On attributes of achromatic and chromatic object‐color perceptions

Adapting luminance dependencies of various color attributes of object colors (lightness, brightness, whiteness‐blackness, whiteness‐blackness strength, chroma, and colorfulness) were clarified under white illumination with various adapting illuminances. The correlation between the perceptions of lightness and brightness and those of whiteness‐blackness and whiteness‐blackness strength is also clarified for achromatic object colors. The difference between the increase of brightness and that of whiteness‐blackness contrast (the effect studied by Stevens and Jameson—Hurvich) by raising their adapting illuminance is resolved without any contradiction. It is also shown that the nonlinear color‐appearance model developed by the author and his colleagues is able to explain the complex characteristics of all the above color attributes of object colors by making minor modifications to it. In addition, two kinds of classifications of various color attributes are given; one is based on the similarity of perception level, and the other on the degree of adapting illuminance dependency. © 2000 John Wiley & Sons, Inc. Col Res Appl, 25, 318–332, 2000     

Field trials on color appearance and brightness of chromatic object colors under different adapting-illuminance levels

Field trials of the nonlinear color-appearance model were done by using chromatic object colors under different illuminance levels. Color-appearance match and brightness match were made for Munsell color pairs by using haploscopic matching. Each color pair was only different in Munsell Chroma. The color-appearance and the brightness match were realized by adjusting the illuminance of one of the two haploscopic fields. Observed illuminances were significantly different between the color-appearance and the brightness matches for the same color pairs. The model accurately predicted the illuminances of color-appearance matches by using the metrics of lightness and chroma Q, T, P of the model, and those of brightness matches by using the metric brightness of the chromatic color B_c. In addition, the estimated contribution of colorfulness to brightness of chromatic colors was generally consistent with that predicted by the formula of Ware and Cowan. To test metric brightness B_c further, an additional experiment on haploscopic matching was done using illuminants with different R_a values. In the experiment, the same samples were used in both fields. Again, matching illuminances in this case were well predicted by using the same contribution factor of colorfulness to brightness already estimated.     

Effect of chromatic components on facial skin whiteness

Although the color measurement of facial skin becomes more common in dermatology and cosmetics, little is known about the relationship between subjective color perception and colorimetric values in facial skin. In this study, the possible relationships among perceived whiteness and the metric lightness, chroma and hue angle of Japanese females' facial skin color were investigated. First, the perceived brightness of the facial skin of Japanese females was evaluated visually and compared with metric lightness, chroma and hue angle, and the effect of hue and chroma on the perceived brightness was discussed. Second, a psychophysical experiment on the whiteness of the facial images and synthesized skin color plate images was conducted for examining the effect of hue and chroma on the perceived whiteness more precisely and independently. The results of two experiments showed that in regard to the facial skin color of the Japanese female, metric lightness disagrees with perceived whiteness or brightness in a narrow lightness range. The reddish facial skin color appeared brighter or whiter than that of a yellowish one in high lightness regions, and the low‐chroma facial skin color appeared brighter or whiter than a high‐chroma one. However, in the color plate images, a change in perceived whiteness by hue could not be confirmed, and the change in perceived whiteness by chroma was weaker than that from facial images. These results indicated that a higher‐level process of face recognition affected whiteness perception, and the criterion of facial skin whiteness was determined by facial skin color distribution. © 2011 Wiley Periodicals, Inc. Col Res Appl, 2011;     

Proposal of a new concept for color‐appearance modeling

A new type of color‐appearance model (CAM) is proposed together with its concept and flow of formulations. The topics described are: (1) The existence of two kinds of color‐appearance models, CAMs previously used and CAMs newly proposed. (2) All the CAMs, previously developed and used, do not predict color‐appearance attribute of perceived lightness of object colors under any illuminations. They may be adequately called “the model for predicting color‐appearance match between object colors under different adapting conditions.” (3) Newly improved CAMs take the Helmholtz–Kohlrausch effect in the VCC method into account. They can determine object colors with the same Tone (equi‐perceived lightness, equi‐whiteness‐blackness, and equi‐perceived chroma) irrespective of hues under reference illuminant. The newly improved models can be named Integrated CAMs. Their applicable fields are described in detail. © 2007 Wiley Periodicals, Inc. Col Res Appl, 32, 113–120, 2007     

Chroma variations and perceived quality of color images of natural scenes

Transformations of natural images in the perceptually uniform CIELUV color space have been investigated with respect to perceptual image quality. To this end, digitized color images of four natural scenes were described on the basis of their color point distributions in the CIELUV color space. A new set of images was created by varying the chroma value of each pixel while the lightness and hue angle were kept constant. The chroma was changed in two different ways: (1) through the addition or subtraction of the same amount of chroma to or from the chroma value of each pixel; (2) through multiplication of the chroma value of each pixel by a constant. In three experiments, subjects judged the perceptual quality, colorfulness, and naturalness of the images on a ten-point numerical category scale. The results indicate that colorfulness is the main perceptual attribute underlying image quality when chroma varies. Colorfulness itself was found to depend on both the average chroma and its variability. In general, the subjects preferred slightly more colorful images to the original ones. The perceptual quality of the images was found to be closely related to the naturalness of the images. © 1997 John Wiley & Sons, Inc. Col Res Appl, 22, 96–110, 1997     

Comparison of color-appearance models

The color-appearance models developed by R. W. G. Hunt (model H) and Y. Nayatoni and his collaborators (model N) are compared. The following color perceptions are analyzed: (1) colorfulness under illuminant C for NCS samples, (2) colorfulness change by changing illuminant C to illuminant A, (3) colorfulness change by changing adaptign illuminance, (4) Helson-Judd effect on achromatic colors under saturated chromatic illuminants, and (5) brightness and lightness. Special features of each model are made clear. In addition, a deatailed discussion is given on the mechanism introbucing the Helson-Judd effect and on the model formulations, especially in model H.     

Equivalent lightness of colored objects at illuminances from the scotopic to the photopic level

Equivalent lightness of four colored objects, blue, green, yellow, and red, was obtained for illuminance levels covering the range 0.01 to 1000 lx by brightness matching to a grey scale. Three field sizes, 20', 1° and 6° arc of visual angle, were investigated. With the smallest size, for which no rod response was expected, the equivalent lightness increased monotonically for incresing illuminance, showing the increasing effect of colorfulness upon brightness. The results obtained with the largest size showed influence from both rods and cones and were interpreted as due to two underlying lightnesses, one lightness based on the achromatic response of rods, cones, or both, and the other lightness based on the chromatic response of cones exhibited as colorfulness.     

Equivalent lightness of colored objects of equal munsell chroma and of equal munsell value at various illuminances

Equivalent lightness was determined for 26 colored surfaces by heterochromatic brightness matching with a grey scale. The illuminance for observation was varied from 0.01 to 1000 lx to cover scotopic, mesopic, and photopic vision, and the equivalent lightness-versus-log illuminance curve was obtained for every stimulus. The shape of the curves did not change if the surfaces had the same Munsell hue and chroma. It differed significantly if they had different hues or different chroma. The curves were interpreted in terms of achromatic lightness and chromatic lightness, which are both subject to change with illuminance level. The achromatic lightness was assumed to follow the Purkinje shift and the chromatic lightness monotonically increased with illuminance. The chromatic lightness was larger for larger Munsell chroma within a single hue.     

Visual clarity and feeling of contrast

The visual clarity of a lighting environment is significantly affected by changing the general color rendering index (Ra) of its illumination. This effect has been studied by a number of researchers, but the cause of this effect has not been thoroughly studied. In order to clarify this, the mutual relations between visual clarity, lightness perception, and feeling of contrast are analyzed by using object colors under illuminations with various Ra values. the results obtained are as follows. (1) the visual clarity of a lighting environment is different from the lightness perception of the object colors in the environment. (2) the change in visual clarity of a lighting environment is highly correlated to a feeling of contrast between object colors under the illumination. (3) the effect of visual clarity is estimated effectively by assessing the feeling of contrast using a specially selected four-color combination. (4) the illuminance for equal visual clarity for any illumination is predicted by the equality of feeling of contrast under the same illumination specified by the gamut area made by the component colors of the four-color combination in a brightness and colorfulness space. (5) the effect of visual clarity under various illuminations cannot be predicted by using their Ra values.     

Relationships among chromatic tone,perceived lightness,and degree of vividness

The present study describes the usefulness and importance of chromatic tone concept on object colors. It is clarified that the concept of a tone category consists of the same perceived lightness and the same degree of vividness of chromatic object colors in the tone irrespective of hue. Prediction equations are given to color attributes on perceived lightness and degree of vividness. They clearly show different functions on metric lightness and metric chroma on the two color attributes. It is also clarified that the theoretical opponent‐colors system by the author (NT system) gives a basis for defining the tone concept, perceived lightness, and degree of vividness. The results of the present study are useful for understanding fundamental color notion “tone,” which is important both in the fields of colorimetry (fundamental color‐perception study) and color design (practical application). In addition, attributes of equivalent whiteness–blackness [W‐Bk]eq and equivalent chroma Ceq are proposed. © 2005 Wiley Periodicals, Inc. Col Res Appl, 30, 221–234, 2005; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/col.20108     

Effects of luminance,wavelength and purity on the color attributes: Brief review with new data and perspectives

A color stimulus may be characterized by three psychophysical dimensions (luminance, dominant wavelength, and purity), whose corresponding color attributes are lightness, hue, and chroma/colorfulness. The 3 × 3 matrix gives nine basic effects of the psychophysical dimensions on the color attributes (e.g. the effect of luminance on hue), but there are 49 possible combinations as more complex effects (e.g. the effect of luminance on hue and chroma, i.e. on chromaticity). Researching and quantifying such effects enables modelling of the underlying neural mechanisms and of color appearance. Using a simple nomenclature to identify the effects (e.g. Ph denotes the effect of Purity on hue), this paper briefly reviews and interrelates 15 of the commonest effects, giving new data or new graphical perspectives to clarify or fill gaps in the literature. Contrast and no‐contrast effects (stimuli viewed simultaneously or singly, respectively) are differentiated. © 2007 Wiley Periodicals, Inc. Col Res Appl, 32, 208–222, 2007     

Which color is more colorful,the lighter one or the darker one?

To answer a question often asked in industrial color reproduction, a series of highly chromatic color samples of the same CIELAB hue but of small variations of CIELAB chroma and lightness were prepared and scaled for perceived colorfulness. The results indicate that lightness contributes to the perceived colorfulness as defined by the observers according to their everyday color experiences. For the samples used, colorfulness can be modeled by factoring in the CIELAB L* value in addition to CIELAB C*. The results show that colorfulness, as implied in our everyday color experiences, can be a complex perceptual attribute. A newer psychophysical scaling model is also presented, since Thurstone's Case V model was shown to be inadequate. © 2003 Wiley Periodicals, Inc. Col Res Appl, 28, 168–174, 2003; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/col.10142     

Appearance variations of textile materials due to different near gray backgrounds

The preliminary experiments were carried out for investigating the effect of lightness of achromatic background (white, grays, and black) on color appearance attributes (lightness and colorfulness) of some colored fabrics. A series of achromatic backgrounds at eight levels of lightness was provided. Each test color was viewed against these backgrounds and the perceived lightness and colorfulness were evaluated by using the pair comparison method. The results indicated that the lightness of achromatic background affects perceived lightness and colorfulness. The lightness of tests colors increases when surrounded by dark backgrounds while a consistent trend was not noticed for perceived colorfulness. The results from visual assessment were compared with the predicted results by the CIECAM97s model. The predictions of the model were in agreement with the perceived lightness by visual assessment but there was not any correlation between the results of model and visual assessment for colorfulness. In the second part of the experiments, a colorimetric match of samples on different backgrounds was carried out by visual judgments as well as implementation of color appearance models in reverse mode. Results from visual trials were significantly different from those predicted by the models. © 2006 Wiley Periodicals, Inc. Col Res Appl, 31, 133–141, 2006; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/col.20190     

Effect of text‐background lightness combination on visual comfort for reading on a tablet display under different surrounds

Many tablets are designed to change display brightness or color with surround for enhancing visual comfort. Although both color and brightness of a surround may vary a lot, few studies investigated how text‐background lightness combination of a tablet display and surround jointly affect visual comfort, and how display white point affects visual comfort. In this study, 20 observers evaluated visual comfort of 20 text‐background lightness combinations of a 9.7‐inch tablet display through paired comparisons under five surrounds—a dark surround and four ambient lighting conditions comprising two levels of correlated color temperature (CCT)—3500 and 6500 K—and illuminance—300 and 3000 lx. The combination of a black text and a light‐gray background (i.e., L^*_background = 75.33; L^*_text = 1.6) was evaluated the most comfortable when there was ambient light regardless of CCT and illuminance. It was also evaluated the third most comfortable under the dark surround. The observers also evaluated the visual comfort of a dark text on five different white backgrounds under 3500 and 6500 K at 1000 lx. The color of the background that was judged as the most comfortable neither had the whitest appearance nor matched the color of the ambient light. The simultaneous adjustment of the display white point and the text‐background lightness combination merits further investigations.     

Chroma,chromatic luminance,and luminous reflectance. Part I: Basic research and illustration of relations

Chromatic luminance carries both wavelength and radiance of light, is the source of all psychophysical dimensions and all color attributes, and is a key to understand their relations. It has long attracted research, hindered in the past by flawed definitions of colorimetric purity (p_c), remedied in a recent article. This article investigates relations between luminous reflectance Y (i.e., total luminance, chromatic plus achromatic), chromatic luminance (calculated from Y per p_c), and chroma/colorfulness. Relations are clarified, illustrated by formulas and graphs including three‐dimensional schemas of luminance Y, chromatic luminance, and color solids. A useful new term, relative chromatic luminance, is introduced. Munsell chroma is shown to resemble inverted log luminance much more closely than inverted log colorimetric purity as claimed by previous researchers. The relationship is used in Part II to model chroma, colorfulness, and brightness. © 2008 Wiley Periodicals, Inc. Col Res Appl, 34, 45–54, 2009.     

On exponents of a nonlinear model of chromatic adaptation

Exponents to be used in a nonlinear model for chromatic adaptation by Nayatani, Takahama, and Sobagaki are discussed and determined as functions of the effective adapting levels by considering several experimental facts of color perception related to a change of adapting illuminance. The analyzed data refer to the color-appearance attribute “colorfulness” studied by Hunt, the color appearance of a high-reflectance sample studied by Hunt, the brightness studied by Stevens, and the chromatic adaptation effect between daylight and tungsten light studied by Breneman. By using the derived functions of the exponents it is shown that the model predicts quite well the following perceptual effects: (1) the chromatic adaptation effect between standard Illuminants D₆₅ and A, (2) the dependence of effect (1) on the adapting luminance, (3) the increase of colorfulness of colored samples (the first Hunt effect), (4) the increase of blueness of high-reflectance samples (the second Hunt effect), (5) the increase of bright and dark contrast of nonselective samples (the Stevens effect), and (6) the color perception of nonselective samples under a colored adapting illumination (the Helson effect). Effects (3)–(5) are observed by increasing the adapting luminance.     

Prediction of color appearance under various adapting conditions

A model of color vision is given to predict color appearance of object colors under various illuminants and illuminance levels. The model constitutes a uniform color-appearance space under any adapting condition, and can predict the metric quantities relating to the following color perceptions: (1) constant-hue loci, (2) the Munsell color scheme, (3) the increase of brightness contrast of nonselective samples with increasing adapting-illuminance level, (4) the Helson-Judd effect, and (5) the increase of colorfulness of chromatic samples with increasing adapting-illuminance level.     

Adequateness of a newly modified opponent‐colors theory

Two features of a newly modified opponent‐colors theory are examined for correctness: (1) The perceived chroma of pure color is different for different hues. This was confirmed by using Ikeda's UCS (Uniform Color Scales) formula and also by the maximum Munsell Chroma Values for different hues. (2) Chromatic colors with the same values of whiteness, blackness, grayness, and perceived chroma have the same perceived lightness and chromatic tone regardless of hue. This was confirmed by a theoretical analysis and observations of the color samples in the Practical Color Co‐ordinate System (PCCS) developed in Japan. Chromatic tone, a complex concept of object colors, is clarified. The structure of the newly modified theory and its corresponding color space were confirmed by observation of object colors. Furthermore, it was found effective for developing a color‐order system and its corresponding standard color charts to the modified theory. © 2003 Wiley Periodicals, Inc. Col Res Appl, 28, 298–307, 2003; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/col.10164     

A comment about the chroma scale of Nayatani‐theoretical color order system

Nayatani‐theoretical (NT) color order system is a Hue–Tone color order systems. It can be used for determining surface colors with the same Tone (equiperceived lightness and equiperceived chroma) irrespective of hues under reference illuminant. The fundamental structure of NT system uses Munsell attributes HV/C for the easiness of its use. Some deviations caused by the approximation on chroma scale are naturally expected in NT system, although the formula used for deriving equivalent lightness V_eq has the same structure as that for deriving L*_eq using the corresponding CIELUV formula. The present article discusses the degree of approximation, and confirms the goodness of approximation. NT system can be used as it is for the field of artistic color design (complete accuracy is not required). The corrections, however, should be introduced in NT system, when higher accuracy is required. © 2007 Wiley Periodicals, Inc. Col Res Appl, 32, 230–233, 2007     

Expanding display color gamut beyond the spectrum locus

Digital video and display media are at a “sweet spot” of growth with brighter and more colorful digital projectors and displays available seemingly every day. Much more is possible in achieving brighter and more vibrant colors, colors that may even transcend our typical experience in terms of dynamic range and an expanded gamut in the perceptual sense. If the full capabilities of these technologies to produce a fuller visual experience are to be realized, new processing and encoding methodologies are required. In this article, the powers of adaptation and the CIECAM02 color appearance model are exploited to define perceptual gamut. The strategy of this methodology is, simply and in effect, to “push down” the white point of the display and demonstrate, both empirically and with a limited set of images, a striking gamut expansion in the perceptions of lightness, chroma, brightness, and colorfulness beyond the locus of pure, spectral color, and the MacAdam limits as observed with traditional display configurations. © 2006 Wiley Periodicals, Inc. Col Res Appl, 31, 475–482, 2006; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/col.20260