20 research outputs found
Model Eye Incorporating a Manually Tunable Polymer Lens and Microfluidics Chamber for Simulation of Vitreous Floaters
No full textThe Effect of the Range of a Modulating Phase Mask on the Retrieval of a Complex Object from Intensity Measurements
No full textThe authors have been supported by the postdoctoral project (1.1.1.2/16/I/001, 1.1.1.2/VIAA/1/16/199), the CAMART2 project (grant agreement ID 739508), the Latvian Investment and Development Agency (LIDA) project (KC-PI-2017/105), and the grant for the Latvian State Emeritus Scientists.In many fields of science, it is often impossible to preserve the information about the phase of the electromagnetic field, and only the information about the magnitude is available. This is known as the phase problem. Various algorithms have been proposed to recover the information about phase from intensity measurements. Nowadays, iterative algorithms of phase retrieval have become popular. Many of these algorithms are based on modulating the object under study with several masks and retrieving the missing information about the phase of an object by applying mathematical optimization methods. Several of these algorithms are able to retrieve not only the phase but also the magnitude of the object under study. In this study, we investigate the effect of the range of modulation of a mask on the accuracy of the retrieved magnitude and phase map. We conclude that there is a sharp boundary of the range of modulation separating the successfully retrieved magnitude and phase maps from those retrieved unsuccessfully. A decrease in the range of modulation affects the accuracy of the retrieved magnitude and phase map differently. © 2021 V. Karitans et al., published by Sciendo. This work is licensed under the Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International License.Postdoctoral project (1.1.1.2/16/I/001, 1.1.1.2/VIAA/1/16/199); the Latvian Investment and Development Agency (LIDA) project (KC-PI-2017/105), and the grant for the Latvian State Emeritus Scientists; Institute of Solid State Physics, University of Latvia as the Center of Excellence has received funding from the European Union’s Horizon 2020 Framework Programme H2020-WIDESPREAD-01-2016-2017-TeamingPhase2 under grant agreement No. 739508, project CAMART2
Dependence of Perceived Purity of a Chromatic Stimulus on Saturation Adaptation
No full textBackground and Objective. The purpose of sensory adaptation of the visual system is to adjust sensitivity of the photoreceptors to optimize the dynamic range of response of the visual system. It has been shown in numerous research papers that chromatic adaptation influences both color appearance and color discrimination. However, there are almost no studies in which the influence of chromatic adaptation on perceived purity has been investigated. Therefore, the aim of this study was to investigate how chromatic adaptation to stimuli with certain saturation influences perceived purity of test stimuli with the same hue but different saturation. Material and Methods. As the stimuli were modulated in saturation only, we refer to the type of chromatic adaptation used in our study as saturation adaptation. Two types of psychophysical methods – the method of adjustment and the method of constant stimuli – were used. Results. The results obtained with the method of constant stimuli reveal that saturation adaptation seems to be nonlinear much the same way as the Bezold-Brücke and the Abney effect. In addition, hysteresis of saturation adaptation can be observed in the method of adjustment. Conclusions. Pronounced hysteresis of perceived color purity was observed when approaching to the point of subjective equality from the side of low saturation of the matching stimulus compared with the side of high saturation of the matching stimulus. There was a time course of mechanisms of saturation adaptation similar to those of chromatic adaptation previously quoted in literature.</jats:p
The effect of noise, a constant background, and bit depth on the phase retrieval of pure phase objects
No full textIn the current study, we investigate the effect of uniform white noise, Poisson noise and a constant background on the phase retrieval of pure phase objects. We also study the influence of the aforementioned factors on phase retrieval at different bit depths of intensity measurements. An algorithm called PhaseLift is used for phase retrieval as it requires a small number of modulating masks and can retrieve the phase of an object from sparse intensity measurements of low bit depth. A test object is modulated by eight random masks generated from a single mask and the phase of the object is retrieved from coded diffraction patterns. Different levels of uniform white noise, Poisson noise and constant background are superimposed on the diffraction patterns and the root-mean-square error (RMSE) of the retrieved object is calculated at each level. The results suggest that Poisson noise and a constant background at the same level cause similar RMSE compared to uniform white noise. Lowering the bit depth from 18-bits to 14-bits resulted in the decrease of the RMSE caused by Poisson noise and a constant background. We conclude that the effects of noise and constant background can be reduced by lowering the bit depth
“SmartGlass” Obstacles for Dynamic Inducing of Light Scattering in Vision Research Experiments
No full textWe describe a technique that allows control of visual stimuli quality through the use of a setup with a polymer dispersed liquid crystal (PDLC) film positioned in the optical pathway of one or both human eyes. Nowadays, PDLC films allow alteration of the resolution and contrast limits of the transmitted light due to continuous change in the light scattering that is obtained by the application of an AC electrical field. In our experimental setup, the use of a wide-aperture up to area of 20 x 15 cm2 PDLC sheet is combined with a flat-screen PC display or with a modified display emission block without its interference filter unit and with an installed individually controllable colored light-emitting diode (LED) backlight. In the latter case, the spatial structure of visual stimulus remains constant, but the PDLC switching-on timing for intensity, color, and contrast of visual stimuli control is done by a PC via an Arduino USB interface. Arduino applies a voltage to the backlight colored LEDs and the low voltage up to 30 – 80 V to light-scattering PDLC sheet. Modifications to this setup can improve the resolution of the timing and screen stimulus intensity and color purity, and increase the flexibility of its application in visual research tasks. A particular use of PDLC scattering sheets involves the altering of the stimuli input strength of the eye in different binocular viewing schemes. In such applications, a restricted-optical-aperture PDLC element is mounted in a goggle frame, and the element is controlled by the application of low-voltage AC field. The efficacy of the setup is demonstrated in experiments of human vision contrast sensitivity adaptation studies. Studies allow to determine the characteristic time of the contrast sensitivity altering of 4 s during adaptation phase and the same order of the characteristic time during recovery.DOI: http://dx.doi.org/10.5755/j01.ms.22.4.12907</p
Phase Retrieval of One-Dimensional Objects by the Multiple-Plane Gerchberg–Saxton Algorithm Implemented into a Digital Signal Processor
No full textIn the current study, we address the phase retrieval of one-dimensional phase objects from near-field diffraction patterns using the multiple-plane Gerchberg–Saxton algorithm, which is still widely used for phase retrieval. The algorithm was implemented in a low-cost digital signal processor capable of fast Fourier transform using Q15 arithmetic, which is used by the previously mentioned algorithm. We demonstrate similarity between one-dimensional phase objects, i.e., vectors cut out of a phase map of the tertiary spherical aberration retrieved by the multiple-plane Gerchberg–Saxton algorithm, and these vectors are measured with a non-contact profiler. The tertiary spherical aberration was induced by a phase plate fabricated using grayscale lithography. After subtracting the vectors retrieved by the algorithm from those measured with the profiler, the root mean square error decreased, while a corresponding increase in the Strehl ratio was observed. A single vector of size 64 pixels was retrieved in about 2 min. The results suggest that digital signal processors that are capable of one-dimensional FFT and fixed-point arithmetic in Q15 format can successfully retrieve the phase of one-dimensional objects, and they can be used for applications that do not require real-time operation, i.e., analyzing the quality of cylindrical micro-optics
Wavefront Recovery: Shack-Hartmann Aberrometry vs PhaseLift Using a Single Binary Amplitude Modulating Mask
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A/V Ratio as Predicted by Full Width at Half Maximum and by Blood Vessel Tracking in Presence of Ocular Aberrations
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