Semantic interpretation of color differences and color‐rendering indices

A series of visual experiments were carried out to rate the similarity of color appearance of two color stimuli on categorical and continuous semantic rating scales. Pairs of color stimuli included two copies of the same colored real or artificial object illuminated by a test light source and a reference light source. A formula was developed to predict a category of color similarity (e.g., “moderate” or “good”) from an instrumentally measured color difference. Given a numeric value of a color difference between the two members of a pair of colors, for example, 2.07, the formula is able to predict a category of color similarity, for example, “good.” Because color‐rendering indices are based on color differences, the formula could be applied to interpret the values of the new color‐rendering index (n‐CRI or CRI2012) in terms of such semantic categories. This semantic interpretation enables nonexpert users of light sources to understand the color‐rendering properties of light sources and the differences on the numeric scale of the color‐rendering index in terms of regular language. For example, a numeric value of 87 can be interpreted as “good.” © 2013 Wiley Periodicals, Inc. Col Res Appl, 39, 252–262, 2014; Published online 14 March 2013 in Wiley Online Library ( wileyonlinelibrary.com ). DOI 10.1002/col.21798     

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     

A Monte Carlo method for assessing color rendering quality with possible application to color rendering standards

The lighting industry has been increasingly challenged to reduce electrical energy consumption while providing illumination with sufficient color rendering quality. As a result, the problem of accurately assessing color rendering quality has gained increased prominence and the introduction of efficient narrow band light emitting diode (LED) sources has further intensified the debate. This study argues that there is a basic problem with the traditional method of quantifying color quality color rendering index (CRI), one that cannot be solved through minor improvements. The CRI relies on a determination of the degree of color distortion that a test source produces for a small number of test samples of specified spectral reflectance distribution, but there is no clear objective rationale for selecting these few samples. Also, any such arbitrary scoring scheme lacks an objective argument for what constitutes an acceptable score. This study proposes a new method for color rendering assessment that determines the color shift of one thousand, or more, representative reflection spectra that span the full multidimensional range of possible spectral distributions and colors. This broad sampling eliminates the intrinsic selection bias of the CRI calculation and its variants and it is compatible with a more objective standard for a color quality score, one that is statistically based on the fraction of the test spectra that experience color shifts that are less than a just noticeable difference (JND), or an agreed upon multiple of it. Since the concept of JNDs in color has been reproducibly quantified, it is hoped that this approach will be widely acceptable. © 2010 Wiley Periodicals, Inc. Col Res Appl, 2012     

Color rendering: Beyond pride and prejudice

It is a truth, universally ignored, that a single metric of color rendering must be in want of another. Evidence presented here, together with those from an earlier study, strongly suggest that the quest for a single metric to quantify color rendering will be in vain. Rather, the strengths of color rendering index (CRI) and of gamut area index (GAI) † seem to counteract the weaknesses of one another, such that together they can be used to guide lighting practitioners in choosing a source that will provide good color rendering of most objects in most applications. The present study was conducted to determine whether sources, both warm and cool, with high levels of both CRI (above 80) and GAI (above 80 and less than 100) were judged better than ones with high levels of just CRI or just GAI. The results support the conclusion that a two‐metric system of color rendering is needed for general illumination applications. © 2010 Wiley Periodicals, Inc. Col Res Appl, 2010     

Optimization of LED‐based light blendings for object presentation

This study looks at the perceived quality of light‐emitting diode (LED)‐based lighting of various colors. The objective was to find out whether LEDs could provide better (i.e., more relevant and acceptable) lighting than that which is obtained with standard halogen or fluorescent sources. The perception of objects was assessed under different lighting schemes. Subjects were invited to add red, cyan and/or amber to white LED‐based light to match the halogen and fluorescence rendering on specific targets: a color chart and a painting. They were also asked to rate the difference between the two, and to express their preference. The results obtained for the perception of LED‐based lighting were quite positive. Color blendings of LED light were found to provide illuminated situations similar to halogens or fluorescent sources. These blendings were well accepted, and indeed often preferred, although the color rendering index (CRI) was always low. This indicates that the CRI as it stands is inadequate to characterize the color rendering of solid‐state light sources, and needs to be updated. LED‐based lighting systems seem to have considerable potential for use in shops and display units, where they may well outperform existing lighting systems. © 2009 Wiley Periodicals, Inc. Col Res Appl, 34, 310–320, 2009     

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     

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.     

Color rendering of supplementary artificial lighting

Many interiors may be lit by daylight and artificial light; the CIE color rendering index (CRI) of the sources alone is not a characteristic of the complex situation. It is the purpose of this article to derive the general principles leading to the determination of the CRI distribution in such an interior. the CRI is expressed as a function of the proportion α of the combination, for several types of daylight and artificial light. It is shown that the dependence of the CRI on α is linear for supplementary artificial lightings which have the same chromaticity as daylight. Otherwise, nonlinearities are introduced. the time variation of the CRI is also analyzed.     

Particle surface temperature measurements with multicolor band pyrometry

A noncontact, color‐band pyrometer, based on widely available, inexpensive digital imaging devices, such as commercial color cameras, and capable of pixel‐by‐pixel resolution of particle‐surface temperature and emissivity is demonstrated and described. This diagnostic instrument is ideally suited to many combustion environments. The devices used in this method include color charge‐coupled device (CCD), or complementary metal oxide semiconductor (CMOS) digital camera, or any other color‐rendering camera. The color camera provides spectrally resolved light intensity data of the image, most commonly for three color bands (Red, Green, and Blue,), but in some cases for four or more bands or for a different set of colors. The CCD or CMOS sensor‐mask combination has a specific spectral response curve for each of these color bands that spans the visible and often near infrared spectral range. A theory is developed, based on radiative heat transfer and camera responsivity that allows quantitative surface temperature distribution calculation, based on a photograph of an object in emitted light. Particle surface temperature calculation is corrected by heat transfer analysis with reflection between the particle and reactor wall for particles located in furnace environments, but such corrections lead to useful results only when the particle temperature is near or below the wall temperatures. Wood particle‐surface temperatures were measured with this color‐band pyrometry during pyrolysis and combustion processes, which agree well with thermocouple measured data. Particle‐surface temperature data simultaneously measured from three orthogonal directions were also mapped onto the surface of a computer generated 3‐D (three‐dimensional) particle model. © 2008 American Institute of Chemical Engineers AIChE J, 2009     

Erbium(III)‐doped polyurethaneureas: Novel broadband ultraviolet‐to‐visible converters

Novel, polymeric UV‐to‐visible converters were prepared by doping elastomeric poly(ether‐urethaneurea) copolymers with 5–25% by weight of ErCl₃ 6H₂O, corresponding to Er³⁺ concentrations of 2.19 to 10.86% by weight. When excited in the UV at 355 nm, the doped films generated a very broad, continuous visible luminescence between 400 and 750 nm. Preparation and spectroscopic characterization of the samples are discussed in detail. The color coordinates, color temperature, color rendering index of the samples, and the degree of overlap of their emission bands with the spectral response of the eye were determined. The color rendering index of samples is in the 57–70 range. The sample containing 2.19% by weight of Er³⁺ was found to give the color coordinates closest to the white‐source region and the highest color rendering index. The color temperatures of the samples were in the 5093–5540 K range. Overlap between the emission bands and the spectral response of the eye improved with increasing erbium concentration. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

Rare‐Earth Free Phosphors with Ultra‐Broad Band Emission Application for High Color Rendering Index Lighting Source

Rare‐earth containing phosphors have been widely applied in lighting and display fields in the past century. Lower cost rare‐earth free phosphors with high performance are highly desired driven by the exhaustion of rare earth resources and the requirement of cheaper production. Herein, Cu⁺ ions doped tetracalcium phosphate (TTCP) yellow emitting phosphors with quantum yield of 21% are exploited. Particularly, ultra‐broad band emission with a full width at half maximum (FWHM) about 200 nm throughout almost entire visible light region is observed for TTCP: Cu⁺ phosphors, evidencing its promising application in high color rendering index (CRI) lighting source. White light emission with CRI value about 94.3 is generated by combining this TTCP: Cu⁺ phosphor with commercial BaMgAl₁₀O₁₇: Eu²⁺ blue phosphor, exhibiting superiority over the traditional trichromatic phosphors. Therefore, we predict great potential application for this cheaper rare‐earth free TTCP: Cu⁺ phosphor in high CRI lighting sources.     

Color rendering properties of light continuously modulated by an electrochromic switchable device

Every chromogenic switchable window gives rise to considerable change of spectral power distribution of the light crossing trough. The filtered light continuously changes during charging/discharging of the device, seen as its coloration/discoloration. As this process occurs continuously, it causes continuous change of colors on an illuminated multicolored scene. The ability of an electrochromic (EC) switchable unit to modulate daylight and different artificial lights passing through it is studied here. The color rendering properties of such a variable light are evaluated. The range of the effect and its consequences on the color of a scene can be adequately described by the change in chroma and hue angle during the modulation of light. These effects were evaluated by different light sources shining through the same EC unit. © 2009 Wiley Periodicals, Inc. Col Res Appl, 34, 321–329, 2009     

Chromaticity‐matched but spectrally different light source effects on simple and complex color judgments

As light‐emitting diode (LED) light sources mature, lighting designers will be able to deliver white light with a variety of spectral power distributions and a variety of color rendering properties. This experiment examined the effects of three spectral power distributions (SPDs) that were matched in illuminance and chromaticity on three measures of color perception: one objective (performance on the Farnsworth‐Munsell 100 hue test) and two subjective (judgments of the attractiveness of one's own skin, and preferences for the saturation of printed images). The three SPDs were a quartz‐halogen (QH) lamp and two LED sources that were matched to the QH lamp in terms of both illuminance and chromaticity; the three light sources were nominally CCT = 3500 K, x = 0.40, y = 0.39 and ～ 400 lx. LED A used three channels (red, green, blue), and had very poor color rendering (R_a = 18). LED B used four channels (red, amber, cyan, white) and had very good color rendering (R_a = 96, whereas the QH had R_a = 98). Secondary hypotheses addressed the effects of age and skin and eye color on the dependent measures. As expected, LED A delivered very different color perceptions on all measures when compared to QH; LED B did not differ from QH. The results show that it is possible for LED sources to match the familiar incandescent sources. However, although it is possible to deliver what appear to be millions of colors with a three‐chip (RGB) device, there is the risk of creating a very poor luminous environment. © 2013 National Research Council Canada and Wiley Periodicals, Inc. Col Res Appl, 39, 263–274, 2014; Published Online 12 April 2013 in Wiley Online Library ( wileyonlinelibrary.com ). DOI 10.1002/col.21811     

Regulation of Phosphor's Color Gamut Area Using Mesoporous Silicate Source—A New Paradigm for the Solid‐State Lighting Segment

The inception of phosphors provoked the need of highly efficient lighting sources for the conceivable next generation applications. In this regards, a striking characteristic material namely the mesoporous silica (MPS) and their series that are quite distinct to each other in their physical properties have been synthesized by adopting a cooperative self‐assemble strategy. Utilizing it, the development of high‐performance Sr_1?xCa_1?yEu_x+ySiO₄ phosphor was traversed using a simple wet‐solid phase process. Systematic investigations on morphological, structural, and few other physical properties were carried out. The derived results are intriguing with a crystal clear alteration in the phosphor morphology when MPS has been used as a silicate source. Subsequent luminescence studies displayed its efficient yellow‐emitting property covering the red spectral components, along with good thermal luminescence stability. Perhaps, the designed prototype LEDs yielded a color correlation temperature (CCT) < 5000 K and color rendering index (CRI) > 80. Therefore, it directs the way for the fabrication of potential warm white LEDs with a long‐lived emission efficacy and high thermal luminescence stability.     

Skin‐color correction method based on hue template mapping for wide color gamut liquid crystal display devices

The development of wide color gamut (WCG) liquid crystal display (LCD) plays an important role in the high‐quality television (TV) field. Nowadays, people want their TV or display devices to have the capability of showing vivid colors while keeping skin colors as natural as they remember. Therefore, it is necessary to develop color‐correction technologies for WCG LCD system. A new color‐correction method named “natural skin‐color mapping algorithm” (NSCMA) for WCG LCD is proposed in this study. It can solve the skin‐color contour problem in color‐corrected images with simple skin‐color detection. Its development is based on the concepts of performing color mapping between source hue colors and target hue colors on each hue page. The polynomial regression is also applied to calculate the color mapping conversion matrices. Two color mapping factors called template‐size factor and tone‐compression factor are designed in NSCMA. The template‐size factor is used to adjust target template sizes adequately. The tone‐compression factor is designed to control the degrees of image enhancement. For facial skin‐color pictures, the appropriate settings of template‐size factor and tone‐compression factor will get suitable color image rendering on the WCG LCD. It is demonstrated that the WCG LCD can be corrected to show vivid color pictures and keep facial skin colors as natural as possible when the proposed NSCMA is performed. © 2010 Wiley Periodicals, Inc. Col Res Appl, 2011     

White light emission of wide-bandgap silicon carbide: A review

White light-emitting diodes (LEDs) are the most promising alternative to the conventional lighting sources due to their high efficacy and energy saving in illumination. Silicon carbide (SiC) has a wide optical bandgap and could be tailored to emit light at different wavelengths across the entire visible spectrum by introducing different dopants. Donor and acceptor (DA) co-doped fluorescent SiC (f-SiC) is a potential candidate for replacing phosphor material in white LEDs, as it has been observed as a good wavelength converter overcoming the disadvantages of rare earth-containing phosphors, such as poor color-rendering index (CRI), short lifetime, and short degradation time. The current study attempts to present an overview on the available approaches to fabricate f-SiC for generating the white light emission and challenges in fundamental research issues to enhance quantum efficiency, color rendering performance, stability, reproducibility of color quality, and lifetime of f-SiC.     

Color-rendering capacity of light

Two types of questions, both related to color-rendering properties of light, can be identified. Type A: Given a spectral reflectance chip under illumination of a certain spectral power distribution, What color can appear? Does the appearing color, in comparison with a reference one, look right or distorted? Type B: Given a great many chips of different spectral reflectance functions under illumination of a certain spectral power distribution, How many different colors, no matter what colors, can appear? Are there a great many different colors or just a few different colors appearing? Questions of type A reflect an important aspect of color-rendering properties and can be tackled with an established measure, the CIE color-rendering index. Questions of type B, related to quite another aspect of color-rendering properties, have particular relevance when illumination's function of making things visible is of most concern. This article discusses a new measure, the color-rendering capacity (CRC), developed for dealing with questions of type B, and explains the measure's derivation, calculation, and implications.     

Testing LED lighting for colour discrimination and colour rendering

Light‐emitting diode (LED) technology offers the possibility of obtaining white light, despite narrow‐band spectra. In order to characterize the colour discrimination efficiency of various LED clusters, we designed a classification test, composed of 32 caps equally distributed along the hue circle at about 3 ΔE* _ab‐unit intervals. Forty normal colour observers were screened under four different LED test light sources adjusted for best colour rendering, and under one control incandescent light of the same colour temperature. We used commercially available red, green, blue, and/or amber LED clusters. These yielded a poor colour rendering index (CRI). They also induced a significantly higher number of erroneous arrangements than did the control light. Errors are located around greenish‐blue and purplish‐red shades, parallel to the yellow‐axis direction, whereas when the distribution of light covers the full spectrum, the LED clusters achieve satisfactory colour discrimination efficiency. With respect to the lights we tested, the colour discrimination is correlated with the CIE CRIs as well as with a CRI based on our sample colours. We stress the fact that increasing the chroma of samples by lighting does not necessarily imply an improvement of colour discrimination. © 2008 Wiley Periodicals, Inc. Col Res Appl, 34, 8–17, 2009.     

Color preference affected by mode of color appearance

Most color preference research focuses on colors in an object color mode. In our daily life, however, colors are perceived not only as an object color mode but also as other modes, such as unnatural object color and light source color modes. To explore the effect of the color appearance mode on color preference, we examined the relationship between color preference and the mode of color appearance. Thirty‐three color chips were chosen from the Munsell notation varying in hues and chromas. The color chips were presented in different color appearance modes by changing the subject's room illuminance and the color chip room illuminance. The experimental results showed that the brightest and most saturated colors were preferred. It was found that the subject preferred color in a light source color mode and unnatural object color mode to color in an object color mode. Moreover, we found that hue had a small effect on color preference in the light source color mode. We also investigated the relationship between color preference and the perceived color attributes (perceived chromaticness, whiteness, and blackness). In a supplementary experiment, elementary color naming was conducted. The results showed that the perceived chromaticness, perceived whiteness, and perceived blackness play a role for the determination of color preference for different color appearance modes. We, consequently, suggest that color preference is dominated not only by color attributes but also by the mode of color appearance. © 2009 Wiley Periodicals, Inc. Col Res Appl, 2010     

Color conversion properties of various thick-film phosphor-in-glasses depending on structural design for white LEDs

Thick-film phosphor-in-glasses (PiGs) were fabricated via a screen-printing method with various phosphor layer structures, to compose a white light emitting diode (LED). Green (Lu₃Al₅O₁₂:Ce³⁺) and red (CaAlSiN₃:Eu²⁺) phosphors were mixed, layered, and patterned on a glass substrate. The chromaticity of each structured PiG was tuned to achieve a white LED by varying phosphor content and thickness. The emission spectra and the related various color conversion properties, including color coordinates, correlated color temperature (CCT), color rendering index (CRI), luminous efficacy (LE) and the color gamut of the mounted PiGs with different phosphor layer structures were examined and compared. Time-resolved photoluminescence (TRPL) measurements of the white LEDs with various phosphor layer structural designs were also obtained and compared. It was observed that spectral variation depended on the PiG layer structure. A proper PiG layer structural design was discussed for practical applications.