A model of colour vision for predicting colour appearance in various viewing conditions

A previously published model of colour vision predicted the hues, lightnesses, saturations, and chromas of colours seen under a medium photopic level of illumination. This model has now been extended to provide, in addition, predictions of brightness and colourfulness, for both related and unrelated colours, at any level of illumination, whether photopic, mesopic, or scotopic. The model has also been extended to cover a wider range of stimulus intensities and the use of various backgrounds.     

Dynamic cone response functions for models of colour appearance

The CIECAM97s type of colour appearance models results in some changes in hue and saturation for series of colours of constant chromaticity but changing luminance factor. To keep hue and saturation constant for such series, a model in which the dynamic cone response is based on a power function has been developed. By optimizing its parameters, its correlates of hue, lightness, colourfulness, brightness, and saturation perform nearly as well as those of CIECAM97s. A similar performance is achieved in a modified power model, which is more physiologically plausible. © 2003 Wiley Periodicals, Inc. Col Res Appl, 28, 82–88, 2003; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/col.10128     

Correcting veiling glare of refined CIECAM02 for mobile display

Small displays are widely used; they are small enough to be carried around and are often viewed under extreme surround conditions. Under bright illumination, mobile display users experience “veiling glare” caused by bright ambient lighting. A refined version of CIECAM02 called “Refined CIECAM02 and original CIECAM02” were tested to predict visual results in terms of lightness (J), colourfulness (M), and brightness (Q) on a 2‐inch sized display (2″) mobile phone under four surround conditions; dark (0 cd/m²), dim (5 cd/m²), average (1000 cd/m²), and bright (10,000 cd/m²). Other than the two versions of CIECAM02 using the original data, a correction to the models' predicted lightness J and a black correction to the original data were developed. Overall, the refined CIECAM02 plus the J correction performed the best for predicting the lightness, brightness and colourfulness under all the viewing conditions, especially for bright surround condition. Furthermore, another experiment was carried out using complex images to verify different versions of CIECAM02. The images were reproduced using JMh (lightness, colourfulness, and hue) spaces from the modified CIECAM02 versions. The experiment was conducted by comparing original images viewed under dim, average, or bright surround conditions and the predicted images were viewed under dark surround condition on two identical mobile displays. The different versions of the CIECAM02 showed similar results to each other for dim and average surround conditions but large differences when predicting the images under bright surround condition. The refined CIECAM02 with the J′ formula performed the best amongst all four CIECAM02 versions. © 2011 Wiley Periodicals, Inc. Col Res Appl, 2013.     

A comparison of colour appearance for surface colours between outdoor and indoor environments

Psychophysical experiments of colour appearance, in terms of lightness, colourfulness, and hue, were conducted outdoors and indoors to investigate whether there was any difference in colour appearance between outdoor and indoor environments. A panel of 10 observers participated in the outdoor experiment, while 13 observers took part in the indoor experiment. The reference white had an average luminance of 12784 cd/m² in the outdoor experiment and 129 cd/m² in the indoor experiment. Test colours included 42 colour patches selected from the Practical Coordinate Color System to achieve a reasonable uniform distribution of samples in CIECAM02. Experimental results show that for both outdoor and indoor environments, there was good agreement between visual data and predicted values by CIECAM02 for the three colour appearance scales, with the coefficient of variation values all lower than 25 and the R² values all higher than 0.73, indicating little difference in the three dimensions of colour appearance between indoor and outdoor viewing conditions. Experimental data also suggest that the observers were more sensitive to variation in lightness for grayish colours than for highly saturated colours, a phenomenon that seems to relate with the Helmholtz-Kohlrausch effect. This phenomenon was modeled for predicting perceived lightness (J′) using the present experimental data. The new J′ model was tested using three extra sets of visual data obtained both outdoors and indoors, showing good predictive performance of the new model, with an average coefficient of variation of 14, an average R² of 0.88, and an average STRESS index of 14.18.     

A comprehensive model of colour appearance for related and unrelated colours of varying size viewed under mesopic to photopic conditions

CIE has recommended two previous appearance models, CIECAM97s and CIECAM02. However, these models are unable to predict the appearance of a comprehensive range of colours. The purpose of this study is to describe a new, comprehensive colour appearance model, which can be used to predict the appearance of colours under various viewing conditions that include a range of stimulus sizes, levels of illumination that range from scotopic through to photopic, and related and unrelated stimuli. In addition, the model has a uniform colour space that provides a colour‐difference formula in terms of colour appearance parameters. © 2016 Wiley Periodicals, Inc. Col Res Appl, 42, 293–304, 2017     

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.     

Unique hue data for colour appearance models. Part I: Loci of unique hues and hue uniformity

Psychophysical experiments were conducted to assess unique hues on a CRT display for a large sample of colour‐normal observers (n = 185). These data were then used to evaluate the most commonly used colour appearance model, CIECAM02, by transforming the CIEXYZ tristimulus values of the unique hues to the CIECAM02 colour appearance attributes, lightness, chroma and hue angle. We report two findings: (1) the hue angles derived from our unique hue data are inconsistent with the commonly used Natural Color System hues that are incorporated in the CIECAM02 model. We argue that our predicted unique hue angles (derived from our large dataset) provide a more reliable standard for colour management applications when the precise specification of these salient colours is important. (2) We test hue uniformity for CIECAM02 in all four unique hues and show significant disagreements for all hues, except for unique red which seems to be invariant under lightness changes. Our dataset is useful to improve the CIECAM02 model as it provides reliable data for benchmarking. © 2010 Wiley Periodicals, Inc. Col Res Appl, 2011     

Color appearance and color connotation models for unrelated colors

The purpose of this research is to investigate the color appearance and color connotation of unrelated colors. To investigate color appearance (i.e., brightness, colorfulness, and hue) for unrelated colors, 22 observers have answered their color appearance for 50 unrelated color stimuli using the magnitude estimation method. Perceptual data obtained by the experiment is compared with the color attributes data estimated by unrelated‐color appearance models, CAM97u and CAM02u. It is found that both models perform reasonably well but the performance of CAM02u is better than that of CAM97u. For investigating color connotation for unrelated colors, 32 observers have judged their color connotation for the 50 unrelated color stimuli using the 10 color connotation scales (i.e., “Warm – Cool,” “Heavy – Light,” “Modern – Classical,” “Clean – Dirty,” “Active – Passive,” “Hard – Soft,” Tense – Relaxed,” “Fresh – Stale,” “Masculine – feminine,” and “like – Dislike”), and semantic differential method is used for measurement. It is found that the color connotation models developed for related colors perform poorly for unrelated colors. Experimental results indicate that brightness attribute is confusing to estimate and does not affect color connotation significantly for unrelated colors. Based on the psychophysical data, new models for “Warm‐Cool”, “Heavy‐Light”, “Active‐Passive” and “Hard‐Soft” were proposed using CAM02u hue, brightness, and colorfulness. Color connotations for unrelated colors are classified into three categories, which “Color solidity,” “Color heat,” and “Color purity.” © 2013 Wiley Periodicals, Inc. Col Res Appl, 40, 40–49, 2015     

Chromatic discrimination in relation to luminance level

We have measured the chromatic‐discrimination ellipses with different luminances for 66 stimuli distributed throughout the CIE‐1931 chromatic diagram. The distribution of these stimuli enabled us to analyze the influence of luminance on discrimination from clearly photopic levels to levels that could enter the mesopic range. The results show a clear influence of the luminance level on the areas of the chromatic‐discrimination ellipses. These areas remain almost constant when the luminance level is clearly photopic, and clearly increase when the luminance diminishes. However, it is not necessary that the luminance level diminish far below 2–3 cd/m² to detect a substantial increase in the area of the ellipses. According to our data, within the photopic range appears a transition interval of luminance above which the most pronounced increase in the area of the ellipse would be appreciable. Other parameters characteristic of discrimination ellipses, such as orientation and relation to semi‐axes, vary with the luminance level, although without clearly following any rule. © 2001 John Wiley & Sons, Inc. Col Res Appl, 26, 123–131, 2001     

Colour appearance shifts in two different‐sized viewing conditions

This study assessed the effect of size on colour appearance, using a colour matching paradigm where two sizes were presented in a setting similar to a normal colour selection interface. Twelve colours sampling the entire range of the colour spectrum were chosen as target stimuli. The target stimuli consisted of either a large (30° by 50°) or a small (0.5° by 0.5°) test field displayed on a cathode ray tube (CRT). In the experiment, a set of small colour samples consisting of the target and its neighboring colours was presented on the screen. Fifty‐seven participants were asked to pick a colour from the sample set that appeared to exactly match the target. Results in CIECAM02 showed a consistent increase in the apparent brightness (Q) but some decrease in saturation (s) for the larger field. Hue shifts were observed to form a systematic pattern. We noticed a discernable trend showing that, for targets of bluish or purplish colour hues, the accuracy of colour matching is lower and colour difference is greater in the condition of the large viewing field. © 2010 Wiley Periodicals, Inc. Col Res Appl, 2010     

An evaluation of the Hunt94 color appearance model under different light sources at low photopic to low mesopic light levels

Color appearance models allow for the quantification of color appearance under a variety of viewing conditions. Such models may ultimately provide a measure for accurate assessments of the color‐rendering properties of light sources. This article evaluates the Hunt94 color appearance model using a new set of color‐naming and magnitude‐rating data. At one photopic level (10 cd/m²), the evaluations showed that for a xenon lamp and an enhanced metal halide lamp that have chromaticities and spectra close to an equal energy spectrum, the Hunt94 model provided good predictions of the primary and secondary color names and hue magnitudes for a wide range of color chips under the two illuminants. However, for other light sources the Hunt94 model predictions deviated considerably from the evaluations. Three modifications were applied to the Hunt94 color appearance model to predict color‐naming and magnitude‐rating data better for all light sources. The modified Hunt94 model gave good predictions (correlation coefficients r ～ 0.95) of the secondary hue magnitude of the color chips used in the experiment at photopic light levels (10 cd/m² and 1 cd/m² background luminances) under “white” light sources. However, the modified model was still unable to predict color appearances at low mesopic light levels (0.1 cd/m² and 0.01 cd/m²). © 2005 Wiley Periodicals, Inc. Col Res Appl, 30, 107–117, 2005; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/col.20088     

Quantifying colour appearance. part V. simultaneous contrast

Experiments were carried out to investigate the effect of simultaneous contrast on colour appearance by varying the lightness, colourfulness, and hue of an induction field surrounding a test colour. A total of 814 test/surround combinations were displayed on high-resolution colour displays. Each was assessed by a panel of five to six observers using a magnitude estimation technique. the results indicate that colours presented on a computer display are affected by simultaneous contrast in a similar way to surface colours. All three colour appearance parameters studied (i.e., lightness, colourfulness, and hue) are affected and these effects are summarized. In general, the results support and add to the findings of the other studies. the Hunt colour appearance model was tested and gave a somewhat poor prediction to this data set. Further modifications are required to improve its performance. © 1995 John Wiley & Sons, Inc.     

Unique hue data for colour appearance models. Part III: Comparison with NCS unique hues

In this study, Swedish Natural Color System (NCS) unique hue data were used to evaluate the performance of unique hue predictions by the CIECAM02 colour appearance model. The colour appearance of 108 NCS unique hue stimuli was predicted using CIECAM02, and their distributions were represented in a CIECAM02 a_c–b_c chromatic diagram. The best‐fitting line for each of the four unique hues was found using orthogonal distance regression in the a_c–b_c chromatic diagram. Comparison of these predicted unique hue lines (based on the NCS data) with the default unique hue loci in CIECAM02 showed that there were significant differences in both unique yellow (UY) and unique blue (UB). The same tendency was found for hue uniformity: hue uniformity is worse for UY and UB stimuli in comparison with unique red (UR) and unique green (UG). A comparison between NCS unique hue stimuli and another set of unique hue stimuli (obtained on a calibrated cathode ray tube) was conducted in CIECAM02 to investigate possible media differences that might affect unique hue predictions. Data for UY and UB are in very good agreement; largest deviations were found for UR. © 2014 Wiley Periodicals, Inc. Col Res Appl, 40, 256–263, 2015     

Grids for Assessing Colour Appearance

A technique of subjective magnitude estimation has been used to assess a set of pseudosurface colours, each having a luminance equal to half that of its adapting surround. Five observers scaled each of the colours three times in each of two adaptation conditions corresponding to daylight and tungsten light at 120 cd/m². Loci of constant hue and saturation were derived in u', v' chromaticity space for daylight and tungsten light. These grids enable the appearance of a colour to be specified from a knowledge of its chromaticity coordinates and they enable such effects as colour constancy and colour fidelity to be investigated in terms of perceived colour appearance.     

Relationship between pupillary size,brightness, and photoreceptor responses for unrelated self-luminous stimuli at low photopic light levels

Research on the correlation between the initial pupillary light reflex, the perceptual brightness, and the excitations of the photoreceptors was performed for unrelated self-luminous 10° stimuli at low photopic light levels (5 to 80 cd/m²). All stimuli were optically characterized using a spectroradiometer. For determining the pupillary diameter, an Eye Tracker was used. The perceived brightness was evaluated with the magnitude estimation method. Test stimuli were presented alternating with a reference stimulus and the Michelson contrast of the pupillary diameter between test and reference conditions was calculated. Six observers (three female and three male) participated, with ages between 30 and 39 years (with an average of 34.5 years). It was found that the coefficient of determination between the pupillary response and the perceived stimulus brightness was higher than the corresponding value for luminance, allowing the suggestion that brightness has a greater effect on pupillary size than luminance. Nevertheless, this is not the complete story: highly saturated red and blue stimuli both induce a high brightness perception but the pupil contraction induced by the blue stimuli is much greater than for the red stimuli. Pupillary diameter was also studied as a function of the relative excitations of the five photoreceptors. The effect of rod and ipRGC excitations on pupillary size variation seems to be larger than that of the other photoreceptors. However, for the working conditions prevailing in this research, it could be expected that rods are not very active and that the ipRGC response might be the major driving mechanism.     

Proposed modification to the CIECAM02 colour appearance model to include the simultaneous contrast effects

A modified CIECAM02 colour appearance model, named CIECAM02‐m2, is proposed to enable CIECAM02 to predict the simultaneous contrast effect. The structure of the CIECAM02‐m2 is a development from CIECAM02, and contains two different procedures for modifying the reference white; one is for lightness and the other is for hue. The model was tested using a data set accumulated in this study and the LUTCHI data. The CV values for three colour attributes between predictions and experimental data were used to evaluate the accuracy of the model. The low CV values obtained show the performance of the CIECAM02‐m2 model to predict the simultaneous contrast effect satisfactorily. © 2007 Wiley Periodicals, Inc. Col Res Appl, 32, 121 – 129, 2007     

Colour appearance of room colours

Psychophysical experiments were conducted to investigate the colour appearance changes between small colour patches and real room colours. The results clearly showed that colours appear lighter and more colourful for room colours. A method based on CIECAM02 was developed to quantify the colour size effect. © 2010 Wiley Periodicals, Inc. Col Res Appl, 2010     

Quantifying colour appearance. part III. Supplementary LUTCHI colour appearance data

The experimental data from this study supplements the LUTCHI Colour Appearance Data as described in Part I of this paper. Two further experiments were carried out: one was to check conflicting results found previously, and another was to extend the range of luminance conditions used earlier. In addition, a brightness attribute was added to the original lightness, colourfulness, and hue scales for colour assessment. Experiment I results verified the uncertainties found previously in the comparison between luminous and nonluminous colours, and between the grey background results with and without a white border. Experiment II results revealed the changes in four perceived attributes under six quite different adapting luminances. The results were then used to test five uniform colour spaces and colour-appearance models used in Part II of this paper. Hunt's 91 model gave more accurate predictions of the experimental visual results, in comparison with the other spaces and models. Its predictive error for all attributes studied is within the accuracy of the typical observer.     

Background luminance and subtense affects color appearance

Two psychophysical experiments were conducted to investigate the color appearance under non‐uniform surround conditions with variation of stimulus luminance, surround luminance, background luminance, background orientation, and background size. The results show that the background size and surround luminance influence the appearance intensively, but that the orientation of background pattern has little effect. A method to determine optimal parameters for the CIECAM02 color appearance model in lighting applications is proposed. An UGR‐based model also is optimized for brightness estimation. The luminance of adapting field can be estimated by Gaussian‐like functions. © 2016 Wiley Periodicals, Inc. Col Res Appl, 42, 440–449, 2017     

The Specification of Colour Appearance. I. Concepts and Terms

Concepts necessary for the specification of the appearance of colours are described, and the corresponding colour terms collected. Psychometric and psychoquantitative terms are listed that correlate with the perceptual and psychophysical terms used. The perceptual term colourfulness is proposed to denote the attribute of a visual sensation according to which an area appears to exhibit more or less chromatic colour; for a given chromaticity, as the luminance increases, colourfulness generally increases. For related colours, the CIE 1976 Colour Spaces are used to systematize some psychometric variables that are independent of absolute luminance level, including metric saturation, metric purity, and metric chroma, and formulae are given for calculating their amounts. (In Part II, to appear in the next issue, the effects of changes in viewing conditions will be discussed).