Color rendering: A tale of two metrics

It was the best measure of color rendering, it was the worst measure of color rendering. Color rendering index (CRI) is the most common metric used by the lighting industry to represent the color rendering properties of electric light sources. CRI was intended to characterize how “true” or “natural” objects appeared when illuminated by a light source, but was never intended to, for example, represent how well object colors could be differentiated under a light source, another important aspect of color rendering. Data presented here demonstrate that CRI in conjunction with another measure of color rendering, gamut area index (GAI), is useful at predicting subjective judgments of how “natural” objects appear as well as how “vivid” objects appear, and how well one can discriminate between subtle differences in hue. Neither measure by itself, however, is sufficient for meeting all of the expectations of a light source for providing good color rendering under all viewing conditions. It remains for future research to determine if just two metrics are sufficient to assure good color rendering from a light source and whether these two metrics (CRI and GAI) are the best for such purpose. In the meantime, CRI and GAI should be used jointly in recommendations as practical, useful, and mutually reinforcing measures of color rendering. The data presented here also demonstrate that total irradiance is important for good color rendering. © 2008 Wiley Periodicals, Inc. Col Res Appl, 33, 192–202, 2008     

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     

Desaturation of color by environment light in cataract eyes

The visual perception of the cataract eyes is composed of three elements, color, brightness, and haze. The color element is related to the reduced transmittance of light at short wavelengths of crystalline lens, the brightness element to the reduced transmittance at entire region of wavelength, and the haze element to the opacity of the crystalline lens. Researchers pay attention to only the color and brightness elements when they discuss on the color perception by the cataract eyes. We will show in this article that the haze element is also related to the color perception. There exists always environment light in our living situation. Light comes to the eyes from every direction. When the light enters the cataract eye it is scattered by the opacity of the crystalline lens and overlaps on the retinal image of objects that the eye is looking at. The environment light is usually white in color, and it reduces the colorimetric purity of the objects. The color appearance of the objects desaturates. This purity change was confirmed colorimetrically by measuring colors of test patches with the cataract simulating goggles placed in front of the lens of a colorimeter. The desaturation was then confirmed psychophysically by subjects wearing the goggles, who matched two color patches with and without the goggles. The same experiment was done by a subject whose right eye was operated for the cataract and the left eye was before the operation. Desaturation of color was found, though the desaturation was not as strong as with the goggles. In another psychophysical experiment the subject did the heterochromatic brightness matching experiment by using his right eye and left eye, respectively, before and after operation. The results were interpreted that all the color patches presented were perceived as being less saturated by eyes having cataracts. The importance was pointed out that, when we do experiments on the color perception by elderly observers we should not neglect the effect of the environment light. © 2008 Wiley Periodicals, Inc. Col Res Appl, 33, 142–147, 2008     

Goniocolorimetry: From measurement to representation in the CIELAB color space

The classical definitions of color are well adapted to diffusing objects, whose color is almost independent of the viewing angle, and to very glossy object observed in the specular direction in respect to the light source. For glossy or iridescent objects, the color is difficult to characterize due to its dependence on the viewing direction. In order to cope with such objects and to represent their angle‐dependent colors in a colorimetric space, we adapt the CIELAB space to “goniocolorimetric” measurements. A crucial point when defining this space is the statement of the viewing solid angle. First, we suggest performing a BRDF measurement at high angular resolution in order to characterize the gloss of the specimen. Then, since for the definition of colors the CIE recommends cones of half‐angle of 2° or 10°, we propose to convert the measured BRDF into a reflectance factor defined in respect to these solid angles. This procedure is eased by a planar multispectral image of the BRDF, where solid angles are specified by the pixel size. At last, the reflectance factors are converted into CIELAB coordinates. By using this procedure, the perfect white diffuser but also the perfect mirror can be represented in this colorimetric space. © 2010 Wiley Periodicals, Inc. Col Res Appl, 36, 169–178, 2011;     

Experimental determination of laws of color harmony. Part 1: Harmony content of different scales with similar hue

In 1956 we came to the decision at the Budapest Technical University to start large scale experiments on color harmony. The experiments and the processing of the experimental results have been completed in 2006, after 50 years of research work. Within the frame of the experiments 95 000 participants have carried out more than 36 million elementary observations and made elementary decisions. Only certain parts of the experimental results have been published up to now. This article starts publishing the results not published yet. Research work on color harmony carried out during these 50 years can be categorized into seven main groups. The present article deals with the group of experiments testing how much the harmony content of the scales found in different locations in various positions of the axial sections of the Coloroid color system differ from each other. Our experiments were focused to three groups: we examined the variations in the extent of harmony content in the following cases (1) scales carried by lines with different angles to the gray axis, consisting of colors having the same number of harmony intervals between them, (2) scales consisting of colors being parallel to the gray axis, featuring various saturations, having different harmony intervals between them, and (3) scales perpendicular to the gray axis, with different luminosity, having different harmony intervals between each other. The examined color scales contained six colors in each experiment. Experiments were carried out for 24 different axial sections of the Coloroid color system. After 15‐years interruption experiments were repeated; however, with compositions of different appearances. © 2007 Wiley Periodicals, Inc. Col Res Appl, 32, 477–488, 2007     

Preparation,Particle Characterizations and Application of Nano-Pigment Suspension

In this work, we focus on preparation of nano-suspension of organic pigment for different milling method to discuss optimum formula, milling effect, particle characterizations (particle size, sedimentation velocity and stability) by analytical centrifugation system. Due to the variation of centrifugal force, time scales and space variations occur in the suspension such that during the migration of particles, the light transmission, and the vanishing coordinates are plotted on a profile. Therefore, objective of the article was evaluated to predict the particle characterizations between the particle distribution state and long-term stability as accelerated test during centrifugation. Besides, we were also used to observe nano-suspension of organic pigment the mean size, aggregation phenomena and dispersibility of the pigment particles by SEM and TEM. Additionally, we applied to manufacture the LCD-based blue filter and discussed performance of the blue filer by transmission of UV light, color evolution and adhesion ability assessments in this study.     

Color meaning and context: Comparisons of semantic ratings of colors on samples and objects

This article reports the results of a study comparing semantic ratings of color samples (chips) with those of the same colors applied to a variety of familiar objects. Subjects rated a set of 13 color chips and 5 sets of objects, each appearing in the same 13 colors, against 5 bipolar, 7-step semantic differential scales. The scales consisted of beautiful-ugly, elegant-vulgar, loud-discreet, masculine-feminine, and warm-cold. Analyses performed on the data indicated that generally few significant differences existed between chip and object ratings for the same color; when such differences existed, the chip was always rated more beautiful, elegant, discreet, feminine, and warm than the object; and differences between chip and object ratings were confined primarily to a limited number of colors, objects, and semantic scales. The results of this study have implications for the use of color chips in color planning and for the generality of results of earlier color meaning research. © 1997 John Wiley & Sons, Inc. Col Res Appl, 22, 40–50, 1997.     

An integrated spectral imaging system for producing color images of static and moving objects

An integrated spectral imaging system constructed by synchronizing a programmable light source, a high‐speed monochrome camera, and a display device is proposed to produce tristimulus images of static and moving objects effectively in real time onto the display. This system is called the CIE‐XYZ display. Active spectral illuminants, containing both the device characteristics of camera and display, are projected onto object surfaces as time sequence. The images are captured synchronously by the camera and quickly transmitted to the display device in the RGB signal form so that the accurate tristimulus images are displayed. First, we describe the principle of the CIE‐XYZ tristimulus display. The theoretically optimal illuminants contain negative parts in the spectral curves. Second, we design practical illuminants with all positive spectral curves. The color images in our system are composed of the time sequence of RGB component images. Then, the synthesized color images on the display contain color artifacts when objects move fast. An image processing algorithm for correcting the motion color artifact is proposed based on optical flow estimation using a graphics processing unit. The comprehensive performance of the proposed system and algorithms is examined using both static and moving objects. © 2014 Wiley Periodicals, Inc. Col Res Appl, 40, 329–340, 2015     

Physical exemplification of color order systems

Color order systems must be distinguished from colorant order systems and other systematic ways of relating colored objects. Abstract systems require definition of dimensions and scales. A concrete system is a collection of specimens exhibiting properties specified by the dimensions and scales of an abstract system. The accurate production of such specimens requires a complete written definition of the system, including the specification of all pertinent observing and measuring conditions. The choice of materials requires consideration of the spectral and geometric aspects of the optical nature, some aspects of the mechanical nature, stability, durability, availability, acceptability to users, and cost. The value of color standards depends on the precision and accuracy with which they are produced. Efficient production and long-term reproducibility depend on well-standardized methods of color measurement.     

A brief history of disk color mixture

Disk color mixture, known since antiquity, represents a special kind of additive color mixture where the stimuli do not represent direct lights from light sources but light reflected from objects. Its complete elucidation took centuries, requiring means to measure reflectance properties and spectral powers of lights. Explanations of the cause of the mixture, the fact that perceptions are not instantaneous but take a certain amount of time were offered in the late 17th and early 18th centuries. The speed of disk rotation required to obtain additive fusion of reflected lights was determined in 1765. For a time in the second half of the 19th century disk mixture was the only means of establishing color matching equations and quantitatively defining color stimuli. The fact that its results neither agree with Newtonian predictions of spectral light mixture nor with the results of pigment mixture produced endless confusion in scientific and artistic circles in the Enlightenment. Today, there is no practical application of disk color mixture left and it has become a curiosity. © 2010 Wiley Periodicals, Inc. Col Res Appl, 2010     

Assessing the affective feelings of two‐ and three‐dimensional objects

The aim of this study is to investigate the impact of physical appearance attributes (in terms of color and shape) on our affective feelings of 2D and 3D objects. Twelve colors were studied, each consisting of 12 two‐dimensional and 12 three‐dimensional shapes. This resulted in 144 2D and 144 3D color‐shape combinations. Each color‐shape combination was assessed using 20 emotion scales in a viewing cabinet by a panel of observers with normal color vision. The results show that there are five underlying factors of these 20 scales, i.e., “activity,” “weight,” “heat,” “softness,” and “complexity”. The first three factors were mainly related to color and the other two were linked with shape. © 2008 Wiley Periodicals, Inc. Col Res Appl, 34, 75–83, 2009.     

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.     

Visual assessment of light source color quality

The color quality of the lit visual environment can be improved by optimizing light source spectral power distributions. For a comprehensive optimization, it is important to identify the relationship among the perceptual properties of color quality. In this work, a colorful still life or tabletop arrangement was constructed from real artificial objects. Thirty observers with normal color vision scaled nine different properties of color quality under three light sources, incandescent, fluorescent and white LED. Six factors were extracted from the correlations among the obtained visual color quality scales. Factors were assigned the following labels: memory, preference, brightness, fidelity, gamut and difference. Mean factor values were computed for each light source. Significant differences were found in case of preference, brightness and fidelity: for preference, INC was rated better than CFL and LED was rated better than CFL, for brightness, LED was better than INC and LED was better than CFL and for fidelity, INC was better than CFL and INC was better than LED. The brightness factor was consistent among the observers. Three clusters of observers were found for preference and fidelity. The memory, gamut and difference factors showed large interobserver variability. © 2011 Wiley Periodicals, Inc. Col Res Appl, 2013     

Texture affects color emotion

Several studies have recorded color emotions in subjects viewing uniform color (UC) samples. We conduct an experiment to measure and model how these color emotions change when texture is added to the color samples. Using a computer monitor, our subjects arrange samples along four scales: warm–cool, masculine–feminine, hard–soft, and heavy–light. Three sample types of increasing visual complexity are used: UC, grayscale textures, and color textures (CTs). To assess the intraobserver variability, the experiment is repeated after 1 week. Our results show that texture fully determines the responses on the Hard‐Soft scale, and plays a role of decreasing weight for the masculine–feminine, heavy–light, and warm–cool scales. Using some 25,000 observer responses, we derive color emotion functions that predict the group‐averaged scale responses from the samples' color and texture parameters. For UC samples, the accuracy of our functions is significantly higher (average R² = 0.88) than that of previously reported functions applied to our data. The functions derived for CT samples have an accuracy of R² = 0.80. We conclude that when textured samples are used in color emotion studies, the psychological responses may be strongly affected by texture. © 2010 Wiley Periodicals, Inc. Col Res Appl, 2010     

Dynamic mechanical spectrometry analysis of modifications in the cure kinetics of polyepoxy composites with particulate fillers

We have performed an analysis of the modifications of a polyepoxy cure kinetics due to the introduction of organic and inorganic particulate fillers. In a previous work¹ we have studied by differential scanning calorimetry the rates of reaction of a polyepoxy system reinforced with particulate fillers. This work revealed very important differences between the kinetics of the polyepoxy and those of the composites (polyepoxy reinforced). As a matter of fact, the results obtained with the composites do not fit with any literature models. For this reason, we have decided to focus our work on the various possible interactions between the polyepoxy system and the fillers. In order to bring to light elements of understanding to the origins of cure kinetics modifications, several factors have been examined by turns. On one hand, the influence of residual curing stresses, water content, variations in thermal conductivity, and viscosity has been observed. This let us show that any modification of these parameters can induce important cure kinetic modifications, but none of them is at the origin of the observed modifications. On the other hand, several TTT diagrams obtained for various fillers contents by mechanical spectrometry have been constructed. The analysis of those diagrams revealed that the presence of particulate fillers is of a great influence on the main phase transitions that occur during the crosslinking reaction. The set of results shows that some physical bonds develop between the polyepoxy network and the organic particles. None but those latter can occasion the consequent modifications of cure kinetics that have been observed.     

Perception of gold materials by projecting a solid color on black materials

Gold gives a unique luxurious impression and therefore golden objects are often desired. Since gold is an expensive metal, due to its rarity, it is not feasible to produce objects made of gold at reasonable prices. In this study, we have developed a simple representation method that makes real objects appear to be made of gold using projection mapping technology. In recent years, this technology has been extensively explored, mainly in the field of entertainment. Most of these studies were focused on technologies where a target image was superimposed onto an object. Our method, unlike the conventional approach, does not project a target gold image but simply projects a solid color onto an object, thereby giving the perception that the object is made of gold. We have conducted two psychophysical experiments to substantiate our representation method, using stimuli having the same color. Our results indicate that objects were more easily perceived as golden objects when a solid color was projected onto black objects rather than white objects. Furthermore, a few samples were perceived as metals, even though they were actually paper or fabric. These results suggest that when superimposing a solid color onto an object, a change in material type is perceived along with the change of its color. Moreover, we analyzed the relationship between psychophysical evaluations on the perception of projected gold objects and the physical properties of the materials. The results show that the average and the standard deviation of reflectance generated by black objects can be considered factors that affect the perception of a gold‐like color. © 2016 Wiley Periodicals, Inc. Col Res Appl, 42, 522–530, 2017     

Study of balance of images using visual weight

The purpose of this research is to analyze the factors that create a sensation of balance or unbalance when a color image is observed depending on the light transmitted, reflected, and/or emitted by the objects that appear in it. This study is carried out by calculating the center of gravity (CG) of each image. The weight and the CG are obtained by calculating the force resulting from the different visual weights that make up the visual image and its position on this. When the visual CG and the geometric center coincide, it is recognized that the image is balanced. The action that acts on the balance of the lightness of an image is labeled as the visual weight. The visual weight of a figure is defined from the amount of visible light coming from it and the space occupied by this in the visual image. The colors show us the amount of light coming from a figure, while the shape provides us with information about the space that it occupies. The study explores how the qualities of appearance of three‐dimensional objects affect visual weight and how it can vary due to changes in light, motion of objects or the perspective of the image. Since these calculations can be very complex, we have developed the software called “PesoVisual” that automatically performs these operations on digital color images. Similarly, this software enables the analysis of the variability of the visual image. © 2015 Wiley Periodicals, Inc. Col Res Appl, 41, 175–187, 2016     

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     

Further improvements to CIECAM97s

The modifications to the CIECAM97s color appearance model suggested by Fairchild⁵ are reviewed in light of some recent experimental results, and some suggestions are made for refining and extending these modifications. © 2002 Wiley Periodicals, Inc. Col Res Appl, 27, 164–170, 2002; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/col.10047     

Judd's contributions to color metrics and evaluation of color differences

The contributions of Dr. Deane Brewster Judd to colorimetry extended over five decades. Among the most crucial contributions to the development of modern colorimetry were his studies of colorimetric purity, relations among color-mixture and luminosity data, visual sensibility to color differences, uniform chromaticity diagrams, Munsell notation and renotation, color names, color blindness and anomalies, color-vision theory, and uniform color scales.