Practical color barrier‐free illumination for deuteranopia using LEDs

Here, we propose a color barrier‐free illumination consisting of white, red, and blue LEDs for people with deuteranopia‐type defects in color perception. Color perceptions of 20 volunteers with normal vision and four examinees of deuteranopia were evaluated by both the Ishihara test for color blindness and the Farnsworth Panel D‐15 test under color barrier‐free illumination. The illumination was comparably effective, not only for discriminating between red and green but also for discrimination of the hues on a color chip continuously. © 2014 Wiley Periodicals, Inc. Col Res Appl, 40, 218–223, 2015     

Comparison of color vision models based on spectral color representation

Recent break‐throughs in retinal imaging have raised new questions for color vision research, and the existing color vision models should be re‐evaluated. Many color vision models are based on an assumption that there are no differences in the detection phase, neither in the spatial configuration nor in the spectral sensitivities of cells. In this article, we have run experiments with four different color vision models. This evaluation gives us more knowledge about the essential properties of the models. We show how the tested color vision models are able to replicate the behaviour of human color vision by evaluating their performance in Farnsworth‐Munsell 100‐Hue color vision test. Also, the wavelength discrimination power of each model is presented and the properties of color spaces spanned by models are examined using samples from Munsell Book of Color. Our experiments show that there are large differences in the properties of different models. © 2009 Wiley Periodicals, Inc. Col Res Appl, 34, 341–350, 2009     

Test/retest and inter‐test agreement of color aptitude measures

The color discrimination capacity of color vision normal observers can be assessed using color aptitude tests (CATs), although data on the test/retest reliability and inter‐test agreement of such tests using appropriate statistical measures is limited. In this study, 32 color vision normal observers twice performed each of five CATs, being the Farnsworth‐Munsell 100 hue test, HVC Color Vision Skills Test, inter‐Society Color Council Color Aptitude Test, the Nagel Anomaloscope (Nagel), and a custom designed two‐color discrimination test. Two methods of determining the Nagel matching range were used based on the initially accepted matching range or on repeated measures. The test/retest performance of each test was determined using the method outlined by Bland and Altman[Lancet 1986;1:307–310] and the inter‐test agreement was analyzed using the same method, after converting test scores into z score units. No test was found to have a significant systematic alteration in test performance between test and retest. Test/retest performance was generally poor, indicating that tests can only reliably classify observers into very broad performance bands. All tests failed to show inter‐test agreement, and our data indicate a primary reason for this: as test/retest performance is generally poor, inter‐test agreement must also be poor as no test can agree more with another test than it agrees with itself. © 2014 Wiley Periodicals, Inc. Col Res Appl, 40, 224–231, 2015     

Light wavelengths of LEDs to improve the color discrimination in Ishihara test and Farnsworth Panel D‐15 test for deutans

This study was performed to determine significant light wavelengths to improve color discrimination ability of subjects with deutan. We conducted both the Ishihara test and the Farnsworth Panel D‐15 test for subjects with deutan and normal color vision. Seven different LED lights from 450 to 660 nm and an additional D65 white lamp were utilized to change the lighting conditions, including the wavelength and intensity. The results of the Ishihara test and D‐15 test showed that color identification of deutans was markedly improved with the longer wavelength LEDs regardless of the intensity of the additional D65 lamp. Notably, the error rates of deutans in the Ishihara test were <25% for LED wavelengths of 630 and 660 nm. In the case of subjects with normal color vision, the D65 lamp abolished the errors in the Ishihara test, regardless of the LED wavelength. Addition of the D65 lamp also decreased the number of crossings in the D‐15 test. These results suggested that illumination by LED light with longer wavelengths, such as 630 and 660 nm, may provide deutans with greater red‐green discrimination ability in both the Ishihara test and the Farnsworth Panel D‐15 test. © 2016 Wiley Periodicals, Inc. Col Res Appl, 42, 424–430, 2017     

Conversion between the reflectance spectra and the Munsell notations

A novel general transformation between reflectance spectra and the corresponding coordinates of the Munsell Color System is presented. The coefficient values of the transformation were experimentally determined by mapping the actual reflectance spectra of the chips in the Munsell Book of Color into the Munsell Color Order System and by minimizing the distance between calculated and actual coordinates. The experiment was repeated with a selected set of points of the Munsell Renotation System. Both the Smith–Pokorny functions and the CIE 1931 standard color‐matching functions were used as a basis of the transformation. There is a good correspondence between calculated and actual coordinates of the Munsell Color System. It is also shown that the linear part of the same transformation applied to the basis functions results in one achromatic response function and two chromatic response functions in accordance with the opponent‐colors theory. © 2005 Wiley Periodicals, Inc. Col Res Appl, 31, 57–66, 2006; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/col.20173     

A novel Color Rendition Chart for digital tongue image calibration

Digital tongue images are usually acquired by a camera under specific illumination environments. In order to guarantee better color representation of the tongue body, we propose a novel tongue Color Rendition Chart acting as a color reference to be used in color calibration algorithms to standardize the captured tongue images. First, based on a large tongue image database captured with our digital tongue image acquisition system, we establish a statistical tongue color gamut. Then, from the first step, different quantities of colors in the Color Rendition Chart are determined via experimentation. Afterwards, results using X‐Rite's ColorChecker® Color Rendition Chart (a standard in the color calibration field) are compared with the proposed tongue Color Rendition Chart by applying the color difference calculation formula of CIELAB and CIEDE2000 as a reference for the mean color calibration error. The results show that the proposed tongue Color Rendition Chart, which has 24 colors, produces a much smaller error (CIELAB —8.0755/CIEDE 2000—6.3482) compared with X‐Rite's ColorChecker® Color Rendition Chart (CIELAB 1976—14.7836/CIEDE 2000—11.7686). This demonstrates the effectiveness of the novel tongue Color Rendition Chart.     

Dependence of the color appearance of some flowers on illumination

Seven flower colors perceived by five color experts using visual color measurement under 2800 K warm white fluorescent lamps, 3500 K plant growth lamps, and 6500 K light‐emitting diodes (LEDs) were compared with those under 6500 K fluorescent lamps, which represented illuminants in florist shops. Fluorescent lamps (6500 K, 1000 lx) were found to be effective for displaying flower colors and were used as the standard condition. The colors of flowers generally shifted in the same direction as those of the illuminants in CIELAB space. The color differences were highest under the 3500 K fluorescent lamp at both 500 and 2000 lx. At 500 lx, the ΔE values under the 6500 K LED were higher than those under the 2800 K lamp. The C* and ΔE values revealed that the 2800 K lamp was unsatisfactory for purple‐blue and purple flowers and was more suitable for floral displays at lower illuminance. Under the 3500 K lamp, the highest color distortion occurred in cool‐colored flowers, but C* increased for purple‐blue and purple flowers. The 6500 K LED tended to decrease C* for warm‐colored flowers under both illuminances, but it was effective for displaying purple‐blue and purple flowers with increased C*. © 2012 Wiley Periodicals, Inc. Col Res Appl, 39, 28–36, 2014     

Color vision assessment‐2: Color assessment outcomes using single and multi‐test protocols

The main purpose of this study was to produce reliable, color assessment outcomes to examine the extent to which single and multi‐test protocols in use meet current clinical and occupational needs. The latter include the detection of small changes in chromatic sensitivity as the earliest signs of retinal and/or systemic disease, and the need to assess the class of color vision in congenital deficiency and to quantify severity of loss. Color vision was assessed using Ishihara (IH), Farnsworth Munsell D‐15, City University (CU, 2nd ed.) and Holmes‐Wright type A (HW‐A) lantern tests. All subjects also carried out Colour Assessment and Diagnosis and Nagel anomaloscope tests. The sample included 350 normal trichromats, 1012 deutans and 465 protans (age 31.1 ± 12.4, range 10‐65 years). The results reveal the trade‐off between sensitivity and specificity, depending on the number of errors accepted as a pass on the IH test. The D‐15 and CU tests pass all normals and almost 50% of subjects with color vision deficiency. The HW‐A lantern passes all normals, 22% of deutans and 1% of protans. The multi‐test protocols designed to identify protans and to pass only subjects with mild color loss, pass over 50% of protans and deutans. Many of the subjects who fail exhibit less severe loss of color vision than others who pass. When high sensitivity for detection of congenital deficiency is achieved, single‐test protocols fail many normal trichromats. Multi‐test protocols produce large variability and fail to achieve desired aims.     

Describing natural colors with Munsell and NCS color systems

The Munsell Color System and the Natural Color System are widely used but they have some limitations due to the manufacturing process and sampling choices. To estimate quantitatively these limitations we compared the colors of natural scenes with the colors represented by these systems under a wide range of illuminants. Spectral data from the two systems and from natural scenes were used in the analysis. It was found that a considerable portion of natural colors are not accounted by these systems, mainly colors with low lightness levels. Under D65 the Munsell Color System color volume corresponds to 72% of the Natural Color System color volume which in turn represents only 53% of the natural scenes color volume. If individual colors are considered, less than half are contained within these systems. To obtain a complete match to the natural colors contained by the color systems thresholds of 7 and 5 CIELAB units would be required for Munsell Color System and Natural Color System, respectively. Variations with the illuminant are generally modest showing that both system work similarly across different illuminations. Although these Color Systems have limitations in describing low lightness colors they perform quite well for medium to high levels of lightness.     

Wpt (waypoint) shift manifold difference metrics for evaluation of varying observing-condition (observer + illuminant) metamerism and color inconstancy

Metamerism is a critical color phenomenon which can cause serious problems for products assembled by various parts. Customers generally expect all parts are color-matched under different observing conditions. This article extends the concepts of illuminant and observer metamerism to observing-condition metamerism, that is, objects are color-matched under one observing condition but not under others. The color inconstancy of a single object is also expanded to be evaluated under multiple observing conditions. Moreover, four Waypoint (Wpt) Shift Manifold difference metrics are proposed to evaluate not only observing-condition metamerism of metamers and paramers but also observing-condition color inconstancy of single objects: The Mean Object Inconstancy Index (MOII), The Mean Object Color Difference (MMOCD), Object Metamer Index (OMI), and Object Hue Similarity Index (OHSI). Existing indices of metamerism and color inconstancy employ appearance matching using a Chromatic Adaptation Transform (CAT) and color difference formulas such as CIEDE76 or CIEDE2000. The proposed metrics utilize material matching based upon the Waypoint Material Adjustment Transform (Wpt-MAT) and Euclidian color difference in the perceptually uniform Material Color Equivalency Space WLab. Conceptual comparisons between these approaches are discussed and evaluated. Additionally, computational evaluation results under observing conditions composed by 99 illuminants and 70 observers show that MOII provides a measure of color inconstancy for single objects, MMOCD provides a measure of metamerism between metamers and paramers with a generalized assessment of color difference between two objects, OMI provides a measure of paramers, and OHSI provides a quantitative measure of hue characteristics for different observing conditions.     

Comparison of color gamuts among several types of paper with the same printing technology

In this work, we have studied the relationship among the colorimetric properties of different types of paper, having different finishing and grammage. Their color reproduction capability has also been analyzed by using the same printing technology (inkjet printing). On the one hand, we have plotted CIELAB data under the illuminant D50 into constant lightness and hue‐angle planes to be compared with MacAdam limits and with Pointer's real‐world surface color. On the other hand, we have calculated the volume gamut of the color solid associated to each color paper gamut. Analyzing the results, we have checked that there is not any clear relationship among the colorimetric properties of paper (for instance, CIE whiteness index, etc.) and the color gamut volume associated. However, the colorimetric parameters associated to the printed sample showed a quite good linear correlation between the minimum lightness (or the maximum blackness value) of the printed color chart and the color gamut volume. In particular, the greatest color gamut volume corresponds to the glossy papers taking into account this correlation for inkjet printing. © 2009 Wiley Periodicals, Inc. Col Res Appl, 34, 330–336, 2009     

Categorical color constancy under RGB‐LED light sources

The light‐emitting diode (LED)‐based light sources have been widely applied across numerous industries and in everyday practical uses. Recently, the LED‐based light source consisting of red, green and blue LEDs with narrow spectral bands (RGB‐LED) has been a more preferred illumination source than the common white phosphor LED and other traditional broadband light sources because the RGB‐LED can create many types of illumination color. The color rendering index of the RGB‐LED, however, is considerably lower compared to the traditional broadband light sources and the multi‐band LED light source (MB‐LED), which is composed of several LEDs and can accurately simulate daylight illuminants. Considering 3 relatively narrow spectral bands of the RGB‐LED light source, the color constancy, which is referred to as the ability of the human visual system to attenuate influences of illumination color change and hold the perception of a surface color constant, may be worse under the RGB‐LED light source than under the traditional broadband light sources or under the MB‐LED. In this study, we investigated categorical color constancy using a color naming method with real Munsell color chips under illumination changes from neutral to red, green, blue, and yellow illuminations. The neutral and 4 chromatic illuminants were produced by the RGB‐LED light source. A modified use of the color constancy index, which describes a centroid shift of each color category, was introduced to evaluate the color constancy performance. The results revealed that categorical color constancy under the 4 chromatic illuminants held relatively well, except for the red, brown, orange, and yellow color categories under the blue illumination and the orange color category under the yellow illumination. Furthermore, the categorical color constancy under red and green illuminations was better than the categorical color constancy under blue and yellow illuminations. The results indicate that a color constancy mechanism in the visual system functions in color categories when the illuminant emits an insufficient spectrum to render the colors of reflecting surfaces accurately. However, it is not recommended to use the RGB‐LED light source to produce blue and yellow illuminations because of the poor color constancy.     

New method for specifying color‐rendering properties of light sources based on feeling of contrast

“Visual clarity” or “feeling of contrast” of object colors under illumination is affected significantly by changing color‐rendering property of light source used. Though the feeling of contrast is considered one of the most important characteristics on color‐rendering properties of light sources, it cannot be estimated adequately by using the present R_a method. The new index FCI is proposed for estimating the effect of feeling of contrast quantitatively under any light sources. The FCI is derived using a simple transformation of the gamut area, which is constituted by a specially selected four‐color combination in CIE LAB color space. The FCI correlates well with the illuminance ratio for equal feeling of contrast (or equal visual clarity) on various light sources reported so far. Using the FCI together with the present CIE R_a, the color‐rendering capability of a light source can be well clarified. © 2007 Wiley Periodicals, Inc. Col Res Appl, 32, 361–371, 2007     

Whiteness perception under different types of fluorescent lamps

The effects of correlated color temperature and the chromaticity of light sources on the perception of surface whiteness were investigated. For the experiment, a Munsell N9.25 chip and 11 nearly white chips (V = 9.25, Munsell chroma ≦ 1.0) were selected. The interval scale of the whiteness of these chips was determined from the results of pair comparisons under eight different fluorescent lamps with correlated color temperatures from 2800 to 6700 K. The Munsell 3PB, 10PB, 7P, and N chips gained high scores under 6700 K illumination, whereas the 3PB, 5B, 7BG, and 9G chips scored higher under the 2800 K illumination. The 12 chips were divided into two groups. In one group, the interval scale from the bottom was found to increase as the correlated color temperature increased, whereas in the other group, it decreased with the temperature. The Munsell 3PB/9.25/1.0 chips fell into the latter group but consistently exhibited the highest or at least high‐order scores for all the illuminations examined. In those cases in which the correlated color temperature was held constant, the chromaticity of the light source was found to have no significant effect on the whiteness interval scale. A high correlation was identified between the interval scale of the whiteness and the two metrics, the metric chroma of CIELAB, and CIECAM97s chroma C. © 2003 Wiley Periodicals, Inc. Col Res Appl, 28, 96–102, 2003; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/col.10129     

Location of Munsell colors in the RAL Design System

Color order systems are used in various areas concerned with color such as education, color design, and industry. Their nomenclature provides a reliable tool for specification and communication of color appearance. Two of the most widely used color order systems are the Munsell Color System and the RAL Design System. When using both color systems, it would be useful to convert the nomenclature from one to the other. The goal of this study is to define solely the location of Munsell colors in the RAL Design System and to obtain helpful data in case of use both color systems. In accordance with this purpose, the RAL equivalents of certain Munsell colors are determined and the possibility of naming RAL colors by unambiguous adjectives is researched. This study is limited with the definition of RAL equivalents of certain Munsell colors, but it is obvious that it can be expanded easily by including RAL equivalents of a large number of Munsell color and vice versa. © 2005 Wiley Periodicals, Inc. Col Res Appl, 30, 130–134, 2005; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/col.20091     

Fixed range scales applied to the evaluation of color: Comparative analysis between the Munsell System and the Natural Color System

In this article, we discuss the effect of anchoring on the estimation of chromatic variables, which occurs when fixed range scales are used in psychophysical measuring. In order to do this, we compare the Munsell System and the Natural Color System (NCS), and an alternative system that employs a scale of grays as a yardstick against which to measure value and saturation. During experimentation, observers evaluated 60 matte surface color samples, following the methodology proposed by each system, but without using their respective color atlases. The data obtained reveal results that are compatible with the measurements based on the Munsell System and the NCS. By applying the gray‐scale method, the value measurements improved, whereas the saturation measurements were overestimated in samples with low reflectance. In all the cases, hue shifts were observed and thought to be attributable to the variation in luminance revealed by the samples. Judging by the similarity of the results obtained, it can be assumed that when fixed range scales are used, measurements will be anchored to their extremes, equaling every measurement made with them. © 2003 Wiley Periodicals, Inc. Col Res Appl, 28, 103–112, 2003; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/col.10130     

The representation of colors in spherical space

Color scaling experiments have established that perceived colors are distributed on the surface of a hypersphere in spherical space. A formal mathematical model of the color space is defined. Color differences as estimated by an observer are equal to the chord distance between corresponding points on the surface in spherical space. In one mathematical model are united: brightness, saturation, and hue as expressed in the empirical Munsell and NCS systems; complementary colors; large color differences; and contrast effects that are not represented in other models. © 2008 Wiley Periodicals, Inc. Col Res Appl, 33, 113–124, 2008.     

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