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.
Automatic dynamic infrared retinoscope was developed, which allows to run procedure at a much higher rate. Our system uses a USB image sensor with up to 180 Hz refresh rate equipped with a long focus objective and 850 nm infrared light emitting diode as light source. Two servo motors driven by microprocessor control the rotation of semitransparent mirror and motion of retinoscope chassis. Image of eye pupil reflex is captured via software and analyzed along the horizontal plane. Algorithm for automatic accommodative state analysis is developed based on the intensity changes of the fundus reflex.
Color of objects is a spectral composition of incident light source, reflection properties of the object itself, and spectral tuning of the eye. Light sources with different spectral characteristics can produce metameric representation of color; however most variable in this regard is vision. Pigments of color vision are continuously bleached by different stimuli and optical density of the pigment is changed, while continuous conditions provide an adaptation and perception of white. Special cases are color vision deficiencies which cover almost 8 % of male population in Europe. Hyperspectral imaging allows obtaining the spectra of the environment and modelling the performance of the dichromatic, anomalous trichromatic, as also normal trichromatic adapted behavior. First, CRI Nuance hyperspectral imaging system was spectrally calibrated for natural continuous spectral illumination of high color rendering index and narrow band fluorescent light sources. Full-scale images of color deficiency tests were acquired in the range of 420 to 720 nm to evaluate the modelling capacity for dichromatic and anomalous trichromatic vision. Hyperspectral images were turned to cone excitation images according to Stockman and Sharpe (2000) 1. Further, model was extended for anomalous trichromacy conditions. Cone sensitivity spectra were shifted by 4 nm according to each anomaly type. LWS and SWS cone signals were balanced in each condition to provide the appropriate appearance of colors in CIE system.
Perception of different color contrast stimuli was studied in the presence of light scattering in a fog chamber in
Clermont-Ferrand and in laboratory conditions where light scattering of similar levels was obtained. Blue (shortest
wavelength) light is scattered in fog to the greatest extent, causing deterioration of vision quality especially for the
monochromatic blue stimuli. We have done spectral measurements of the light source in different density fog conditions
and no spectral changes were found produced by the fog. Psychophysical measurements of the acuity in fog were done
for two subjects with optotypes analyzed for red and blue stimuli.
Multispectral color analysis was used for spectral scanning of Ishihara and Rabkin color deficiency test book images. It
was done using tunable liquid-crystal LC filters built in the Nuance II analyzer. Multispectral analysis keeps both,
information on spatial content of tests and on spectral content. Images were taken in the range of 420-720nm with a
10nm step. We calculated retina neural activity charts taking into account cone sensitivity functions, and processed
charts in order to find the visibility of latent symbols in color deficiency plates using cross-correlation technique. In such
way the quantitative measure is found for each of diagnostics plate for three different color deficiency carrier types -
protanopes, deutanopes and tritanopes. Multispectral color analysis allows to determine the CIE xyz color coordinates of
pseudoisochromatic plate design elements and to perform statistical analysis of these data to compare the color quality of
available color deficiency test books.
Amorphous chalcogenide thin films are excellent materials for holographic recordings. AsSeS thin film coating is a
useful optical material for it's thickness to be easily corrected with the use of exposure to light and consecutive chemical
etching. Following properties allow to treat the surface of AsSeS chalcogenide films and to use them in adaptive optics
systems for correction of the optical wavefront. Hereby, we characterize AsSeS film properties to be used for correction
of optical aberrations of the human eye. The thickness of the film is characterized with the method of spectrodensitometry and the surface profile depth with a Hartman- Shack waveform analyzator.
Following aspects related to human colour vision are included in experimental lessons for optometry students of University of Latvia. Characteristics of coloured stimuli (emitting and reflective), determination their coordinates in different colour spaces. Objective characteristics of transmitting of colour stimuli through the optical system of eye together with various types of appliances (lenses, prisms, Fresnel prisms). Psychophysical determination of mono- and polychromatic stimuli perception taking into account physiology of eye, retinal colour photoreceptor topography and spectral sensitivity, spatial and temporal characteristics of retinal receptive fields. Ergonomics of visual perception, influence of illumination and glare effects, testing of colour vision deficiencies.
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