White lighting: A provisional model for predicting perceived tint in “white” illumination

Recently, a line of minimum tint in chromaticity space for sources of illumination of different correlated color temperatures (CCTs) from 2700 K to 6500 K has been reported. This line of minimum tint did not correspond to the line of blackbody radiation implicitly associated with sources of white illumination used in architectural applications. It was noted that chromaticities along the line of minimum tint were not metamers but, rather, should represent, for a given CCT, chromaticities where the neural signals from the two spectral opponent channels were minimized. Earlier work provided a theoretical framework for representing the four unique hues where the neural signals from one spectral opponent channel are minimized. This framework was used here to quantitatively represent the line of minimum tint and then to empirically test whether this representation could predict the amount of tint perceived in six different sources of “white” Illumination.” Based on these results, a provisional model is proposed for describing the tint and the amount of tint perceived in “white” Illumination used in architectural applications. © 2013 Wiley Periodicals, Inc. Col Res Appl, 39, 466–479, 2014     

Effects of correlated color temperature on focused and sustained attention under white LED desk lighting

Since the Kruithof's work on general illumination, the relationships between correlated color temperatures (CCTs) and human behavior are well documented. In the recent years, because of the high growth of light‐emitting diode (LED) lighting products, studies on the effects of LED lighting on human behavior have emerged. This study examines the effects of CCTs on focused and sustained attention under white LED desk lighting. Three CCT conditions (2700, 4300, and 6500 K) are examined, and the Chu Attention Test was used to measure focused and sustained attention. Results show that CCTs affect attention. In specific, the 4300 K condition resulted in a significantly better focused and sustained attention. Comfortableness and clarity are rated by the subjects. The self‐reported comfort shows no significant differences among the three conditions, but clarity shows significant differences. Gender differences are also discussed. It is found that the self‐reported clarity by males show no significant differences among the three lighting conditions, whereas the self‐reported clarity by females is significantly lower when CCT is 2700 K. © 2014 Wiley Periodicals, Inc. Col Res Appl, 40, 281–286, 2015     

Chromaticity changes of inorganic pigments in Chinese traditional paintings due to the illumination of frequently‐used light sources in museum

Optical radiation can cause permanent damage to the color of Chinese traditional paintings, which have extremely high responsivity in the museum illumination. In order to obtain quantitative influence laws of artificial illumination on inorganic pigments in Chinese traditional paintings and provide basis for choosing light sources, a long periodic illumination experiment was carried out to examine how prolonged exposure of three light sources (tungsten halogen, metal halide and white light emitting diodes), frequently‐used in museums, affect the chromaticity of five inorganic pigments [azurite (blue), cinnabar (red), orpiment (yellow), ancient graphite (black), clam shell powder (white)], commonly used in Chinese traditional paintings. Through the experiment, we found that the illumination causes a color change to inorganic pigments, and the degree of which is related to the photochemical stability of pigments and the high energy shortwave radiation in the light source spectrum. We also obtain the color change laws of the five pigments and the quantitative relationships of color damage for different painting types. These results can provide data basis for related research on Chinese traditional painting illumination and choice basis for light sources in museum lighting designs.     

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.     

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

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

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     

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     

Determination of correlated color temperature based on a color-appearance model

The present method of determining the correlated color temperature of an illuminant is based on the obsolete CIE 1960 u,v chromaticity diagram. A new method is proposed that is based on Hunt's color-appearance model for unrelated colors. Analytic functions were derived for the blackbody locus in appearance space to simplify the computation. CCT was calculated for some light sources and the results were compared to those obtained by the present method.     

Highly thermally stable and emission color tunable borate glass for white‐light‐emitting diodes with zero organic resin

White‐light‐emitting glass is a kind of potential bireplacement of both phosphors and epoxy resin in high‐power white‐light‐emitting diodes (WLEDs) because of its high thermal conductivity and excellent thermal stability in aspect of luminescence, CIE chromaticity, and transmittance. In this study, a series of tunable white‐light‐emitting strontium borate (SBO) glass SBO:Tb³⁺,Eu³⁺ were prepared by conventional melt quenching method, and their luminescence properties were systematically studied through their photoluminescence excitation and emission spectra, decay curves, and quantum efficiency. Intense white light emitting can be achieved by in situ mixing of yellowish green and reddish orange emissions from Tb³⁺ and Eu³⁺, respectively, in single glass component SBO, and mixed white emissions can be tuned by Tb³⁺→Eu³⁺ energy transfer with the increasing concentration of energy acceptor Eu³⁺. The quantum efficiency of optimal glass SBO:10%Tb³⁺,6%Eu³⁺ was measured as 36.78%. And the excellent thermal stability of this glass can remain its luminescence intensity above 80% at the temperatures below 523 K. Its chromaticity shift is less than 0.01 at the temperature below 548 K, which is far smaller than that of commercial DS‐200 and triple‐color white‐emitting phosphor mixture. Except all above, the transmittance of this glass hardly shows loss after thermal aging at 120°C for 240 hours, which is superior to the only remaining 58.8% transmittance of epoxy resin. The thermal conductivity of this glass is 0.65 W/mK much better than the 0.16 W/mK of epoxy resin. Based on above research results, SBO:Tb³⁺,Eu³⁺ glass is considered as a promising candidate for high‐power WLEDs, thus a SBO‐WLED is simply assembled by SBO:10%Tb³⁺,6%Eu³⁺ glass and 378 nm LED chip that can present excellent luminescence performance with V=10 V, I=600 mA.     

A high efficacy and tunable white light-emitting diode cluster with both color fidelity and nonvisual performances close to natural lights

A luminous efficacy model for a white light-emitting diode (LED) cluster was developed, and five α-opic efficacy ratios (γ_α,Vs) of intrinsically photosensitive retinal ganglion cells were adopted in order to quantify its nonvisual performance relative to natural light. A tunable C/A/R LED cluster, which consists of the cyan LEDs with green phosphor excited by a blue LED, the amber LEDs with green and orange phosphors excited by same blue chip, and the red LEDs, was proposed by maximizing the average luminous efficacy at correlated color temperatures (CCTs) of 2700 to 6500 K under the condition of color fidelity index (R_f) above 96 and five values of γ_α,V of 0.95 to 1.05. The real tunable C/A/R LED cluster with a R_f of 95 to 96, five values of γ_α,V of 0.97 to 1.01, and a luminous efficacy of 114 to 120 lm/W at a CCT of 2670 to 6456 K were demonstrated. Furthermore, five values of γ_α,V can be adjusted according to the distance from the Planckian or daylight locus on the CIE 1960 uv chromaticity diagram at different CCTs. Thus, the tunability of γ_α,V value of the C/A/R LED cluster at a specific CCT could be applied to different requirements for nonvisual performance.     

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     

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     

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     

Concept of correlated color temperature

The concept of color temperature has been used for many years to characterize the relative spectral distribution of energy from light sources. Since not all sources of light are blackbody radiators, the term “correlated color temperature” was devised to indicate the temperature of the blackbody with chromaticity nearest that of the source. Since the CIE has recently recommended two additional color spaces superseding the existing universally agreed system, it is important to know what effect the new recommendation will have on the correlated color temperature determination. In view of this, it has seemed appropriate to design and carry out some fundamental studies in acceptable color matching among color-normal observers. The isotemperature lines derived from the experimental data described here are within one standard deviation of the isotemperature lines derived from the 1960 UCS diagram. Hence, there is no need to revise the isotemperature lines derived formerly.     

Optimal fluorescent colors

This article poses and discusses a maximization problem: for a given chromaticity, and in a given illumination, how light a fluorescent color is possible. The article does not present a solution, but does offer a conjecture that for many illuminants and many chromaticities the greatest lightness is achieved by a simple form of bispectral function. The output is in two points plus a tail, so we call it a “PPT” function. © 2007 Wiley Periodicals, Inc. Col Res Appl, 32, 85 – 91, 2007     

Estimating black‐level emissions of computer‐controlled displays

It is quite common for computer‐controlled displays to emit light in image areas set to digital values of zero, referred to as their black level. This is expected for liquid–crystal displays and also can occur for cathode‐ray tube displays when the “brightness” (gun‐amplifier offset) is set excessively high. For either display, the light emission at the black level results in color channels whose chromaticities vary with luminance level. Consequently, typical methods of colorimetric characterization result in large error. When this black‐level emission is measured and accounted for suitably, characterization accuracy is dramatically improved. Unfortunately, many instruments used to measure displays have too low a sensitivity to measure black‐level emission with sufficient precision and accuracy. A method of estimating black‐level emissions was derived and tested. Because the optimal black‐level results in channel chromaticities that are invariant to the greatest extent with luminance level, an objective function was defined as the sum of chromaticity variances of each channel over a range of measurements. Minimizing this objective function resulted is an estimate of a display's black level. The estimated black level resulted in equivalent or superior performance to direct measurements. © 2003 Wiley Periodicals, Inc. Col Res Appl, 28, 379–383, 2003; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/col.10181     

Observer preference for perceived illumination chromaticity

Subjects assessed white tone preference and tint percentage perception (e.g., a faint greenish tint perceived in the white tone) in a viewing booth containing small colored objects at the illuminance level of 1000 lx for cool white, neutral white and warm white spectra. White points with the lowest perceived tint percentage were most preferred. White tone preference was rated on interval scales labeled by semantic categories, for example, “very good,” “good,” and “moderate” The location of the white points of different preference levels (e.g., “good‐very good” level or “moderate‐good” level) was depicted in the u′‐v′ chromaticity diagram. The most preferred white points were located below the Planckian locus, inside the region of the white points required by CIE Publication 13.3 (1995) with the criterion of the chromaticity difference DC being <0.0054. The most preferred correlated color temperature (CCT) was at 3400, 4500, and 5800 K in the warm, neutral, and cool white CCT groups, respectively. Results were interpreted in terms of the Rea and Freyssinier (2014) model.     

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     

Color discrimination subject to illuminant and color transition

Due to technological influence on the production process there are variations of the chromaticity coordinates that may cause visible differences of the perceived color of LEDs of the same production batch. To minimize the negative impact of such variations for the user the manufacturers carry out colorimetric measurements of LEDs and sort them into tolerance groups of similar chromaticity coordinates. The corresponding groups are defined by their respective center point surrounded by a quadrangular tolerance limit whereas the tolerance limits are based on tolerance ellipses gained in the 1940s by MacAdam. However, due to another scientific target of MacAdam's experiments, applying his ellipses as a base for grouping the chromaticity coordinates has to be questioned. With a view toward a method for grouping according to chromaticity coordinates for white LEDs with phosphor conversion, the results of research with test subjects regarding their discrimination and magnitude perception of chromatic differences are presented. In this research three chromaticities with the correlated color temperatures of 2700, 4000, and 6500 K and variations of color transitions are investigated, which match realistic luminaire designs for practical applications. The empirically determined thresholds are expanded according to the transformation to different chromatic adaptation conditions as per von Kries that are then compared to the results by MacAdam and the influence on adaptation is being discussed. A new concept for the calculation of tolerance limits for LED binning is presented. © 2015 Wiley Periodicals, Inc. Col Res Appl, 41, 457–467, 2016     

Affective matches of fabric and lighting chromaticity

This study investigates the impact of lighting colors on subjective judgments of fabric: in particular, whether the influence of lighting varies depending on fabric types and color combinations. We conducted two visual assessments. In Study 1 (N = 44), eight illuminants and six types of fabric were presented as cloth stimuli. Derived from the literature review, four sets of adjectives (humble-luxurious, cool-warm, old-new, and not preferred-preferred) were used as metrics. In Study 2 (N = 41), five sets of fabric color combination swatches were assessed under lighting conditions that were identical to those of Study 1. Three bipolar scales (ordinary-characterful, classic-modern, and soft-rigid), were employed from factor analysis along with three unipolar scales (luxurious, preferred, harmonious with lighting). The results showed that hue characteristics of lighting and cloth types influenced participants' perceptions of the fabric. Overall, white lighting with 4000 K was the most preferred and luxurious lighting across various types of clothes, while a pinkish white with 4700 K (duv = −0.0127) was the best matched in every color combination. In addition, there were interaction effects between lighting colors, clothes types, and fabric color combinations with regard to each of the perceptual qualities. This study provides empirical evidence for optimally match lighting colors with fabric in the presentation of fabric goods.