Françoise Viénot

Computerized simulation of color appearance for dichromats

We propose an algorithm that transforms a digitized color image so as to simulate for normal observers the appearance of the image for people who have dichromatic forms of color blindness. The dichromats color confusions are deduced from colorimetry, and the residual hues in the transformed image are derived from the reports of unilateral dichromats described in the literature. We represent color stimuli as vectors in a three-dimensional LMS space, and the simulation algorithm is expressed in terms of transformations of this space. The algorithm replaces each stimulus by its projection onto a reduced stimulus surface. This surface is defined by a neutral axis and by the LMS locations of those monochromatic stimuli that are perceived as the same hue by normal trichromats and a given type of dichromat. These monochromatic stimuli were a yellow of 575 nm and a blue of 475 nm for the protan and deutan simulations, and a red of 660 nm and a blue-green of 485 nm for the tritan simulation. The operation of the algorithm is demonstrated with a mosaic of square color patches. A protanope and a deuteranope accepted the match between the original and the appropriate image, confirming that the reduction is colorimetrically accurate. Although we can never be certain of anothers sensations, the simulation provides a means of quantifying and illustrating the residual color information available to dichromats in any digitized image.

Frugivory and colour vision in Alouatta seniculus, a trichromatic platyrrhine monkey

It is a long-standing hypothesis that primate trichromacy evolved to help fruit-eating primates find fruits amongst leaves. We measured the reflectance spectra of fruits eaten by a trichromatic primate, Alouatta seniculus, in the rainforest of French Guiana, as well as those of the leaves that form the natural background to fruits. We develop a method of specifying these natural colour signals in a chromaticity diagram appropriate for A. seniculus. By treating the task facing frugivorous monkeys as a signal detection task, we show that the spectral tuning of the L and M cone pigments in A. seniculus is optimal for detecting fruits amongst leaves.

Digital video colourmaps for checking the legibility of displays by dichromats

We propose replacement colourmaps that allow a designer to check the colours seen by protanopes and deu- teranopes. Construction of the colourmaps is based on the LMS specification of the primaries of a standard video monitor and has been carried out for 256 colours, including 216 colours that are common to many graphics applications of MS Windows and Macintosh computing environments.

Kruithof's rule revisited using LED illumination

We have investigated the effect of colour temperature and illuminance on the visual response. Since Kruithofs (1941) work on general illumination, it has been proposed that users would find ‘pleasing’ high correlated colour temperature (CCT) illumination at high illuminance and low CCT illumination at low illuminance, although it is not unanimously accepted. Here, we question whether the pleasing sensation comes from facilitation of visual performance or from subjective appraisal. We have conducted experiments at various illuminance levels and various CCTs including performance tasks and subjective scaling of illumination quality. The change of illumination was obtained using adjustable LED clusters with high colour rendering index (R a > 90). Whereas the performance tasks yield results which depend on luminance and only slightly on CCT, observers reported that high CCT illuminations look brighter than low CCT illuminations. We investigate the effect of the various spectral power distributions on the intrinsically photoreceptive retinal ganglion cells and on the pupil response.

Recognizing real materials from their glossy appearance.

In everyday life, the visual system is remarkably good at recognizing materials across a wide range of viewing conditions. This paper addresses the problem of identifying real samples of materials from appearance. Here, we consider gloss as an appearance attribute that could reveal certain information about object properties. We prepared twelve samples of glass and PMMA and eroded these using different agents. The gloss and haze of the samples were measured at 60 degrees via a gloss meter. For all samples, the surface roughness properties were measured. Microfacet distributions were derived from measured BRDFs using an inverted microfacet model. We conducted a visual ranking experiment using the pair comparison method. The psychophysical gloss ratings correlate well with the 60 degrees gloss index. Principal component analysis of the psychophysical results revealed a somewhat more complicated picture in which three components seem to play a role. We conclude that observers can apprehend the physical nature of the surface of real objects from features that are included in the BRDF and available in the gloss appearance.

The effect of controlled photopigment excitations on pupil aperture.

In addition to rods and cones, the human retina contains melanopsin which has been identified recently in the body and dendrites of a few ganglion cells. The intrinsically photosensitive retinal ganglion cells (ipRGCs) are good candidates for controlling the tonic pupil aperture but their spectral sensitivity is close to those of rods and S‐cones which are other candidates. Our study aims at identifying the stimulus for the pupil response when the luminance is constant and the spectrum of the light changes. A light booth was equipped with five types of coloured light emitting diodes (LEDs): Blue, Cyan, Green, Orange and Red. The intensity of each LED type could be adjusted to control the light spectrum. Illumination pairs were prepared ensuring the exclusive variation of excitation of one receptor type and silent substitution for others. Because the range of the possible controlled changes of excitation was narrow, we also prepared illumination pairs ensuring silent substitution for luminance rather than for L‐cones and M‐cones independently. Photographs of the observer’s eyes were taken following one minute of adaptation to each illumination and the ratio of pupil to iris diameter was measured. No differential pupillary response was observed with a variation of rod, melanopsin or S‐cone excitation alone. A differential pupillary response could only be obtained with a variation of the melanopsin stimulus of sufficient high contrast with or without a concurrent variation of rods.

Relations between inter- and intra-individual variability of color-matching functions. Experimental results

Color-matching functions, as well as the variances and inter- and intra-individual correlations, are given for ten subjects. The dispersion found in the blue region is greater than that indicated by Stiles and Burch. The color-matching technique used is thought to be the cause. Consequences of intra-individual variability are evaluated. A comparison is made with scores of subjects on the Davidson and Hemmendinger color rule. The chromaticity diagram is shown to give an unsatisfactory representation of individual variations.

Bidirectional reflectance distribution factor and gloss scales

Gloss is a visual attribute, which, as well as color, provides qualitative information on the surrounding objets. The relevant physical quantity for gloss measurement is the BRDF that characterizes the geometrical distribution of the reflected light on the sample. We hypothesize that the light reflection on a glossy sample is split in 2 parts. The volume diffusion and the surface reflection. We assume that the surface of the sample consists of tiny mirror facets the orientation of which is specific of the surface microstructure. Each facet is the source of specular reflection. With this hypothesis, we have calculated a theoretical model of the BRDF according to Fresnel laws. We have applied the model to black and white painted samples of various gloss indexes. In addition, we have acquired real measurements of the BRDF of the same samples by using a new device which gives the compete measurement of the luminance geometrical distribution in the hemisphere. The comparison of the measurements and the theoretical model shows that, for this series of samples, the facet hypothesis is reliable and makes it possible to model the real BRDF in all directions.

Brightness matching and flicker photometric data obtained over the full mesopic range.

Three normal observers made visual matches from 0.03 to 100 td on a 10 degree central field at 445, 560 and 630 nm, using two different methods: heterochromatic flicker photometry and direct comparison brightness matching. In a relative sensitivity vs illuminance plot, the brightness matches underwent smooth changes, while the flicker curve exhibited a step-like transition around 1 td, between two separate branches. A reverse Purkinje shift was found on the high mesopic branch at 630 nm, possibly due to cone-cone interactions, and on the low mesopic branch at 445 nm, with questionable origin. At 630 nm and 0.03 td, both methods yielded matches that were not truly scotopic but might not have addressed the same mechanisms.

Domain of metamers exciting intrinsically photosensitive retinal ganglion cells (ipRGCs) and rods

Any stimulus can be described as composed of two components-a fundamental color stimulus that controls the three cone responses and a metameric black that has no effect on cones but can drive photoreceptors other than cones [e.g., rods and melanopsin expressing retinal ganglion cells (ipRGCs)]. The Cohen and Kappauf [Am. J. Psychol. 95, 537 (1982)] method is extended to calculate the black metamer basis for a limited set of band spectra. Using seven colored LEDs, the method is exploited to produce real metamer illuminations that stimulate in parallel melanopsin expressing ipRGCs and rods, at most or at least. We have verified that the pupil diameter increases when the ipRGC and rod excitation is at a minimum. For 14 observers, the average relative increase is 12%.