neutralise, reverse or double the natural S-C function. Mean accommodative gain was not reduced significantly when the normal S-C function was either neutralised or reversed. For the present subject, the average S-C effect does not mediate the accommodation response to defocus, but more subjects should be examined. Other methods by which directionally sensitive cone receptors could detect light vergence are discussed.
Transient glare reduces the perceived brightness for stimulus luminances within the scotopic-mesopic range. This work studies this effect in myopic observers with lens and spectacles corrections and in one emmetrope. The task of the subject consists of compare brightness of two uniform luminance fields sequentially displayed, one of which had a reference luminance of 1 cd/m2. The reference stimulus was presented under glare condition the angle between the glare source and the lie of sight was 10 degrees. The reference field and he glare source were onset at the same time. Subjects had to report which field was brighter with respect to one another. A YES-NO method with constant stimulus was adopted to determine the perceptual matching luminance. Then duration of each stimulus was 300 msec and the interval between both of them was 1.2 msec. We found the effect of the transient glare is stronger as myopic subjects wear glasses corrections than they wear contact lens and the effect is even less for emmetropic subject with the naked eyes. Results could be explained considering an additional ectoptic light veil due to scattering on the glasses and lens.
- Feb 2000
- Vision Science and its Applications
The double-pass method has been widely used to obtain accurate measurements of the retinal image quality under different conditions. In this paper, we describe the method in some detail and we present a general formulation of its image formation theory. The most important assumptions and limitations of the technique are revised as well. Finally, some of the main contributions of the method to the understanding of the ocular optics are also reviewed.
- Jul 1999
We used the crossed-cylinder aberroscope technique to obtain the near infrared (784 nm) wave-front aberration of the human eye. We compared the results with those obtained under the same conditions using red light (633 nm). Other than the greater retinal scattering of the near infrared light, third- and fourth-order wave-front aberrations are similar in both wavelengths. Values of the calculated near infrared point spread function show a typical half-height width of around 2 arcmin, which is in good agreement with previous work.
A low cost pupil-meter and tracking apparatus was developed. It is based in digital analysis of images of the anterior eye to measure the diameter of the eye pupil and to track the position of its geometrical center at video rate. The system consists in an array of infrared LEDs to illuminate the eye and a CCD video camera that captures the images of the pupil to be grabbed in a computer. A fast algorithm analyses the digital images (512 X 512 pixels) in real time (25 images per second) providing the pupil diameter and the location of the geometrical center within that temporal rate. The algorithm was implemented to run under DOS, Win 3.x or Win95 and is compatible with any frame-grabber. the reliability of the apparatus was tested in different subjects for the particular problems of measuring the pupil diameter during accommodation and tracking the eye position during fixation. Under the measuring range, the system is robust to moderate lateral and axial movements of the subject. The spatial resolution is 0.02 mm and the linearity in measuring the pupil diameter is better than 1% in the complete useful range. The apparatus was adapted to operate without interfering either the visual task or possible simultaneous measurements.