Puzzling colours

Previous research has shown the effects of hue and lightness on judgments of perceived size. Findings are that red stimuli appear larger than blue ones, and that lighter stimuli seem to be larger than darker ones. Applicability of these results in product or interface design is limited, since the size-effect of colour has not been studied in relation to the user's task. In this article, the effect of colour on perceived fit between objects is studied in a puzzle task. Subjects are asked to select a peg fitting to a hole. the response mode is varied by pointing or actually handling the pegs, feedback being only available in the latter condition. A comparison between colour appearance is made by using surface (real puzzle) and self-luminous colours (computer screen). the hypotheses are that red pegs that are too small are chosen more often than blue ones, and that lighter coloured pegs that are too small are chosen more often than darker pegs. In the real puzzle task, perceived fit is not influenced by hue but is influenced by brightness when using a fluorescent colour. Hypotheses on the effects of hue and brightness are both confirmed when judging self-luminous colours on a computer screen.     

On the conditions affecting the contribution of color to brightness perception

Colored stimuli appear to be brighter than an achromatic stimulus of the same luminance as the colored stimuli. We have studied this effect using similarity judgments in a triadic scaling task. Several instructions and stimulus configurations were used. the color sets contained colors at three luminance levels. When subjects are instructed to attend to brightness differences, the contribution of the brightness axis is strongly increased relative to the contribution of the color channels. Moreover, a correction to the color input is necessary when subjects are instructed to attend to brightness similarities between saturated colors. For a fixed adaptation level, the chromatic input is relatively stronger for the lower luminance levels. This correction is absent in all other investigated conditions. the correction found here confirms an earlier report by Fairchild and Pirotta (1991).     

Evaluation of a simple method for color monitor recalibration

An algorithm for recalibrating a color monitor's RGB input-output relations is presented that requires only a single measurement of a properly chosen reference stimulus. For the application under concern, i.e., reproduction of 35 different colored patches that were used as stimuli for psychophysical experiments on color constancy, the reference stimulus was a white (D65) presented at a luminance corresponding to the mean of the test stimuli. Three sets of data were obtained for evaluating the algorithm's error reduction power for a given stimulus configuration. These relate to different ways in which the monitor can get out of calibration. That is, slow, but cummulative changes over time, fast changes dur to gun interaction (resulting from changed stimulus conditions), and error introduced by a different setting of the monitor's brightness control. Additional experiments were performed to evaluate the effect of background intensity and color. The algorithm was found to be quite effective in dealing with the instantaneous changes (gun interaction, brightness control), and also for keeping track of the slow changes that may finally necessitate a full recalibration of the monitor.     

Age-related changes in the color appearance of broadband surfaces

Chromatic and achromatic components of 15 broadband surfaces were scaled by 15 younger (mean age = 21.3 yrs) and 15 older (mean age = 71.9 yrs) subjects. Colored surfaces were obtained from the OSA Uniform Color Scales and presented as 2°-diameter test stimuli that were freely viewed. Stimuli were presented in a grey (L = 0) ganzfeld-like hemisphere with an illuminant having a correlated color temperature of 6200 K. Five stimuli were presented at three lightness levels (L = ?4, 0 and +4). Each lightness level included samples chosen so that one pair plotted approximately along a tritan axis in MacLeod-Boynton receptor-excitation space and another pair plotted on an axis of constant S-cone stimulation. The appearance of each test stimulus was scaled using the hue terms red, green, yellow and blue, and in terms of the proportion of overall chromatic content. The results indicate that hue names were used similarly for the two groups of subjects, however, older subjects perceived all stimuli as having less chromatic content than demonstrated by younger observers. Differences between the two age groups in perceived chromatic content of the test stimuli progressively increased as the lightness and luminance levels of the test stimuli decreased. Results obtained from control experiments suggest that this difference between the two age groups is not due to the influence of senile miosis or lenticular senescence leading to variations in the illuminances and spectral compositions of the test lights reaching the retina. © 1993 John Wiley & Sons, Inc.     

The distimulus method of assessing luminous efficiency of the spectrally pure stimuli

In the tristimulus method of photometry, the luminance of any monochromatic stimulus can be assessed by matching it in hue and brightness to a mixture of two primaries and then reducing the saturation of the test stimulus to obtain a complete match. The luminances of the stimuli are considered to be additive, and the luminances of the two mixtures to be equal. In this article it is shown that if the primaries are properly chosen, it is not necessary to desaturate the test stimulus. The difference in saturation between the test stimulus and the mixture of the two primaries can be ignored and it may be assumed that the luminance of the test stimulus is equal to the sum of the luminances of the two primaries when the mixture and the test stimulus match in hue and brightness. Luminous efficiencies assessed in this way agree well with assessments based on flicker photometry. Flicker photometry has been used to assess the luminances of the primaries.     

Nonlinear nature of the opponent-color channels

Heterochromatic brightness matches were made between bichromatic mixtures of Λ₁ and Λ₂, and a 100-trolands white reference light of 2° visual angle. A remarkable brightness reduction was found in the bichromatic mixtures, showing brightness additivity failure. The failure was especially large at the luminance ratio where hue cancellation occurred. The results were explained by assuming the brightness sensation to be mediated not only by the luminance channel but also by the red-green and yellow-blue opponent-color channels, and by assuming further nonlinear transformation of responses in the opponent-color channels.     

Assessing the application of an image color appearance model to basic self‐luminous scenes

Image color appearance models (Image CAMs) have been developed to predict the perception of complex scenes and are mainly used for image rendering and video reproduction applications. Among these Image CAMs, iCAM is an Image CAM that takes an image as the input and provides the perceptual attributes for each pixel. On the other hand, nonimaging CAMs are widely used and validated, but they always assume a simple test scene of a uniform flat stimulus, a quasi‐neutral background, and a surround. This study presents an evaluation of the performance of iCAM when applied to these simple self‐luminous scenes in predicting the influence of background luminance, background size, saturation, and stimulus size on stimulus brightness. The results show that iCAM is capable of predicting the effect of background luminance and some background size scenarios. However, for unrelated self‐luminous stimuli (dark background), the model predictions do not match the reference data. An evaluation of the effect of the filter kernel size and its relation to the physiological mechanism of image processing inside the visual system has been investigated. Furthermore, the impact of saturation and stimulus size on brightness seems to be underestimated by the model, because the Helmholtz‐Kohlrausch and stimulus size effects are not included. Hence, these findings call for an enhanced Image CAM.     

Effects of surrounding brightness on visual search for safety colors

The present article reports the investigation of the effects of surrounding brightness on a visual search for three safety colors: red, orange, and yellow. Images of visual stimuli consisting of an array of colored circles placed on a large visual field (a visual angle of 40°) were displayed on an 80‐in. screen with a DLP projector. Experiment I examined the search efficiency under three levels of background luminance that were equivalent to the three target luminances. The results showed that the search efficiency for the orange target decreased as the number of distractors increased, under each of the background luminance levels, whereas the efficiency scarcely decreased for the red and yellow targets. Although a reduction in background luminance increased the search efficiency for the orange target, it is suggested that the effect of background luminance is smaller than the effect of the target color in search efficiency. Experiment II examined the search efficiency under three conditions of low levels of incident illuminance, which were matched with a linear regression to the luminance of color chips of safety colors measured twilight conditions. The results showed that the search efficiency for the orange target decreased as the number of distractors increased under each of the different illuminance conditions, whereas the efficiency scarcely decreased for red and yellow targets. Furthermore, as illuminance decreased, the search time for the orange target was more greatly impacted than for red or yellow. These results imply that the recognition of orange tends to be influenced by the surrounding brightness. © 2005 Wiley Periodicals, Inc. Col Res Appl, 30, 400–409, 2005; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/col.20152     

Simple reaction time to chromatic changes along L&M‐constant and S‐constant cone axes

The present study uses simple reaction time (RT) to examine the temporal response to chromatic changes of the red‐green (L‐2M) and yellow‐blue (S‐(L+M)) chromatic opponent channels. The hue‐substitution method was adopted to negate the luminance channel and generate changes in the chromatic input. The equiluminant stimuli were distributed along three tritanopic and three red‐green confusion lines. RTs were measured at equiluminance from three reference stimuli with chromatic changes occurring in either the tritan or red‐green directions. These reference stimuli also allowed evaluation of the influence of chromatic adaptation level on RT. The results showed that the mean value of RT, for the cone‐input variation either of the L‐2M channel (S‐constant cone axis) or of the S‐(L+M) channel (L&M‐constant cone axis), was described by a similar law as for luminance variations (Piéron's law). Then, the sensitivity of the two channels for chromatic changes was equalized to study their relative response speed. In addition, for both axes, an increase in the chromatic adaptation level lengthens response delay. © 2001 John Wiley & Sons, Inc. Col Res Appl, 26, 223–233, 2001     

Scaling procedures for specifying color appearance

Colors of objects are often specified by color matches in some standard colorimetric system. While color-matching allows a color to be reproduced accurately, it does not specify what the color actually looks like. Color appearance (i.e., hue, saturation, brightness) can be measured by direct psychophysical scaling techniques; however, these methods have not been standardized and have been confined mostly to the laboratory. We have examined several variants of continuous, numerical scaling of hue and saturation to find a procedure that is usable, even by untrained observers. Appearances of monochromatic, equal-luminance light flashes were scaled along the dimensions of the unique hue sensations (red, yellow, green, blue) and apparent saturation. These functions were obtained for various stimulus conditions, including changes in stimulus luminance, size, and eccentricity. Multidimensional scaling methods were used to derive metrically uniform color appearance diagrams from the hue and saturation scaling data. Validity of the scaling procedures was assessed by deriving well-known visual functions, such as wavelength discrimination, from the color appearance diagrams. The direct scaling method was shown to be generally useful, and both trained and untrained observers readily and reliably used the technique.     

The purkinje effect: Some considerations on the interplay of receptoral and postreceptoral mechanisms

A heterochromatic brightness or lightness match, established at photopic levels, is not expected to hold at lower adapting luminances. This applies, in particular, to the Purkinje Effect, concerning the red-blue comparison. The fact that a “blue” stimulus mainly involves the shorter wavelengths favors the interpretation in terms of rod intrusion, coming gradually into play as the eye, pre-adapted to a photopic level, is becoming adapted to a mesopic or scotopic one. The two experiments described in this paper aim to put into evidence some effects that are likely to be due to postreceptoral mechanisms, and which may confuse the student faced with a practical demonstration of the Purkinje Effect. In particular, we consider the blue lightness enhancement after an abrupt decrease in adapting luminance, and the decay of lightness-luminance discrepancy of narrow-band stimuli during the recovery period.     

Evaluation of the visibility of colored objects under led lighting with various correlated color temperatures

This study proposes a “visibility” concept of colored objects that includes a combination of color brightness and appearance perception of colored objects that are illuminated by seven LED lights (each with a CIE color rendering index under 80 but with various correlated color temperatures). To determine the brightness perception of colored objects, luminance calculations and measurements were conducted. The areas of the “color brightness graph” in this study correspond to the results of eight different color sample papers under each of the LED lights with different CCTs. The luminance values were calculated by multiplying the SPD by the spectral luminous efficiency and spectral reflectance of a colored object. The luminance values were measured under these conditions to identify the relationship between the luminance calculations and the measured values. Efficient CCTs were identified for each color sample in terms of the brightness of the colored object. Further subjective evaluations were also conducted to identify the relationship between luminance values and subjective brightness perception. In addition, subjective evaluations of the color appearance perception were conducted to identify the overall visibility concept of colored objects. The subjective evaluations included brightness perception, color appearance, and similarity of the reference light source. The visibility of the colored objects was analyzed according to the results of the brightness and appearance perception of the colored objects that were illuminated by LED lights with various CCTs. © 2016 Wiley Periodicals, Inc. Col Res Appl, 42, 78–88, 2017     

Toward a more accurate and extensible colorimetry. Part Ii. Discussion

Technical advances in the forty years since the Stiles-Burch color-matching work-among them the bright split-field visual colorimeter coupled by quartz light pipe to a spectroradiometer-allow measurement of an accurate absolute spectral power distribution (SPD) of every viewed light. Furthermore, the ability to measure accurately the power-content of each spectral constituent of a viewed light has clarified both colorimetric procedures and signficance, for example, of color-matching functions (CMFs) and tristimulus values. Three disparate sets of three spectral primaries were used throughout the work. This has not been done before, and its use elucidated much of the difficulty (as described by Judd, Stiles, MacAdam, Wyszecki, and others) inherent in traditional colorimetry. Errors in computed chromaticities of sets of 28 visually-matching lights depend strongly on their spectral content, and are particularly large in the presence of high spectral content in the violet, deep red, and near 500 nm or 580 nm. Peaks of visual efficiency, and minima in chromaticity errors, were found with power in three intermediate spectral regions near 450 nm, 530 nm, and 610 nm. Computed luminance (1964 CIE Standard Observer) and power-content also depend strongly on spectral content of the visually-matching white lights. That this is true of luminance is generally unknown. Perceived brightness of single spectral lights in isolation, and of mixtures, was examined at length: As a rule, removing one spectral component from a mixture increases perceived brightness, thus invalidating both luminance and R + G + B as correlates of brightness. Visual tests of the Grassmann proportionality and additivity assumptions, with highly metameric mixtures, tended to confirm them. Nevertheless, numerous tests-substituting pairs of actual visually-matching lights from one primary-set for pairs from another, which is how “transformation of primaries” is (mathematically) carried out-led to the inescapable conclusion that the normal human visual system does not honor such transformation. At the same time, numerous indications supported a particular set of primaries as an invariant of the visual system, making transformation needless as well as invalid. Chromaticity errors are not the fault of the particular maximum-saturation CMFs utilized; the observer's own CMFs do no better, whatever the primary-set. Soundness of traditional construction of the chromaticity diagram is shown to depend on the primary-set, breakdown occurring for some otherwise legitimate primary-sets.     

An investigation of colour appearance for unrelated colours under photopic and mesopic vision

Data were obtained for the colour appearance of unrelated colours under photopic and mesopic conditions. The effects of changes in luminance level and stimulus size were investigated. The method used was magnitude scaling of brightness, colourfulness, and hue. Two stimulus sizes (10° and 0.5°) and four starting luminance levels (60, 5, 1, and 0.1, cd/m²) were used. The results at 0.1 cd/m² had large variations, so data were obtained for two additional stimulus sizes (1° and 2°) at this luminance level. Ten observers judged 50 unrelated colours. A total of 17,820 estimations were made. The observations were carried out in a completely darkened room, after 20 min adaptation; each test colour was presented on its own. Brightness and colourfulness were found to decrease with decreases of both luminance level and stimulus size. The CAM97u model predicted brightness more accurately than CIECAM02 but gave worse performance in predicting colorfulness. For hue, CAM97u and CIECAM02 both gave satisfactory predictions. Using the brightness correlate from CAM97u, a new colour‐appearance model based on CIECAM02 was developed specifically for unrelated colours under photopic and mesopic conditions, with parameters to allow for the effects of luminance level and stimulus size. © 2011 Wiley Periodicals, Inc. Col Res Appl, 2011;     

Successive heterochromatic brightness matches for a LED display

An experiment is described in which red and green pointlike sources are flashed in succession on nearby locations on the retina of the eye. While keeping the intensity of the green light fixed, the red's intensity is varied until a heterochromatic brightness match is attained. As the retinal image is more and more defocussed by the use of a blurring lens, less and less red light is needed to match the green reference light, both in the central retina and at moderate eccentricities, while the far periphery shows an insensitivity to blur. The result is tentatively explained by extending to peripheral vision some speculations concerning the brightness–luminance discrepancy in central vision. As a practical implication, it is noted that the photometric assessment of small sources, holding for the plane in which the eye is at focus, may exhibit deviations even as large as 0.5 log units when passing to defocussed locations.     

Perception of brightness when the eyes are closed

To describe the lighting environment during sleep accurately, it is necessary to know the extent to which light is still perceived when the eyelids are closed. In this study, we measured the perception of equivalent brightness under closed-eye and open-eye conditions in binocular photopic vision. The light sources used were monochromatic red (peak wavelength 630 nm), yellow (593 nm), green (515 nm), and blue (460 nm) LEDs, as well as a white LED (T_cp = 4188 K, R_a = 93). A total of 33 subjects (Asian adults aged 22 years on average) with eyelids of approximately the same color were recruited. The average effective transmittances of their closed eyes were 52.4% ± 31.5% for red, 26.2% ± 18.2% for yellow, 21.6% ± 16.7% for green, 4.5% ± 3.9% for blue, and 42.7% ± 24.8% for white light. These values were up to 10 times higher than eyelid transmittances reported previously, so eyelid transmittance cannot by itself explain perceived closed-eye brightness. There were also significant individual differences. As it is unlikely that the physical transmittances of the eyelids differed from subject to subject, psychological factors may be responsible. This phenomenon should be further investigated in the future.     

A system of photometry and colorimetry based on cone excitations

A system of photometry and colorimetry is proposed that is based upon cone action spectra. Instead of X, Y, Z tristimulus values, the new system divides the visual stimulus into L, M, and S components, which are related to the relative excitation levels of the three classes of human cone photoreceptors (long-wavelength-sensitive L, middle-wavelength-sensitive M, and short-wavelength-sensitive S). On the assumption that luminance is proportional to L + M, with S cones making no contribution to it, a chromaticity diagram results in which the relation between chromaticity coordinates and cone excitations is transparent, rather than inadvertently obscured as in the CIE system.     

Testing the performance of CIECAM02 from 100 to 3500 cd/m2

Color appearance models were developed to characterize the color attributes of stimuli under different viewing conditions based on data collected through magnitude estimation or color matching experiments. Although human beings experience very high light levels under daylight and the reproduction of colors under daylight is important in the color and imaging industries, the existing color appearance models were developed based on the data that were collected under the conditions with luminance levels below 700 cd/m² due to the lack of facilities to produce stable illumination at high light levels. A recent study investigating color preference of an artwork under a wide range of light levels from 20 to 15 000 lx suggested that CIECAM02 cannot accurately characterize the color appearance under extremely high light levels. This study was designed to directly test the performance of CIECAM02 from 100 to 3500 cd/m². Human observers performed color match for four hues under a series pairs of adapting conditions with a haploscopic viewing condition. It was found that CIECAM02 had the best performance in characterizing the hue angles but the worse performance in characterizing the brightness with a maximum underprediction around 200% across a wide range of luminance. This was mainly due to the fact that CIECAM02 was developed based on the data collected under relatively low adapting luminance levels. The color appearance model that was proposed to use the adapting luminance levels in characterizing the cone compression in the postadaptation process was found to have a much better performance in characterizing the brightness.     

Toward a more accurate and extensible colorimetry. Part Iii. Discussion (continued)

The instrument used to gather the data of this article, a bright split-field visual colorimeter coupled by quartz light pipe to a spectroradiometer, generated many pairs of lights visually-matching to one of six normal human observers. an accurate absolute spectral power distribution (SPD) was obtained for every such viewed light. Three disparate primary-sets were used. Large errors in chromaticity and brightness (represented by luminance), computed by the 1964 CIE Standard Observer, were found to pervade the results. Problems are introduced by the use of maximum-saturation CMFs, by transformation of primaries, by the structure of the CIE chromaticity diagram, and by measurement inaccuracies of the far past. In Part III, alternative derivations of weighting functions, multiplied against the absolute SPDs of the visually-matching lights to obtain what are called tristimulus values, are shown to reduce errors in computed chromaticity. an alternative brightness function, of complexity commensurate with that of the three-input normal human visual system, is shown to correlate considerably better to perceived brightness of white and colored lights than does computed luminance. an alternative chromaticity diagram is constructed of two chromatic terms of the brightness function, as a constant-perceived-brightness plane (which conventional diagrams are not), and avoids problems inherent in the conventional diagram.     

Categorical formation of Mandarin color terms at different luminance levels

This study presents the categorical formation of a set of Mandarin color terms on the International Commission on Illumination (CIE) 1931 chromaticity diagram across six luminance levels. This article conducted a study that employed 44 native Mandarin speakers to perform a force–choice sorting task. The Mandarin color terms for sorting were determined by a free‐recall pretest and are consistent with basic color terms proposed by Berlin and Kay. The square‐sampled stimuli were generated by evenly sweeping the x–y diagram of 5, 10, 25, 50, 100, and 170 cd/m² planes. The categorical sorting results and response time (RT) measurements suggest that: (1) the concepts of green, blue, purple, and gray stably exist at most luminance levels. The voting RT for the green, blue, and purple categories is particularly short. (2) Red, orange, yellow, and pink are highly luminance‐dependent; these can be identified without difficulty only at some restricted luminance levels. (3) The chromaticity areas designated as orange, partial yellow, red, and pink are recognized as brown when the luminance level decreases. (4) Brown and gray serve as representations of two distinct tints in the low saturation condition. (5) The location of boundaries between blue and green are remarkably different than those in a similar study that employed Japanese speakers. © 2011 Wiley Periodicals, Inc. Col Res Appl, 2011.