Geometrical theory to predict eccentric photorefraction intensity profiles in the human eye

School of Optometry, University of Waterloo, Ontario, Canada.
Journal of the Optical Society of America A (Impact Factor: 1.56). 09/1995; 12(8):1647-56. DOI: 10.1364/JOSAA.12.001647
Source: PubMed


In eccentric photorefraction, light returning from the retina of the eye is photographed by a camera focused on the eye's pupil. We use a geometrical model of eccentric photorefraction to generate intensity profiles across the pupil image. The intensity profiles for three different monochromatic aberration functions induced in a single eye are predicted and show good agreement with the measured eccentric photorefraction intensity profiles. A directional reflection from the retina is incorporated into the calculation. Intensity profiles for symmetric and asymmetric aberrations are generated and measured. The latter profile shows a dependency on the source position and the meridian. The magnitude of the effect of thresholding on measured pattern extents is predicted. Monochromatic aberrations in human eyes will cause deviations in the eccentric photorefraction measurements from traditional crescents caused by defocus and may cause misdiagnoses of ametropia or anisometropia. Our results suggest that measuring refraction along the vertical meridian is preferred for screening studies with the eccentric photorefractor.

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    • "We do not know whether these non-uniformities have an impact on other methods of measuring the eye’s optics, such as IR photorefraction. Geometrical theory to predict eccentric photorefraction intensity profiles assumes a single retinal layer which diffusely scatters light, with reflection properties independent of eccentricity [49,50]. It might be possible that the multilayer properties of the mouse retina may also contribute to the non-uniform intensity distribution observed during IR photorefraction. "
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    • "For photorefractor positions outside of this region , the slope of the pupil reflex increases with refractive error [12] [9]. It has been shown that monochromatic aberrations also affect the shape of the photorefractive profile [13]. In the eccentric photo-optometer, effects of aberrations are minimized by using a slope-based instrument and by using a 5 mm artificial pupil at the subject's eye. "
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    ABSTRACT: An objective infrared optometer has been designed, based on the optical principles of eccentric photorefraction. A CCD camera with an eccentric infrared light source images the subject's pupil through a Badal optometer. The slope of the light distribution across the pupil is continuously recorded. Accommodative state is measured by moving the camera behind the Badal lens until the slope is zero. This position corresponds to the case where the camera is conjugate with the retina of the observer. In this Badal optometer, the irradiance of light at the pupil plane, the sensitivity of the photorefractor, and the focal setting of the camera lens remain constant for all positions of the camera from the eye. The repeatability of a single measure of refractive state in a cyclopleged eye was less than 0.05 D. Static accommodative responses taken from 3 subjects in both closed and open loop conditions provided expected stimulus/response measures. The instrument can also be adapted to measure dynamic accommodation.
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    ABSTRACT: A geometrical-optical analysis is developed to predict the reflex observed in retinoscopy. The analysis can be expanded to explain the reflex for an eye with aberrations. The succession of reflexes across the pupil for each position of the retinoscope is represented in a contour plot. The plots demonstrate that retinoscopy can be considered a measure of the transverse ray aberration of the eye. For an eye with simple defocus this causes the typical with and against motions observed with hyperopic and myopic refractive errors. For an eye with aberrations we predict more-complex retinoscopic reflexes. This theory is confirmed by actual measurements on a human eye with known aberrations.
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