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PubMed2006

On-line simultaneous monitoring of polarization mode dispersion and chromatic dispersion.

We have developed new expressions for power fading and average power fading owing to polarization mode dispersion (PMD) and chromatic dispersion in terms of the angle of precession of the output state of polarization about the PMD vector. Based on these expressions, a simple and novel pilot-tone-based technique for simultaneous monitoring of chromatic dispersion and PMD is proposed. Experimental results demonstrate the effectiveness of the proposed technique; these results agree well with the theoretical analysis.

Journal Article↗
PubMed1979

Opponent chromatic mechanisms: relation to photopigments and hue naming.

Opponent chromatic response functions were determined from monochromatic, equal-luminance stimuli from 400 to 700 nm for three observers using a hue cancellation procedure. The same observers scaled the hue of the stimuli using the terms red, green, yellow, and blue. The results showed that the hue scaling was accurately predicted from the cancellation functions using the model of Hurvich and Jameson. Theoretical curves were generated to fit the chromatic response functions with a linear combination of three cone photopigments. The theoretical photopigments were based on an idopsin nomogram with lambdamax at a = 435, beta = 530, and lambda = 562 nm. An estimate of the density of each observer's preretinal optic media was obtained in order to relate the photopigment absorption spectra to the psychophysical data. Good linear fits were obtained for each observer's red-green curve, but not for the yellow-blue curves. A nonlinear model with an expansive exponent was used to fit the yellow-blue response functions with the three theoretical photopigments.

Adult↗
PubMed1979

Using color substitution pupil response to expose chromatic mechanisms.

It is well known that pupillary threshold shows a scotopic spectral behavior, even for foveal stimuli. However, when two scotopically balanced fields at different wavelengths are alternated, the pupil shows a constriction response at each transition, exposing innervation from chromatic mechanisms. Using a suitable model for pupillary innervation, this substitution response is systematically studied for different wavelengths and radiant power levels to yield (within the accuracy of the assumptions of the model) spectral threshold curves for the chromatic mechanism as indicated by the pupil. Four mechanisms have thus been identified, having peak sensitivities near 450, 525, 580, and 495 nm. We propose that these represent the blue, green, red and scotopic mechanisms as maintained before the level of the lateral geniculate body.

Color Perception↗
PubMed1983

Chromatic induction as a function of wavelength of inducing stimulus.

Induced chromatic effects were determined for monochromatic, equal-luminance inducing stimuli from 460 to 680 nm by using a hue-cancellation procedure. The observed red-green-and yellow-blue-induced chromatic-response functions, which were different from the prediction based on the opponent-color hypothesis, could accurately explain the characteristics of the simultaneous color contrast effect. Good linear fits were obtained for the red-green function with a linear combination of R and G cones and for the yellow-blue function with a linear combination of R and B cones. These findings suggest that the opponent mechanisms for color contrast may be different from those for homogeneous color.

Adult↗
PubMed1983

Chromatic valence and hue sensation.

Red-green and yellow-blue chromaticnesses were scaled for various monochromatic lights by a just-noticeable-difference method. The just-noticeable difference of each chromaticness, i.e., redness, greenness, yellowness, or blueness, was defined by the change of the canceling light intensity that was required to produce a just-noticeable difference in the amount of the opponent-hue attribute of each monochromatic light. The results showed that an approximately logarithmic transformation took place at the two opponent-color coding systems and that there existed an interaction between red-green and yellow-blue opponent-color coding systems in such a manner that the effective contribution of one opponent-color response to the perceived opponent-hue attribute was reduced by increasing the magnitude of the other opponent-color response. This interaction is considered to be responsible for the well-known veiling effect.

Adult↗
PubMed1993

Role of chromatic and luminance contrast in inferring structure from motion.

We measured the ability to infer structure from motion (SFM) in several directions in three-dimensional color space. Only motion cues are useful to subjects in performing this three-dimensional shape-identification task. We report the following results: (1) SFM performance is at chance for equiluminant stimuli that isolate short-wavelength-sensitive cones. Hence the short-wavelength-sensitive-cone input to SFM is negligible. (2) SFM performance increases with the magnitude of delta L - delta M signal when delta L + delta M = 0 (i.e., only chromatic and no luminance contrast is available). We reject the hypothesis that SFM obtains input from a single chromatic mechanism combining the long- and medium-wavelength-sensitive cones linearly. Our data are compatible with SFM that uses the output of two mechanisms, one taking the difference between the long- and medium-wavelength-sensitive-cone signals and the other taking the respective sum. We reject the particular hypothesis that SFM utilizes only the magnitude and not the sign of the long- and medium-wavelength-sensitive-cone signal. (3) We compare SFM performance with threshold performance for velocity and motion discrimination. Stimuli with luminance contrast yield SFM performance that is superior to stimuli without luminance contrast when they are expressed as multiples of velocity discrimination threshold. This superiority is even greater when SFM performance is compared with motion-direction discrimination thresholds.

Color Perception↗
PubMed1993

Individual differences of the contribution of chromatic channels to brightness.

Perceived brightness is considered to be a combined consequence of outputs of the luminance channel and the chromatic channels in the visual system. The differences of logarithmic spectral luminous efficiencies between heterochromatic brightness matching and flicker photometry that were obtained from 16 subjects were examined by using principal component analysis. The luminous-efficiency difference between the two methods is described by only two principal components. Individual characteristics of the contribution of chromatic channels to brightness can be specified by measuring luminous efficiencies at 470 and 660 nm.

Adult↗
PubMed1995

Saccadic suppression of achromatic and chromatic responses measured by increment-threshold spectral sensitivity.

We measured spectral-sensitivity functions during saccadic eye movement by the increment-threshold method to test whether saccades selectively suppressed achromatic or chromatic responses. A circular monochromatic test stimulus of 12-deg diameter was presented for 10 ms on a 62 deg x 43 deg white background, and observations were made under three conditions: during fixation, during 6-deg saccades, and immediately after saccades. In two additional conditions the test stimulus was made to move during fixation and during 6-deg saccades at the same speed and in the same direction as the saccades. The during-fixation spectral-sensitivity function was found to resemble the relative luminous efficiency V (lambda) function in shape except for the case of short wavelengths, whereas the during-saccade spectral-sensitivity function showed lower sensitivity for all wavelengths and had three prominent peaks at approximately 440, 530, and 600 nm. These characteristics did not depend on whether the stimulus was stationary or moving. These results indicated that saccadic suppression was greater for achromatic than for chromatic response. A possible suppression mechanism was discussed involving the magno and parvo pathways.

Adult↗
PubMed1996

Luminosity thresholds: effects of test chromaticity and ambient illumination.

Color constancy is often modeled on the assumption that color appearance in natural scenes is a function of the visual system's estimates of surface reflectance. Some stimuli, however, do not look like illuminated surfaces. Instead, they appear to be self-luminous. We hypothesized that the appearance of luminosity occurs when the visual system estimates a reflectance spectrum that is outside the gamut of physically realizable surfaces. To test this idea, we measured luminosity thresholds as a function of stimulus chromaticity and illuminant spectral power distribution. Observers adjusted the luminance of a test patch until it just appeared self-luminous. The test patch was spot illuminated by a computer-controlled projection colorimeter viewed in an experimental room lit diffusely by computer-controlled theater lamps. Luminosity thresholds were determined for a number of test patch chromaticities under five experimental illuminants. The luminosity thresholds define a surface in color space. The shape of this surface depends on the illuminant. We were able to describe much of the luminosity threshold variation with a simple model whose parameters define an equivalent illuminant. In the context of our model, the equivalent illuminant may be interpreted as the illuminant perceived by the observer. As part of our model calculations we generalized the classic notion of optimal stimuli by incorporating linear-model constraints. Given the equivalent illuminant, the model predicts that a patch will appear self-luminous when it is not consistent with any physically realizable surface seen under that illuminant. In addition, we show that much of the variation of the equivalent illuminant with the physical illuminant can be modeled with a simple linearity principle. The fact that our model provides a good account of our data extends the physics-based approach to judgements of self-luminosity. This in turn might be taken as support for the notion that the visual system has internalized the physics of reflectance.

Color↗
PubMed1998

Detection of chromatic and luminance contrast modulation by the visual system.

The data presented in this paper examine the ability of observers to detect a modulation in the contrast of chromatic and luminance gratings as a function of the carrier contrast, duration, and spatial frequency. The nature of the signal underlying this ability is investigated by examining both the paradigm used to make the measurement and the effect of grating masks on performance in the tasks. The results show that observers' ability to discriminate amplitude modulation from an unmodulated carrier is dependent on carrier contrast but only up to approximately 5-8 times carrier-detection threshold. Discrimination is, however, independent of spatial frequency [10-1 cycles per degree (cpd) component-frequency range], carrier color, and, most surprisingly, stimulus duration (1000-30 ms). This set of experiments compliments data from previous papers and assimilates many of the conclusions drawn from this previous data. There is absolutely no evidence for the existence of a distortion product mediating performance under any of the current conditions, and the data seriously question whether the visual system might use such a signal even if it does exist under more extreme conditions than those used here. The evidence suggests that the visual system detects variations in both chromatic and luminance contrast by means of a mechanism operating locally upon the spatial structure of the carrier.

Color Perception↗
PubMed2004

Spectral switches of partially coherent light focused by a filter-lens system with chromatic aberration.

It is shown that when partially coherent polychromatic light is focused by a filter-lens system with chromatic aberration, a spectral shift exists in the focused field, and a spectral switch that is defined as a sharp transition of the spectral shift also takes place at some positions of the focused field. The influence of the chromatic aberration of the lens, the coherence of the partially coherent light in the filter (a circular aperture), the radius of the aperture, and the spectral width of the partially coherent light in the aperture on the spectral shift and the spectral switch are investigated in detail. The numerical results show that these parameters affect the spectral shift and the spectral switch significantly. Potential applications of the spectral shift and the spectral switch of the partially coherent light are discussed.

Journal Article↗
PubMed2004

Chromatic compensation of broadband light diffraction: ABCD-matrix approach.

Compensation of chromatic dispersion for the optical implementation of mathematical transformations has proved to be an important tool in the design of new optical methods for full-color signal processing. A novel approach for designing dispersion-compensated, broadband optical transformers, both Fourier and Fresnel, based on the collimated Fresnel number is introduced. In a second stage, the above framework is fully exploited to achieve the optical implementation of the fractional Fourier transform (FRT) of any diffracting screen with broadband illumination. Moreover, we demonstrate that the amount of shift variance of the dispersion-compensated FRT can be tuned continuously from the spatial domain, which is totally space variant, to the spectral domain, which is totally space invariant, with the chromatic correction remaining unaltered.

Journal Article↗
PubMed2005

Chromatic dispersions of the ocular media of human eyes.

Of the commonly used chromatic dispersion equations, only the Sellmeier and the Cauchy equations seem to be theoretically based. Cauchy's equation is derived from the Sellmeier equation, is simpler to implement, and was found to give an excellent fit to published refractive-index data of the human eye. We used Cauchy's equation to model the chromatic difference in refraction of the Gullstrand number 1 schematic eye with a gradient-index lens. To estimate the dispersion at different refractive-index levels within the lens, a single dispersion equation at one nominal refractive index was linearly scaled. This scaling was justified after exploring the effect of mean refractive index on dispersion by using Sellmeier's equation and finding that a dispersion equation for one wavelength is just a linearly scaled version of the dispersion equation at any other wavlength. Because Cauchy's equation is theoretically based and gives excellent fit to data in the visible spectrum, it can be used to extrapolate results into the near infrared with confidence.

Color↗
PubMed2005

Chromatic shadow compatibility and cone-excitation ratios.

Logvinenko [Perception 31, 201 (2002)] asserts that Adelson's wall-of-blocks illusion [Science 262, 2042 (1993)], where identical gray-cube surface tops appear to differ in brightness, arises when the surfaces surrounding the cube tops are shadow compatible, creating a concomitant illusion of transparency. We replicated Logvinenko's main findings in the chromatic domain across three experiments in which observers match cube tops in hue, saturation, and brightness. A second set of stimuli adjusted cone-excitation ratios across the apparent transparency border [Proc. R. Soc. London 257, 115 (1994)], which enhanced lightness and brightness constancy but only when the stimuli varied in both chromaticity and intensity.

Adult↗
PubMed2005

Sensory recoding via neural synchronization: integrating hue and luminance into chromatic brightness and saturation.

If neural spike trains carry information in the frequency and timing of the spikes, then neural interactions--such as oscillatory synchronization--that alter spike frequency and timing can alter the encoded information. Using coupled oscillator theory, we show that synchronization-based processing can be used to integrate sensory information, resulting in new second-order sensory percepts signaled by the compromise frequency of the coupled system. If the signals to be coupled are nonlinearly compressed, the coupled system behaves as if it signals the product or ratio of the uncoupled signals, e.g., chromatic brightness can be signaled by the compromise frequency of coupled neurons responding to hue and luminance, and chromatic saturation can be signaled by the coupled frequency of neurons responding to hue and brightness, with a power- (Stevens's) law scaling like that observed psychophysically. These emergent properties of coupled sensory systems are intriguing because multiplicative processing and power-law scaling are fundamental aspects of sensory processing.

Action Potentials↗
PubMed2006

New approach to determine the effects of polarization mode dispersion and chromatic dispersion on pulse and RF signals.

We present a novel analytical expression relating the output state of polarization (SOP) and the polarization mode dispersion (PMD) vector, including polarization-dependent chromatic dispersion (PCD), in terms of the angle of precession of the output SOP around the PMD vector. We derive a number of new expressions incorporating for the first time this angle of precession. First, a general relation to study the effect of differential group delay, PCD, and chromatic dispersion on pulses of arbitrary shapes is given. From this general relation, we derive expressions for pulse broadening and power penalty for Gaussian pulses. Moreover, a new expression for PMD-induced power fading for single-sideband modulated radio frequency signals is also derived. Measured experimental results are presented to support the derived expressions.

Journal Article↗
PubMed1986

Chromaticity discrimination: effects of luminance contrast and spatial frequency.

We examined the effects of luminance contrast and spatial frequency on chromaticity discrimination of grating bars. Alternate bars of gratings were filled with light of a standard wavelength and light that could be varied in wavelength. The observer set the variable bars to match the standard bars in hue. Discrimination, as measured by the standard deviation of the matches, decreased as spatial frequency increased. Luminance contrast did not improve chromaticity discrimination but did lead to Bezold-Brücke hue shifts that were spatial-frequency dependent.

Color Perception↗
PubMed1991

Texture segregation by chromatic and achromatic visual pathways: an analogy with motion processing.

We present results to show that texture segregation can be obtained through the so-called coherent spatial grouping of local shape (orientation) and of local color under both nonequiluminant and equiluminant conditions. Color grouping entails texture segregation independent of orientation grouping, while the reverse is not true under equiluminant conditions. The experiments permit the isolation of chromatic- and luminance-oriented mechanisms, as well as of chromatic nonoriented mechanisms, all of which contribute to texture discrimination. As a general rule, the present results (including the asymmetry between color and orientation grouping) are similar to those obtained by us in a series of motion-perception experiments. This similarity suggests that the perceptual rules governing spatial grouping are analogous (if not identical) to those governing spatiotemporal grouping. As in the case of directional discrimination, texture-discrimination performances may be accounted for by the activation of higher-order units receiving inputs from subunits, all of which display similar tuning properties within a multidimensional space.

Color Perception↗