Article
To read the full-text of this research, you can request a copy directly from the authors.

No full-text available

Request Full-text Paper PDF

To read the full-text of this research,
you can request a copy directly from the authors.

ResearchGate has not been able to resolve any citations for this publication.
Full-text available
Article
In this work we propose Jacobi-Fourier phase masks for wavefront coding-based imaging systems. The optical properties of the phase mask is study in detail and numerical simulation are shown. Pixel size and noise are taken into account for the deconvolution of images. Numerical simulations indicate that overall performance is better than of the well-known and commonly used trefoil phase.
Full-text available
Article
Wavefront coding involves the use of an asymmetrical phase mask to extend the depth of field of incoherent imaging systems. The performance of wavefront coding systems depends on designing a suitable phase profile to generate the defocus-invariant imaging characteristic. We proposed a square-root phase mask with two profile factors for achieving a steadier defocused modulation transfer function (MTF). Several evaluation methods are employed for the purpose of performance comparison between the proposed phase mask and the previously suggested phase masks under the constraint condition that the phase parameters are optimized at the same level of noise gain. Numerical results show that the square-root phase mask yields better properties in extended depth of field imaging, especially in acquiring defocus-invariant MTFs and eliminating image artifacts associated with the decoded images.
Full-text available
Article
We present outcomes of an imaging experiment using the refractive light sword lens (LSL) as a contact lens in an optical system that serves as a simplified model of the presbyopic eye. The results show that the LSL produces significant improvements in visual acuity of the simplified presbyopic eye model over a wide range of defocus. Therefore, this element can be an interesting alternative for the multifocal contact and intraocular lenses currently used in ophthalmology. The second part of the article discusses possible modifications of the LSL profile in order to render it more suitable for fabrication and ophthalmological applications.
Full-text available
Article
We present a new optical technique to suppress the unwanted zero-order diffraction (ZOD) in holograms produced by the digital micromirror device (DMD). The proposed optical architecture consists of two light beams illuminating the DMD in an interferometer configuration. The two beams are incident from different angles, +24° and -24°, in order to utilize light diffracted from all the pixels to produce a binary Fresnel hologram. The relation between these two beams diffracting from the DMD was found to be complementary, and they both generated the same reconstructed image pattern. With π phase difference between the two beams, the diffracted beams had their ZOD components out of phase while the reconstructed holograms were identical and in phase. Experiments were conducted to demonstrate ZOD suppression by destructive interference and simultaneous hologram enhancement by constructive interference. The method was shown to suppress the ZOD by a factor of 2.9 in a Fresnel hologram.
Full-text available
Chapter
In recent years live cell fluorescence microscopy has become increasingly important in biological and medical studies. This is largely due to new genetic engineering techniques which allow cell features to grow their own fluorescent markers. A popular example is green fluorescent protein. This avoids the need to stain, and thereby kill, a cell specimen before taking fluorescence images, and thus provides a major new method for observing live cell dynamics.
Full-text available
Article
Wavefront coding (WFC) is a powerful hybrid optical-numerical technique for increasing the depth of focus of imaging systems. It is based on two components: (1) an optical phase element that codifies the wavefront, and (2) a numerical deconvolution algorithm that reconstructs the image. Traditionally, some sophisticated optical WFC designs have been used to obtain approximate focus-invariant point spread functions (PSFs). Instead, we present a simple and low cost solution, implemented on infrared (IR) cameras, which uses a decentred lens inducing coma as an adjustable and removable phase element. We have used an advanced deconvolution algorithm for the image reconstruction, which is very robust against high noise levels. These features allow its application to low cost imaging systems. We show encouraging preliminary results based on realistic simulations using optical PSFs and noise power spectral density (PSD) laboratory models of two IR imaging systems. Without induced optical phase, the reconstruction algorithm improves the image quality in all cases, but it performs poorly when there are both in and out-of-focus objects in the scene. When using our coding/decoding scheme with low-noise detectors, the proposed solution provides high quality and robust recovery even for severe defocus. As sensor noise increases, the image suffers a graceful degradation, its quality being still acceptable even when using highly noisy sensors, such as microbolometers. We have experienced that the amount of induced coma is a key design parameter: while it only slightly affects the in-focus image quality, it is determinant for the final depth of focus.
Full-text available
Article
A practical activity designed to introduce wavefront coding techniques as a method to extend the depth of field in optical systems is presented. The activity is suitable for advanced undergraduate students since it combines different topics in optical engineering such as optical system design, aberration theory, Fourier optics, and digital image processing. This paper provides the theoretical background and technical information for performing the experiment. The proposed activity requires students able to develop a wide range of skills since they are expected to deal with optical components, including spatial light modulators, and develop scripts to perform some calculations.
Full-text available
Article
We designed an optical-digital system that delivers near-diffraction-limited imaging performance with a large depth of field. This system is the standard incoherent optical system modified by a phase mask with digital processing of the resulting intermediate image. The phase mask alters or codes the received incoherent wave front in such a way that the point-spread function and the optical transfer function do not change appreciably as a function of misfocus. Focus-independent digital filtering of the intermediate image is used to produce a combined optical-digital system that has a nearly diffraction limited point-spread function. This high-resolution extended depth of field is obtained through the expense of an increased dynamic range of the incoherent system. We use both the ambiguity function and the stationary-phase method to design these phase masks.
Full-text available
Article
We present the first analytical analysis of image artifacts in defocused hybrid imaging systems that employ a cubic phase-modulation function. We show that defocus artifacts have the form of image replications and are caused by a net phase modulation of the optical transfer function. Both numerical simulations and experimental images are presented that exhibit replication artifacts that are compatible with the analytical expressions.
Full-text available
Article
We describe the use of wavefront coding for the mitigation of optical aberrations in a thermal imaging system. Diffraction-limited imaging is demonstrated with a simple singlet which enables an approximate halving in length and mass of the optical system compared to an equivalent two-element lens.
Full-text available
Article
Traditional methods of optical design trade optical system complexity for image quality. High quality imagers often require high system complexity. A new imaging methodology called Wavefront Coding uses aspheric optics and signal processing in order to reduce system complexity and deliver high quality imagery. An example in terms of a conformal IR imaging system is given.
Full-text available
Article
We describe a new paradigm for designing hybrid imaging systems. These imaging systems use optics with a special aspheric surface to code the image so that the point-spread function or the modulation transfer function has specified characteristics. Signal processing then decodes the detected image. The coding can be done so that the depth of focus can be extended. This allows the manufacturing tolerance to be reduced, focus-related aberrations to be controlled, and imaging systems to be constructed with only one optical element plus some signal processing.
Full-text available
Article
The use of the human iris as a biometric has recently attracted significant interest in the area of security applications. The need to capture an iris without active user cooperation places demands on the optical system. Unlike a traditional optical design, in which a large imaging volume is traded off for diminished imaging resolution and capacity for collecting light, Wavefront Coded imaging is a computational imaging technology capable of expanding the imaging volume while maintaining an accurate and robust iris identification capability. We apply Wavefront Coded imaging to extend the imaging volume of the iris recognition application.
Full-text available
Article
We report experimental verification of an extended depth of focus (EDF) system with near-diffraction-limited performance capabilities. Dowski and Cathey [Appl. Opt. 34, 1859-1866 (1995)] described the theory of this system in detail. We can create an EDF system by modifying a standard incoherent optical system with a special cubic phase plate placed at the aperture stop. We briefly review the theory and present the first optical experimental verification of this EDF system. The phase plate codes the wave front, producing a modified optical transfer function. Once the image is transformed into digital form, a signal-processing step decodes the image and produces the final in-focus image. We have produced a number of images from various optical systems using the phase plate, thus demonstrating the success of this EDF system.
Article
We propose a hybrid optical-digital imaging system that can provide high-resolution retinal images without wavefront sensing or correction of the spatial and dynamic variations of eye aberrations. A methodology based on wavefront coding is implemented in a fundus camera in order to obtain a high-quality image of retinal detail. Wavefront-coded systems rely simply on the use of a cubic-phase plate in the pupil of the optical system. The phase element is intended to blur images in such a way that invariance to optical aberrations is achieved. The blur is then removed by image postprocessing. Thus, the system can provide high-resolution retinal images, avoiding all the optics needed to sense and correct ocular aberration, i.e., wavefront sensors and deformable mirrors.
Article
Wavefront coding as an optical-digital hybrid imaging technique can be used to extend the depth of field. The key to wavefront coding lies in the design of suitable phase masks to achieve the invariant imaging properties over a wide range of defocus. In this Letter, we propose another phase mask with a tangent function to enrich the odd symmetrical kind of phase masks. The performance of the tangent phase mask is evaluated by comparison with a cubic mask, improved-1 logarithmic mask, improved-2 logarithmic mask, and sinusoidal mask. The results demonstrate that the tangent phase mask has superior performance in extending the depth of field.
Article
A simple graphical interpretation at the pupil of an optical system is proposed to analyse the properties of the cubic-phase wavefront-coding function. The approach explains the invariance achieved against optical defocus of wavefront coding imaging systems with simplicity and clarity. Next, an analytical approximation of the modulation-transfer function of general wavefront coding systems is derived and used, in combination with the graphical interpretation, for selecting the optimal strength of the cubic-phase encoding function.
Book
Radiometric considerations. Basic optics. Primary aberrations. Wave aberrations. Special optical surfaces and components. Design examples. Thermal effects. Optical coatings. Image evaluation. Diamond turning. Appendix A.1: Paraxial ray tracing. Appendix A.2: Spherical aberration of a thin lens.
Article
Three-dimensional (3-D) objects recognition and localization is of major importance in a wide range of applications. A number of implementations concerning the complex D object recognition and localization have been achieved using some heuristic approaches. However, theoretical considerations concerning the construction of invariant (or quasi invariant) relations between the types of ojbect (identifying those objects) and the position in 3D space of those objects is still a problem. That is why, a marker based method for recognition and localization of 3D objects from their 2D image is suggested in this paper. Theorems relative to the features and conditions of such markers are proposed and demonstrated. Examples are given and discussed.
Article
By properly designing a phase pupil mask to modulate or encode the optical images and then digitally restoring them, one can greatly extend the depth of field and improve image quality. The original works done by Dowski and Cathey introduce the use of a cubic phase pupil mask to extend the depth of field. The theoretical and experimental researches all verified its effectiveness. In this paper, we suggest the use of an exponential phase pupil mask to extend the depth of field. This phase mask has two variable parameters for designing to control the shape of the mask so as to modulate the wave-front more flexible. We employ an optimization procedure based on the Fisher information metric to obtain the optimum values of the parameters for the exponential and the cubic masks, respectively. A series of performance comparisons between these two optimized phase masks in extending the depth of field are then done. The results show that the exponential phase mask provide slight advantage to the cubic one in several aspects.
Article
In a previous Letter [Opt. Lett. 33, 1171 (2008)], we proposed an improved logarithmic phase mask by making modifications to the original one designed by Sherif. However, further studies in another paper [Appl. Opt. 49, 229 (2010)] show that even when the Sherif mask and the improved one are optimized, their corresponding defocused modulation transfer functions (MTFs) are still not stable with respect to focus errors. So, by further modifying their phase profiles, we design another two logarithmic phase masks that exhibit more stable defocused MTF. However, with the defocus-induced phase effect considered, we find that the performance of the two masks proposed in this Letter is better than the Sherif mask, but worse than our previously proposed phase mask, according to the Hilbert space angle.
Article
A significant hurdle for the widespread adoption of iris recognition in security applications is that the typically small imaging volume for eye placement results in systems that are not user friendly. Separable cubic phase plates at the lens pupil have been shown to ameliorate this disadvantage by increasing the depth of field. However, these phase masks have limitations on how efficiently they can capture the information-bearing spatial frequencies in iris images. The performance gains in information acquisition that can be achieved by more general, nonseparable phase masks is demonstrated. A detailed design method is presented, and simulations using representative designs allow for performance comparisons.
Article
In this paper we derive an approximate analytical representation for the modulation transfer function (MTF) of an imaging system possessing a defocused cubic-phase pupil function. This expression is based on an approximation using the Arctan function and significantly reduces the computational time required to calculate the resulting MTF. We derive rigorous bounds on the minimum and average accuracy of our approximation. Using this approximate representation of the MTF, the analytical solution of the problem of calculating the extension of the depth of field for a circular aperture with a cubic phase mask becomes possible. We also comment on how one can modify our method to construct a lower-bound or an upper-bound approximate analytical expression for the MTF.
Article
To achieve a further extension of the depth of field in wavefront-coded imaging by reducing the impact of focus error in the optical transfer function, we propose the use of a free-form phase mask (FPM) instead of a conventional cubic phase mask (CPM). We optimized the shape of the FPM using the simulated annealing algorithm and confirmed that the optimized FPM provides a much larger focal tolerance and better final images than the CPM in the noise-free case. (C) 2008 Optical Society of America.
Article
This paper gives a brief introduction into the background, application, and design of Wavefront Coding imaging systems. Wavefront Coding is a general technique of using generalized aspheric optics and digital signal processing to greatly increase the performance and/or reduce the cost of imaging systems. The type of aspheric optics employed results in optical imaging characteristics that are very insensitive to misfocus related aberrations. A sharp and clear image is not directly produced from the optics, however, digital signal processing applied to the sampled image produces a sharp and clear final image that is also insensitive to misfocus related aberrations. This paper gives an overview of Wavefront Coding and example images related to the two applications of machine vision/label reading and biometric imaging. Design techniques of Wavefront Coding are unique from that of traditional imaging system design since both the optics and digital processing characteristics of the system ar...