Knowledge of the structure of vitreous floaters is crucial to evaluate the need for surgical removal of these floaters. We simulated the phase retrieval of microstructures simulating vitreous floaters by an algorithm PhaseLift and investigate the effects of various parameters on the retrieved phase. The object under test was modulated and the coded diffraction patterns were calculated. Next, PhaseLift was used to retrieve the phase. In the current study, we simulate the effect of Gaussian and Poison noise on the phase retrieval of pure phase objects. We apply an iterative algorithm PhaseLift for phase retrieval as this algorithm requires a very few modulating masks and is able to retrieve the phase of an object from very sparse data. Both types of noise are added to the intensity measurements and their effect on the retrieved phase is assessed in terms of the root-mean-square error. We conclude that Poisson noise compromises the accuracy of phase retrieval more compared to Gaussian noise given that the levels of both types of noise are equal. We also conclude that the thermal noise having nearly Gaussian distribution is of less importance compared to the shot noise having Poisson distribution and dominating at low-light levels.
Light scattering caused by Tiffen ProMist photographic filters of various grades at different laser radiation 640 nm, 532 nm and 405 nm was objectively studied and compared. The lack of removal of speckles from the scene of laser radiation scattering did not allow an accurate assessment of the effect of scattering on the decrease in image contrast. Subjective deterioration in visual acuity using various contrast optotypes and contrast sensitivity was assessed by scattering induction using filters up to grade #5. Vision contrast sensitivity diminishes within all studied spatial frequency range 0.5-18 cpd. The degradation of the impact factor to visual acuity without scattering filters when the contrast of the optotype was reduced from 100% to 12.5% for optotypes with black-white letters was up to 30%, which was similar to the level of degradation (25% for a #5 degree filter) from scattering caused by filters modeling cataracts.
In recent years, phase retrieval methods recovering the phase of an object from coded diffraction patterns have gained popularity. A numerical phase retrieval method called PhaseLift that recovers the phase of an object from a very limited number of coded diffraction patterns was recently proposed. Performance of PhaseLift has been analyzed for different types and the number of masks modulating an object. We present a unique application of PhaseLift that uses four rotations of a single mask, modulating only the amplitude of an object. In simulations, a phase screen with the root-mean-square (RMS) value 0.294 μm was used as the test object. The RMS value of the retrieved phase screen after smoothing was 0.257 μm. In experiments, the RMS value of a wavefront measured with a Shack–Hartmann wavefront sensor was 0.094 while that of the retrieved wavefront after smoothing was 0.054 μm. While PhaseLift is able to recover a wavefront using this kind of modulation, a serious limitation to applicability of this method is its high computational cost and time.
In signal processing one often faces the phase problem, i.e., when an image is formed information about the phase is lost so that only information about intensity is available. This is often an issue in astronomy, biology, crystallography, speckle imaging, diffractive imaging where the phase of the object must be known. While there have been many approaches how to find a solution to the phase problem, numerical algorithms recovering the phase from intensity measurements become more and more popular. One of such algorithms called PhaseLift has been recently proposed. In this study, we show that even 4 masks may be sufficient for reasonable recovery of the phase. The original wavefront and the recovered wavefront were visually indistinguishable and showed very high correlation. In addition, the four masks are essentially one and the same mask rotated around in steps of 90 degrees. By using just four rotated versions of a single mask, the PhaseLift could be easily implemented in real optical systems thus simplifying the wavefront sensing in astronomy, biology etc.
Ocular aberrations can be corrected with wavefront correctors created in a photoresist layer. The simplest type of the mask used in optical lithography is a binary amplitude mask. It is known that such a mask has a periodic hole pattern. The purpose of this research was to assess applicability of a binary amplitude mask for creating ocular wavefront correctors. The photoresist was applied to the substrate by using the dip-coating method. The photoresist layer was illuminated through a mask printed on a transparent film by using a laser printer. The surface of the wavefront correctors was evaluated by aberrometry, scanning electron microscopy and profilometry method. The dip-coating method can be used to apply an uniform photoresist layer on the substrate. Despite rapid variations in the surface depth the required shape of the wavefronts can still be obtained. Because of strong light scattering the wavefront correctors manufactured by using a binary amplitude mask aren't suited for studying visual functions. However, the wavefront correctors manufactured by such a type of the mask may be used for calibration of aberrometers.
In pediatric ophthalmology 2 - 3 % of all the children are impacted by a visual pathology - amblyopia. It develops if a
clear image isn't presented to the retina during an early stage of the development of the visual system. A common way of
treating this pathology is to cover the better-seeing eye to force the "lazy" eye to learn seeing. However, children are
often reluctant to wear such an occluder because they are ashamed or simply because they find it inconvenient. This fact
requires to find a way how to track the regime of occlusion because results of occlusion is a hint that the actual regime of
occlusion isn't that what the optometrist has recommended. We design an electronic eye occluder that allows to track the
regime of eye occlusion. We employ real-time clock DS1302 providing time information from seconds to years. Data is
stored in the internal memory of the CPU (EEPROM). The MCU (PIC16F676) switches on only if a mechanical switch
is closed and temperature has reached a satisfactory level. The occlusion is registered between time moments when the
infrared signal appeared and disappeared.
Simulation of vision pathologies and adverse viewing conditions in laboratory conditions requires optical phantoms with different level of light scattering. Such obstacles are designed as passive or active elements applying several technologies. We used for studies two kinds of solid state smart materials with electrically controllable light scattering - electrooptic PLZT ceramics, polymer dispersed liquid crystals PDLC and obstacles with fixed light scattering - composite of polymer methylmethaacrilat PMM together with grinded glass microparticles. Report analyzes optical characteristics of such obstacles - attenuation, scattering, depolarization of different wavelength light at various scattering levels and changes of visual performance applying obstacles in vision science studies.