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     

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

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

Color identification data obtained from photopic to mesopic illuminance levels

To use colors properly as an aid in visual tasks, it is necessary to know how colors are identified under various illuminating environments. In this study color identification was examined under a wide range of illuminances, from photopic to mesopic levels. Fifteen subjects named a color chip using one of the preselected color terms: red, orange, yellow, yellow‐green, green, blue‐green, blue, purple, pink, brown, white, gray, and black. The 256 color chips were selected from value planes of 4, 6, and 8 of the Munsell color space. The illuminance levels tested were 1000, 10, 1, and 0.1 lx. At 1000 lx the color chips were identified consistently by each of the color terms. At 10 lx the pattern of color identification was very similar to that at 1000 lx, though the consistency of the identification evidently declined. At 1 lx great changes in color identification occurred. By 0.1 lx reliable color identification was completely lost, though blue and red responses remained. At the lower illuminances green was replaced with blue, and red, orange, and pink were frequently confused with each other. However, the border between blue and purple was almost constant. These results provide a scientific basis for the appropriate use of colors in various illuminating environments. Also, they are useful for studies in color appearance modeling. © 2002 Wiley Periodicals, Inc. Col Res Appl, 27, 252–259, 2002; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/col.10065     

Unique hue judgments using saturated and desaturated Munsell samples under different light sources

Past studies investigating the unique hues only used samples with a relatively high saturation levels under standard illuminants. In this study, 10 observers selected the four samples with unique hues from 40 V6C8 (Value 6 Chroma 8) and 40 V8C4 (Value 8 Chroma 4) Munsell samples under six light sources, comprising three levels of D_uv (i.e., 0, ?0.02, and ?0.04) and two levels of correlated color temperature (i.e., 2700 and 3500 K). Significant differences were found between the two chroma levels for unique blue and yellow, with the hue angles of unique yellow and blue judged using the desaturated samples being significantly different from those defined in CIECAM02. The iso‐lines of unique yellow, blue, and green did not always go through the origin of the a*‐b* or a′‐b′ planes in CIELAB and CAM02‐UCS. Thus, the problems of CIECAM02, CIELAB, and CAM02‐UCS identified in this study need further investigations.     

Chromatic adaptation to illumination investigated with adapting and adapted color

The state of chromatic adaptation was investigated by using the two‐room technique. This technique involves a subject in a room who looks a white board in a separate test room through a window and judges the color of the window using the elementary color naming method. When the subject room is illuminated with a colored light and the test room with a white light, the window appears to be a very vivid color, for which the apparent hue depends on the color of the subject room. The color is referred to as the adapted color. The subject also evaluated the appearance of the illumination color of the subject room, which is called the adapting color. Two types of illuminating light in the subject room, fluorescent lamps with 7 colors and LED lamps with 19 colors, were employed. The adapting and the adapted colors were plotted on a polar diagram that was used in the opponent color theory, from which the hue angles were obtained. The hue angle difference between the two colors did not appear to be 180° except for one pair of adapting and the adapted colors, which implies that chromatic adaptation does not follow the opponent color concept. An improvement was achieved to explain the results by introducing complementary colors relation between the adapting and adapted color.     

What can we learn from a dress with ambiguous colors?

We performed objective spectroradiometric measurements on an LCD image of the recently famous Tumblr dress which is typically perceived by people as blue/black or white/gold. The average ± standard deviation of the CIELAB coordinates was as follows: For a set of 33 points in the areas considered as blue/white, L* = 46 ± 6, C*_ab = 33 ± 6, and h_ab = 282 ± 3°, and for a set of 36 points in the areas considered as black/gold, L* = 29 ± 6; C*_ab = 10 ± 4; h_ab = 16 ± 34°. Initially, this first set of values has low variability and corresponds to a blue color, whereas the second set of values has a very large hue‐angle range, including points which can be considered as both gold and black colors. We also performed spectrophotometric measurements on an original model of this dress, and, assuming D65 illuminant and CIE 1931 colorimetric standard observer, the average results were L* = 26, C*_ab = 39, and h_ab = 289°, and L* = 10, C*_ab = 1, and h_ab = 290° for the blue/white and black/gold points, respectively. We discuss the influence of different factors on the blue/black and white/gold perceptions of different people, including observers' variability in color‐matching functions, Bezold–Brücke and Abney effects, background influence, and illumination assumptions. Although more research on the effect shown in this dress is needed, we think that from this example we can learn that objects do not have specific colors; that is, color is a human perception, and many times the answer of the human visual system is not simple and relies on assumptions of unknown, and variable, origin. © 2015 The Authors Color Research & Application Published by Wiley Periodicals, Inc., 40, 525–529, 2015     

Computational tongue color simulation in tongue diagnosis

Tongue color is one of the important indices for tongue diagnosis in traditional Chinese medicine. This study aims to analyze tongue colors of computational tongue diagnosis in traditional Chinese medicine using scientific quantification and computational simulation. The tongue color data are established according to the experiment in which the doctors who use Chinese traditional medicine assessed standard Munsell color charts under the standard lighting environment. Tongue color is classified into six color names, which are pale red, light red, red, crimson, dark red, and purple. The doctors assessed Munsell color charts to find the corresponding color charts' distributions of each color name in tongue color. The hue-lightness-chroma data of the chosen Munsell color charts were transformed to CIE xyY using the look-up table computation, then further were converted to CIELAB values and sRGB data. Based on the 95% confidence ellipses formed on CIELAB (a^*, b^*) plane and CIELAB (C^*, L^*) plane, the comparisons between tongue colors and general colors were analyzed. The computational tongue image simulation combining the elements of color, texture, and moisture was successfully established. This computational simulation method could potentially become a useful tool for teaching and learning diagnoses in the education of Chinese medicine.     

Associations of visual forms with colors: The minor role of emotion as the mediator

Associations of colors with a wide variety of visual forms were examined. Fifty visual forms were presented to Japanese participants, who were asked to report a color suitable to each visual form out of 15 colors. Correspondence analysis was applied to the responses, and four dimensions of configurations were obtained. The first and second dimensions were related to chromaticity or hue. In these dimensions, red and orange were associated with round symmetric shapes, yellow with shapes with sharp corners, violet and purple with irregular shapes with smooth curves, and cold colors such as blue and green with forms that consisted of simple geometrical elements such as triangles, squares, and circles. The third dimension was related to L* and b* in the CIELAB color space and to the stability of visual forms. Dimension 4 was not significantly related to any of L*, a*, and b*. In another experiment, participants rated the affective impressions of the colors and visual forms on semantic differential (SD) scales. The analyses of the SD data combined with the results of the color‐form associations suggest that the role of emotional meanings of colors and visual forms in mediating the associations was rather minor, and that world‐knowledge contributed considerably to the observed color‐form associations. Kandinsky's color‐form assignment was also examined, and except for the yellow‐triangle association, the assignment was not supported.     

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     

Three‐dimensional color prediction modeling of single‐ and double‐layered woven fabrics

In this research, the three‐dimensional structural and colorimetric modeling of three‐dimensional woven fabrics was conducted for accurate color predictions. One‐hundred forty single‐ and double‐layered woven samples in a wide range of colors were produced. With the consideration of their three‐dimensional structural parameters, three‐dimensional color prediction models, K/S‐, R‐, and L*a*b*‐based models, were developed through the optimization of previous two‐dimensional models which have been reported to be the three most accurate models for single‐layered woven structures. The accuracy of the new three‐dimensional models was evaluated by calculating the color differences ΔL*, ΔC*, Δh°, and ΔE_CMC(2:1) between the measured and the predicted colors of the samples, and then the error values were compared to those of the two‐dimensional models. As a result, there has been an overall improvement in color predictions of all models with a decrease in ΔE_CMC(2:1) from 10.30 to 5.25 units on average after the three‐dimensional modeling.     

Precision and accuracy in the color specification of virgin olive oils from the bromthymol blue method

Twenty experienced observers with nondefective color vision judged 27 virgin olive oil samples within an acceptable color range, using the bromthymol blue (BTB) method, under controlled observation conditions (daylight source with a correlated color temperature of 6500 K, and standard gray back-ground). On the average, 44.8% of the observers agreed in their selections of the BTB standard solution matching a given oil sample, and this percentage increased to 88.2% considering ±one step in the two dimensions (pH and concentration) of the BTB scale. On the average, the lowest color difference between oil samples and available BTB solutions was 6.6 Commission Internationale de l'éclairage 1976-(L^*a^*b^*) (CIELAB) units, but this color difference was approximately two times greater for the color difference between oil samples and BTB solutions selected by our observers. The colors of the BTB standard solutions in the CIELAB space are not uniformly distributed, and thus one step in pH or concentration is equivalent to CIELAB color differences varying in a wide range (1.7–13.5 and 1.7–26.3 CIELAB units, respectively). From these values, indicating low precision, accuracy, and uniformity, some suggestions are made for future improvements of the current BTB method.     

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.     

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.     

Colored apparel and its potential influence on heterosexual attraction

The objective of this study is to determine if men would follow the “red effect” when choosing colors for women to wear on a date, and also to determine if the colors that men would wear when going on a date would be the same as the colors that females (their date) would wish them to wear. A set of psychophysical data was generated from this experiment, where participants were asked to rank a set of 10 colored samples based on preference for each question asked. There were three different sets of colored samples. The set of colored samples given to the participant depended on the question. A total of five questions were asked. Scaling analysis was done on the data to organize a set of items according to preferences providing values, an interval scale (Z values), that correspond to the relative perceptual differences among the stimuli. The Z values were graphed to show the general preference of colors for women to wear, and the preference of colors for men to wear. A Spearman's rank-order correlation coefficient (SRCC) was calculated comparing each individual's rank order with the mean rank order for that specific question. An average Spearman's rank order was calculated for each question and each gender in order to determine the variability in answers. Scaling results indicate that men follow the “red effect,” but women preferred to wear other colors such as turquoise, blue, or yellow depending on the outfit. Males and females agreed that no matter the colored bottoms (denim or black), blue was the preferred color top for men to wear. SRCC results showed a lot of variability between individual answers and the mean answer indicating that participants' rankings did not necessarily agree with general color preferences presented in the scaling analysis. While scaling analysis might suggest certain color preferences such as men following the “red effect” and women preferring to wear blue, the poor correlation found using SRCC between the individual answers and the mean rank orders suggests that color preferences for each individual are inherently unique.     

Effect of Boron Nitride (BN) on Luminescent Properties of Y3Al5O12:Ce Phosphors and their White Light‐Emitting Diode Characteristics

This article reports a low‐cost yellow‐emitting Y₃Al_5‐xB_xO_12‐xN_x:Ce³⁺ phosphor with an enhanced luminescent intensity and excellent thermal stability for white light‐emitting diodes (LEDs). It was synthesized by a simple gas‐pressure sintering (GPS) process. The effect of B³⁺–N^3? incorporation on the optical properties of Y₃Al₅O₁₂:Ce³⁺ phosphor was investigated. The addition of appropriate amounts of boron nitride (BN) leads to a marked increase in photoluminescent intensity and a slight shift of its emission spectra toward the blue region, which is assigned to the improved crystallinity and increased particle size. Especially, the prepared oxynitride phosphor does not exhibit any thermal quenching under high temperature, and the emission intensity at 250°C even increases up to 175% of that measured at 20°C. Finally, the white LED flat lamp with luminous efficiency as high as 101 lm/W, color rendering index of 72, and correlated color temperature of about 6600 K is successfully realized by using YAG:Ce³⁺ phosphor doped with 0.5 molar ratio BN, which is acceptable and promising for general indoor illuminations to replace fluorescent or incandescent lamps.     

Quantitative representation of floral colors

Human and insect pollinator perceived floral colors of 81 species of angiosperms (flowering plants) from Trivandrum (Kerala, India) was represented using the CIE 1976 L*a*b* color space and color hexagon, respectively. The floral color difference among human perceived red, yellow, and blue‐hued flowers and that of each flower from its respective pure hue was calculated using the CIE ΔE 2000 formula. Human perceived floral color difference values were consistently higher than 3.5, indicating the uniqueness of floral colors. Flowers perceived red and yellow by humans were dominant and of comparable proportions. Insect pollinators perceive most of the flowers as blue‐green. Quantitative representation of human and pollinator perceived floral colors would be invaluable to understand the information broadcasted by flowers. It can form the basis of flower grading in the floriculture industry and underpin objectivity in evolving the framework for national pollinator strategies.     

Predicting personality associations evoked by multicolored appearance of virtual agents: An exploratory study

Designing agents' color appearance that can evoke desired color‐personality associations (CPAs) is still a challenge. To respond to the challenge, this study proposed a multicolor personality index (MCPI) model to predict the CPAs evoked by agents using the color‐personality indices for single colors together with the histogram of agents' image. Specifically, the color personality index for each bin of an agent's histogram was first calculated based on the color‐personality indices for single colors; then, the predicted values of CPAs were obtained by calculating the weighted average of color‐personality indices of all bins in the histogram. To improve the MCPI model, a simple color‐attention index model was proposed to evaluate the effects of color on attention and the effects were introduced into the MCPI model, that is, the multi‐color personality index based on color attention (MCPI‐CA) model. To validate the two models, an experiment was conducted to collect the CPAs of five traits (i.e., extraverted‐introverted, moody‐unemotional, agreeable‐disagreeable, organized‐disorganized, and wide interests‐narrow interests) evoked by the multi‐colored appearance of agents. The results showed that (a) observers in general perceived the CPAs evoked by color appearance in similar way; (b) for most agents, the CPAs evoked by multi‐colored appearance could be predicted well by the MCPI model; and (c) the MCPI‐CA model usually had better performance than the MCPI model due to the inclusion of the effects of color attributes on attention. In addition, the limitations of the proposed models and their implications for designers were also discussed.     

An attempt to reconstruct the meaning of al‐Tusi's color words

In a text dating between 1259 and 1277, the Persian scholar al‐Tusi presented a systematic arrangement of 26 color terms. We propose a reconstruction of all color terms from al‐Tusi's scheme, in terms of preferred translation, mean CIEL*a^*b^* coordinates and digital representation. This reconstruction is based on a visual experiment with 30 subjects, who identified the Munsell chip best representing each color term. Persian words for which the meaning changed since the time of al‐Tusi were substituted by direct translations. The results show considerable interobserver variability in the colors selected when identifying color terms. This relatively large variation was shown to be a characteristic for memory matching experiments in general. Several specific color terms for which the resulting color variation was particularly large are discussed in more detail, and possible explanations for these variations are proposed. The proposed reconstruction suggests that al‐Tusi's list is largely consistent in modern colorimetric terms, although some large hue shifts are observed for color terms corresponding to green. We found no evidence for blue‐green (“grue”) confusion. © 2015 Wiley Periodicals, Inc. Col Res Appl, 41, 206–216, 2016     

The perception of static colored noise: Detection and masking described by CIE94

We present psychophysical data on the perception of static colored noise. In our experiments, we use the CIE94 color difference formula to quantify the noise strength and for describing our threshold data. In Experiment 1 we measure the visual detection thresholds for fixed pattern noise on a uniform background color. The noise was present in one of three perceptual color dimensions lightness (L*), chroma (C*), or hue (h). Results show that the average detection threshold for noise in L* is independent of hue angle and significantly lower than that for noise in C* or h. Thresholds for noise in C* and h depend on hue angle in an opponent fashion. The measured detection thresholds, expressed in terms of the components ΔL*/k_LS_L, ΔC*/k_CS_C, and ΔH*/k_HS_H that build up the CIE94 color difference formula are used to tune CIE94 to our experimental conditions by adjusting the parametric scaling factors k_L, k_C, and k_H. In Experiment 2, we measure thresholds for recognizing the orientation (left, right, up, down) of a test symbol that was incremental in L*, C*, or h, masked by supra‐threshold background noise levels in L*, C*, or h. On the basis of the CIE94 color difference formula we hypothesized (a) a constant ratio between recognition threshold and noise level when the test symbol and background noise are in the same perceptual dimension, and (b) a constant recognition threshold when in different dimensions. The first hypothesis was confirmed for each color dimension, the second however, was only confirmed for background noise in L*. The L*, C*, h recognition thresholds increase with increasing background noise in C* or h. On the basis of some 16,200 visual observations we conclude that the three perceptual dimensions L*, C*, and h require different scaling factors (hue dependent for C* and h) in the CIE94 color difference formula, to predict detection threshold data for color noise. In addition these dimensions are not independent for symbol recognition in color noise. © 2008 Wiley Periodicals, Inc. Col Res Appl, 33, 178–191, 2008     

Hue,chroma, and lightness preference in Chinese adults: Age and gender differences

Recent studies have shown cultural differences in color preference. However, the color preference of people in China, which was found to have its own pattern, was yet to be studied in depth. The current study investigated color preference and the associated age and gender differences in an adult national sample (N = 1290) to provide a culture‐specific characteristic of color perception. Participants rated how much they liked each of 31 colors (four chroma‐lightness levels of red, orange, yellow, green, cyan, blue, and purple, plus three achromatic colors). We found a unique saturated color preference pattern characterized by red, cyan, and blue being preferred the most and orange as the least preferred chromatic color. The “red preference” phenomenon was observed in Chinese adults. Light colors were preferred the most in terms of chroma‐lightness level, followed by saturated, muted, and dark colors. The results of a principal component analysis of the 28 chromatic colors showed that blue‐green‐like colors (cool colors) constituted the largest proportion of color preference. The preference for orange and several dark colors increased with age, while that for bluish colors, purple, yellow, white, black, and light colors decreased. In terms of gender, women liked cyan, white, pink, and light colors and disliked red, orange, and dark colors more than men did. Our findings provide new empirical evidence about the color preference of Chinese and may offer some insight into the study of color preference and lay the foundations for future theoretical and practical research.