Counterphase modulation photometry: comparison of two instruments.

The ratio of long-wavelength to medium-wavelength sensitive cones varies significantly among people. In order to investigate the possible effect of this variation in large numbers of participants, a quick and efficient method to estimate the ratio is required. The OSCAR test has been utilized previously for this purpose, but it is no longer available commercially. Having access to one of the few remaining OSCAR instruments, we compared the observers' mean settings to those obtained with the Medmont C100, a newer but apparently similar device. We also obtained Rayleigh matches for each participant. One hundred volunteers took part in the study. Settings on the OSCAR test were highly correlated with those on the Medmont C100. Both tests appeared to be influenced not only by L∶M cone ratios but also by the spectral positions of the cone photopigments, since anomaloscope midmatch points accounted for a significant proportion of the variance. We conclude that the Medmont C100 can be used as a suitable replacement for the OSCAR test and has a role in the rapid estimation of L∶M cone ratios.

The effects of surface gloss and roughness on color constancy for real 3-D objects.

Color constancy denotes the phenomenon that the appearance of an object remains fairly stable under changes in illumination and background color. Most of what we know about color constancy comes from experiments using flat, matte surfaces placed on a single plane under diffuse illumination simulated on a computer monitor. Here we investigate whether material properties (glossiness and roughness) have an effect on color constancy for real objects. Subjects matched the color and brightness of cylinders (painted red, green, or blue) illuminated by simulated daylight (D65) or by a reddish light with a Munsell color book illuminated by a tungsten lamp. The cylinders were either glossy or matte and either smooth or rough. The object was placed in front of a black background or a colored checkerboard. We found that color constancy was significantly higher for the glossy objects compared to the matte objects, and higher for the smooth objects compared to the rough objects. This was independent of the background. We conclude that material properties like glossiness and roughness can have significant effects on color constancy.

Variations in daylight as a contextual cue for estimating season, time of day, and weather conditions.

Experience and experiments on human color constancy (i.e., Arend & Reeves, 1986; Craven & Foster, 1992) tell us that we are capable of judging the illumination. However, when asked to make a match of the illuminant's color and brightness, human observers seem to be quite poor (Granzier, Brenner, & Smeets, 2009a). Here we investigate whether human observers use (rather than match) daylight for estimating ecologically important dimensions: time of year, time of day, and outdoor temperature. In the first three experiments we had our observers evaluate calibrated color images of an outdoor urban scene acquired throughout a year. Although some observers could estimate the month and the temperature, overall they were quite poor at judging the time of day. In particular, observers were not able to discriminate between morning and afternoon pictures even when they were allowed to compare multiple images captured on the same day (Experiment 3). However, observers could distinguish between midday and sunset and sunrise daylight. Classification analysis showed that, given a perfect knowledge of its variation, an ideal observer could have performed the task over chance only considering the average chromatic variation in the picture. Instead, our observers reported using shadows to detect the position of the sun in order to estimate the time of day. However, this information is highly unreliable without knowledge of the orientation of the scene. In Experiment 4 we used an LED chamber in order to present our observers with lights whose chromaticity and illuminance varied along the daylight locus, thus isolating the light cues from the sun position cue. We conclude that discriminating the slight variations in chromaticity and brightness, which potentially distinguish morning and afternoon illuminations, lies beyond the ability of human observers.

Effects of memory colour on colour constancy for unknown coloured objects.

The perception of an object's colour remains constant despite large variations in the chromaticity of the illumination-colour constancy. Hering suggested that memory colours, the typical colours of objects, could help in estimating the illuminant's colour and therefore be an important factor in establishing colour constancy. Here we test whether the presence of objects with diagnostical colours (fruits, vegetables, etc) within a scene influence colour constancy for unknown coloured objects in the scene. Subjects matched one of four Munsell papers placed in a scene illuminated under either a reddish or a greenish lamp with the Munsell book of colour illuminated by a neutral lamp. The Munsell papers were embedded in four different scenes-one scene containing diagnostically coloured objects, one scene containing incongruent coloured objects, a third scene with geometrical objects of the same colour as the diagnostically coloured objects, and one scene containing non-diagnostically coloured objects (eg, a yellow coffee mug). All objects were placed against a black background. Colour constancy was on average significantly higher for the scene containing the diagnostically coloured objects compared with the other scenes tested. We conclude that the colours of familiar objects help in obtaining colour constancy for unknown objects.

Role of eye movements in chromatic induction.

There exist large interindividual differences in the amount of chromatic induction [Vis. Res. 49, 2261 (2009)]. One possible reason for these differences between subjects could be differences in subjects' eye movements. In experiment 1, subjects either had to look exclusively at the background or at the adjustable disk while they set the disk to a neutral gray as their eye position was being recorded. We found a significant difference in the amount of induction between the two viewing conditions. In a second experiment, subjects were freely looking at the display. We found no correlation between subjects' eye movements and the amount of induction. We conclude that eye movements only play a role under artificial (forced looking) viewing conditions and that eye movements do not seem to play a large role for chromatic induction under natural viewing conditions.

Color naming reveals our ability to distinguish between a colored background and colored light.

Objects appear to have much the same color under quite diverse illumination. This phenomenon, which is known as color constancy, can only be achieved by considering the color of surrounding surfaces. However, considering surrounding surfaces will yield errors if the chromaticity in such surfaces arises from the surface reflectance rather than from the illumination. Does the visual system treat chromaticity in the direct surrounding differently when it is evident that such chromaticity arises from the illumination than when it is evident that it arises from the surrounding surface reflectance? To investigate this, we briefly presented target patches on a simulation of a colorful ball rotating slowly under a lamp. Target patches were shown on differently colored surfaces, both under the lamp and in the shade. When naming the target patches' colors, surrounding colors had a larger influence on the named color when the simulated illumination was different than when the simulated reflectance of the surrounding surface of the ball was different. When matching the color rather than naming it, this distinction was only evident if the matching stimulus encouraged people to match the appropriate contrast. We propose that matching can reveal the sensed color, whereas naming reveals the interpretation in terms of surface reflectance.

Can illumination estimates provide the basis for color constancy?

Objects hardly appear to change color when the spectral distribution of the illumination changes: a phenomenon known as color constancy. Color constancy could either be achieved by relying on properties that are insensitive to changes in the illumination (such as spatial color contrast) or by compensating for the estimated chromaticity of the illuminant. We examined whether subjects can judge the illuminant's color well enough to account for their own color constancy. We found that subjects were very poor at judging the color of a lamp from the light reflected by the scene it illuminated. They were much better at judging the color of a surface within the scene. We conclude that color constancy must be achieved by relying on relationships that are insensitive to the illumination rather than by explicitly judging the color of the illumination.

Reliable identification by color under natural conditions.

In order to recognize objects on the basis of the way in which they reflect different wavelengths of light, the visual system must deal with the different illuminant and background conditions under which the objects are seen. To test this ability under natural conditions, subjects were asked to name 6 uniformly colored papers. The experiment started by showing subjects six papers simultaneously in a normally illuminated room, and instructing them about how to name them. The papers were easy to differentiate when seen together but they were so similar that subjects only identified 87% correctly when they were presented in isolation under otherwise identical conditions to those during the instruction. During the main part of the experiment subjects walked between several indoor and outdoor locations that differed considerably in lighting and background colors. At each location subjects were asked to identify one paper. They correctly identified the paper on 55% of the trials (well above chance level), despite the fact that the variation in the light reaching their eyes from the same paper at different positions was much larger than that from different papers at the same position. We discuss that under natural conditions color constancy is probably as good as it can be considering the theoretical limitations.

Do people match surface reflectance fundamentally differently than they match emitted light?

We compared matches between colours that were both presented on a computer monitor or both as pieces of paper, with matching the colour of a piece of paper with a colour presented on a computer monitor and vice versa. Performance was specifically poor when setting an image on a computer monitor to match the colour of a piece of paper. This cannot be due to any of the individual judgments because subjects readily selected a matching piece of paper to match another piece of paper and set the image on the monitor to match another image on a monitor. We propose that matching the light reaching the eye and matching surface reflectance are fundamentally different judgments and that subjects can sometimes but not always choose which to match.

Perceiving colour at a glimpse: the relevance of where one fixates.

We used classification images to examine whether certain parts of a surface are particularly important when judging its colour, such as its centre, its edges, or where one is looking. The scene consisted of a regular pattern of square tiles with random colours from along a short line in colour space. Targets defined by a square array of brighter tiles were presented for 200ms. The colours of the tiles within the target were biased by an amount that led to about 70% of the responses being correct. Subjects fixated a point that fell within the target's lower left quadrant and reported each target's colour. They tended to report the colour of the tiles near the fixation point. The influence of the tiles' colour reversed at the target's border and was weaker outside the target. The colour at the border itself was not particularly important. When coloured tiles were also presented before (and after) target presentation they had an opposite (but weaker) effect, indicating that the change in colour is important. Comparing the influence of tiles outside the target with that of tiles at the position at which the target would soon appear suggests that when judging surface colours during the short "glimpses" between saccades, temporal comparisons can be at least as important as spatial ones. We conclude that eye movements are important for colour vision, both because they determine which part of the surface of interest will be given most weight and because the perceived colour of such a surface also depends on what one looked at last.

Combining local and global contributions to perceived colour: an analysis of the variability in symmetric and asymmetric colour matching.

Are surfaces' colours judged from weighted averages of the light that they reflect to the eyes and the colour contrast at their borders? To find out we asked subjects to set the colour and luminance of test disks to match reference disks, on various backgrounds, and analysed the variability in their settings. Most of the variability between repeated settings was in luminance. The standard deviations in the set colour were smallest when the disk and background were the same colour, irrespective of the colour itself. Matches were equally precise for greenish or reddish disks on a grey background, as for grey disks on a greenish or reddish background. The precision was less dependent on the colour contrast at the disks' borders when the backgrounds were more complex and when there was a large luminance contrast at the disks' borders. Subjects were less precise when different colours surrounded the two disks. These findings are consistent with the perceived colour at any position being a weighted average of the local cone excitation ratio and the change in the cone excitation ratio at the borders of the surface in question. However, the involved weights must be variable and depend systematically on parameters such as the luminance contrast at the surface's borders and other chromatic contrasts within the scene.

Luminance-color correlation is not used to estimate the color of the illumination.

Humans can identify the colors of objects fairly consistently, despite considerable variations in the spectral composition of the illumination. It has been suggested that the correlation between luminance and color within a scene helps to disentangle the influences of illumination and reflectance, because the surfaces that reflect the light of the illuminant well will normally be bright. Because the reliability of the luminance-color correlation as an indicator of the chromaticity of the illuminant depends on the number of surfaces that are considered, we expected the correlation to be determined across large parts of the scene. To examine whether this is so, we compared different scenes with matched luminance and chromaticity, but in which the correlation between luminance and chromaticity was manipulated locally. Our results confirm that there is a bias in perceived color away from the chromaticity of bright surfaces. However, the results show that only the correlation within about 1 degree of the target is relevant. Thus, it is unlikely that the visual system uses the correlation between luminance and color to explicitly determine the chromaticity of the illuminant. Instead, this correlation is presumably implicitly considered in the way that the color contrast at borders is determined.