The usability of the CIE colour‐matching functions in the case of CRT monitors

During the past years, several papers have been published that question the use of the CIE colour‐matching functions in the case of metameric samples. Visually matching samples produced on CRT (Cathode Ray Tube) monitors are metameric to most colour stimuli created by illuminating reflecting materials. As CRT monitors are often used in colour design applications, it seemed important to check how well CIE colorimetry will predict such colour matches. To investigate this problem, we set up an experiment in which painted samples were matched with samples produced on a CRT monitor. The colour of incandescent lamp irradiated Munsell samples were visually matched to the mixture of the RGB primaries of a CRT monitor. Both the reflected colour stimuli of the Munsell samples and the emitted stimuli of the monitor were measured spectroradiometrically. Our results imply that there is an observer‐dependent variability among the matches, but we could not find a major difference between the tristimulus data of the hard copy and soft copy presentations that would indicate errors in the CIE colour‐matching functions. The measurement accuracy, quantization errors of the monitor, and the achieved accuracy of the colour matches are treated in this study. © 2001 John Wiley & Sons, Inc. Col Res Appl, 26, 436–441, 2001     

Quantifying the suitability of CIE D50 and a simulators based on LED light sources

In this study, two psychophysical experiments, colour‐difference assessment and colour‐appearance estimation, were conducted to investigate the perceived quality of four CIE illuminant simulators, including two simulators based on LED light sources. The perceived colour‐differences of 30 metameric pairs, seen under the different simulators, were evaluated by 10 colour‐normal observers using the gray scale method and the colour appearance of the colour inconstant sample in each pair assessed in a magnitude estimation method. Colorimetric measures revealed that LED simulators can achieve the desired quality according to the relevant ISO and CIE standards. The results of the experiments showed that an LED simulator outperformed the conventional fluorescent lamp‐based simulator for the CIE illuminant D50 condition. In addition, an LED simulator worked almost equally well as a conventional simulator for simulating CIE illuminant A. These findings strongly indicated the good quality of LED simulators based on a limited number of channels, and the superiority of LED technology. © 2016 Wiley Periodicals, Inc. Col Res Appl, 42, 408–418, 2017     

Inter‐observer comparison of color‐matching functions

The colorimetric difference between pairs of observers is simulated by a proper filtering of the stimulating radiation, and their comparison is made on properly defined Common Reference Frames in the tristimulus space. As examples, two comparisons are proposed: (1) Comparison between the Vos modification of the CIE 1931 Standard Colorimetric Observer and the CIE 1964 Supplementary Standard Observer: in this case, it is supposed that the difference between these two color‐vision systems is due to the macula lutea only, which with a spectral selective absorbance alters the power spectral distribution of the color stimuli. The optical density of the macular pigment is well reproduced. (2) Comparison between the Vos modification of the CIE 1931 Standard Colorimetric Observer and the CIE 1931 Standard Colorimetric Observer: in this case, the difference between these two observers could be simulated by different calibration of the photodetectors. © 1999 John Wiley & Sons, Inc. Col Res Appl, 24, 177–184, 1999     

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.     

A general metric for the magnitude of observer metamerism

Observer metamerism is defined as a property of a pair of spectrally different stimuli having the same colour sensation for an individual (reference) observer. Frequently, samples in this pair no longer match if the observer is changed. In this article, a linear approximation formula is developed that predicts a metameric effect caused by small changes in the observer's colour‐matching functions. This approximation formula enables a general metric of observer metamerism, the observer metamerism potential, to be defined that is independent of any particular deviated observer but still provides a close link to ‘observer‐metameric’ colour difference. Numerical experiments were conducted to investigate the correlation between the observer metamerism potential and the maximum of 53 metameric colour differences caused by the change from the colour‐matching functions of CIE standard 10° observer to the colour‐matching functions of 49 Stiles and Burch's real 10° test observers. The proposed general metric, together with a previous metric proposed by the present authors, the illuminant metamerism potential, could be taken as a quantitative measure of the performance of spectral approximation methods.     

Comparing observers

Color‐matchingfunctions may be considered dimension reduction functions that project a spectral reflectance function into the desired space of colors. Using a gray metameric pair with maximal spectral difference we compare the abilities of various human and other observers with regard to the transition wavelengths for that metameric pair. Transition wavelengths are shown to be a convenient tool for comparing and classifying observers regardless of the number of dimension reduction functions. Four human observers were identified as differing in a comparable manner from the CIE 2° standard observer. © 2004 Wiley Periodicals, Inc. Col Res Appl, 29, 183–186, 2004; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/col.20004     

Evaluation of discomfort glare from color leds and its correlation with individual variations in brightness sensitivity

The light‐emitting diode (LED) has attracted attention as an alternative light source to fluorescent and incandescent lighting. The characteristics of LED light are different from other sources, but regulations for LED products have not been completely established. Common LED lights, such as automotive lamps, street lighting systems and traffic lights, are produced under the existing glare regulations for other light sources, and some organizations are seeking to establish standardized regulations for LED products. Glare can impair vision and cause discomfort and must be considered when establishing regulations for lights. In this study, we measured the sensitivity of observers to the discomfort glare from color LEDs and analyzed the correlation between discomfort glare sensitivity and brightness sensitivity using heterochromatic brightness matching and flicker photometry. The results indicate a correlation between discomfort glare sensitivity and brightness sensitivity using blue LEDs and mild correlations with green and red LEDs. © 2010 Wiley Periodicals, Inc. Col Res Appl, 2010.     

Judd's 1951 color-mixture diagram

Vos demonstrated that the Judd 1951 color-mixture diagram is a projective transformation of the CIE 1931 mixture diagram. He has provided transformation formulas for computing values of x′ and y′ from x and y, but he does not explain how the one set of data is derived from the other. Neither does Judd. Judd wanted the CIE to replace the 1931 V(Λ) curve with a new V(Λ) curve and to replace the 1931 XYZ diagram with a new X′Y′Z′ diagram. I have assumed that Judd started with the CIE 1931 RGB diagram. His X′Y′Z′ diagram can be derived from the RGB diagram if we use a different set of luminosity coefficients than those specified by the CIE. This change in luminosity coefficients was no doubt intended to compensate for the use of a new V(Λ) curve, but Judd has ignored the effect that this change in the luminosity coefficients should have on the alychne of the RGB diagram and the locus of spectral colors. This makes Judd's X′Y′Z′ diagram inappropriate for use in colorimetry and for use in building color-vision models. I have proposed a new solution to this problem.     

Testing the behavior for metamerism of the new deviate observer (JF‐DO)

Recently,in our laboratories, a set of color‐matching functions (cmfs) has been formulated for small fields by using two groups of real observers: JAM, MM, CF and AY, JR, MR, JL, JA, FP. The measurements of these cmfs have been made using different experimental devices and methods and it has enabled us to propose a New Deviate Observer for small fields (JF‐DO). This new JF‐DO was derived from the average observer of our nine real observers, following the technique used by the CIE to establish the Standard Deviate Observer (CIE‐1989 SDO), which was established for fields of 10°, despite the CIE's assumption that it can be applied to smaller fields. In the present work, we report experimental results of the JF‐DO using metameric reflectances in comparison to the CIE‐1931 Standard Observer and to the CIE‐1989 SDO. © 2005 Wiley Periodicals, Inc. Col Res Appl, 30, 363–370, 2005; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/col.     

Determination of unique hues using Munsell color chips

Forty observers determined their unique hues from arrays of Munsell chips in a standard surround under artificial daylight. There was some discrepancy in the results of males and females. Essentially no variability due to age of observer was found. The standard deviation around the total mean was less than 1 Munsell 40 hue step. Simple linear opponent‐color a and b values were calculated. The ranges were found to straddle in all but the red color the system axes for the CIE 2°observer, but not for the 10°observer. NCS unique hues determined at similar chroma and lightness values fell in all cases within the ranges. The shifts in cross‐over wavelengths of the color‐matching functions necessary to match the extreme range values were determined to be between 6–11 nm. The results provide support for an opponent‐color system based on subtractions of color‐matching functions. They also point to significant variation in color normal observers. © 2001 John Wiley & Sons, Inc. Col Res Appl, 26, 61–66, 2001     

The use of metamers to compare the color vision of observers

In this research we compare the colorimetric behavior of several observers. For color centers recommended by CIE we have produced large sets of spectral distributions, which are metameric for the CIE 1931 standard observer. For each one of the color centers, we compare the clouds of chromaticity coordinates with the chromaticity thresholds. We define a parameter that provides a quantitative measure of the interobserver variability. This parameter is used to arrange the observers by their degree of likeness. A similar procedure has been used to compare two real observers. It is shown how there is no reciprocity between the colorimetric behavior of two real observers. © 2001 John Wiley & Sons, Inc. Col Res Appl, 26, 262–269, 2001     

Colour matching experiments with RGB‐LEDs

CIE colorimetry breaks down when lights produced by narrow band RGB‐LEDs are matched with broad‐band lights. A colour matching experiment was set up and matches in a number of parts of the chromaticity diagram have been made, to determine the magnitude of the discrepancy. Differences between visual and instrumental matches increase as one moves in the chromaticity diagram from yellowish white lights toward greenish and bluish lights. CIE TC 1‐36 recently suggested newly defined cone fundamentals: Applying a transformation of these to a space similar to the CIE XYZ space enables a much better prediction of the matches to be made. The difference between the visual match and its instrumental prediction decreases by a factor of two or even more. The use of a cone fundamental based colorimetric system is recommended for LED colorimetry. © 2008 Wiley Periodicals, Inc. Col Res Appl, 33, 108–112, 2008     

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     

Categorical observers for metamerism

A method is proposed to generate categorical colour observer functions (individual colour matching functions) for any field size based on the CIE 2006 system of physiological observer functions. The method combines proposed categorical observer techniques of Sarkar et al with a physiologically-based individual observer model of Asano et al and a clustering technique to produce the optimal set of categorical observers. The number of required categorical observers varies depending on an application with as many as 50 required to predict individual observers' matches when a laser projector is viewed. However, 10 categorical observers are sufficient to represent colour-normal populations for personalized colour imaging. The proposed and recommended categorical observers represent a robust and inclusive technique to examine and quantify observer metamerism in any application of colorimetry.     

Comparison of real colour gamuts using a new reflectance database

A large set of data, comprising the spectral reflectances of real surface colours, has been accumulated. The data comprise 16 groups with different materials and include 85,879 measured spectra. From these data, CIELAB colorimetric coordinates were calculated under CIE illuminant D50 and the CIE 1931 standard colorimetric (2°) observer. Several published colour gamuts including those developed by Pointer and ISO reference colour gamut [ISO Graphic Technology Standard 12640‐3:2007] were compared using the present data set. It was found that the Pointer gamut is smaller than the new real data in most of the colour regions. The results also showed that the ISO reference colour gamut is larger than the new real accumulated data in most regions. The present finding indicates that there is a need to derive a new colour gamut based on the newly accumulated data for common applications. © 2013 Wiley Periodicals, Inc. Col Res Appl, 39, 442–451, 2014     

New deviate observer (JF‐DO) obtained from experimental color‐matching functions for small fields of real observers

The CIE established the Standard Deviate Observer (SDO) CIE 1989 for fields of 10°, enabling the evaluation of discrepancies caused by the variability among these observers. This observer could also be applied to smaller fields, depending on the physiological causes of this variability in color‐matching functions (cmf's) among observers. Here, we have obtained a new Deviate Observer (which we call JF‐DO) established from the cmf's for small fields (2°) corresponding to two groups of real observers: JAM, MM and CF; AY, JR, MR, JL, JA and FA. Both groups of cmf's were measured experimentally in our laboratories using one for each of the different experimental methods and devices. All the new cmf's of the 9 real observers were referred to a new, unique system of unreal primaries, which we call X′Y′Z′ (derived in a way similar to that of the CIE 1931 XYZ system of unreal primaries). To establish a new JF‐DO for small fields, we followed a procedure similar to the one used by the CIE to establish the CIE 1989 SDO. A comparative study was also made between the cmf's of the CIE 1989 SDO (established for fields of 10°), the SDO from Stiles‐Burch (which we call Poza‐SDO, developed for small fields), and our JF‐DO. For this comparison, the cmf's of all these deviate observers were referred to the new system of unreal primaries X′Y′Z′. © 2003 Wiley Periodicals, Inc. Col Res Appl, 28, 209–215, 2003; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/col.10146     

The impact of viewing conditions on observer variability for cross‐media colour reproduction

This study investigates observer metamerism in cross‐media colour reproduction (CMCR) between monitor and physical colours. An LCD display was placed inside a viewing cabinet. The cabinet had a mid‐grey interior. Observers were asked to match a colour displayed on the monitor to a printed colour patch, which was also inside the viewing cabinet in two configurations, one consisting of two samples separated by a hairline gap (Configuration 1), and the other consisting of two samples separated by a large gap (Configuration 2). Eleven observers were asked to first match the background colour and then 10 test colours for each configuration, and this was repeated five times. The observers’ variability results showed little difference between inter‐ and intra‐variability, and between the two configurations. Comparing the observers’ results with the CIE 1964 standard colorimetric observer, Configuration 1 and Configuration 2 each showed similar agreement. In both configurations, the results of observer variability were smaller than those obtained by Oicherman et al. in 2008. In addition, each configuration's results showed better agreement with the CIE standard colorimetric observer than Oicherman et al.'s results. This implies that both configurations, one with two samples having a hairline gap and with two samples separated by a large gap in a viewing cabinet, could be recommended for future CMCR systems. However, if choosing between the two, then Configuration 2 is recommended rather than Configuration 1.     

Adaptation and colour matching of display and surface colours

In an asymmetric colour matching experiment, eleven observers adjusted computer displays to colour‐match surface samples in a viewing booth. We found systematic discrepancies between the observers' judgments and the predictions of the CIE 1964 Standard Colorimetric Observer. The features of the discrepancies are consistent with previous reports on adaptation in colour matching and on failures of colorimetric additivity, but have never been confirmed to be significant in practical colorimetry. We attribute the discrepancies to post‐receptoral adaptation mainly of the blue‐yellow chromatic channel, and report a framework of an adaptation transform based on the MacLeod‐Boynton chromaticity diagram which can compensate for them without abandoning traditional colorimetry and the use of tristimulus values. © 2009 Wiley Periodicals, Inc. Col Res Appl, 34, 182–193, 2009     

Color matching functions when one primary wavelength is changed

Color‐matching functions (cmfs) produced by monochromatic primaries change in an orderly way when the wavelengths of the primaries are changed. When only one of the three wavelengths is varied, the corresponding cmf changes in scale but not in shape. That is, changing one primary changes all three cmfs, but if only the red primary is changed, for example, then the red cmf changes only in scale. A set of primaries will exist such that each cmf has a maximum value of 1, and that peak occurs at the primary wavelength, by a prior theorem. Those are Thornton's prime colors, and if they are the initial primaries, then changing one primary wavelength can only increase the scale of its cmf. Precise prime color sets have been calculated: (603, 538, 446) for the CIE 2° observer, and (600, 536, 445) for the CIE 10° observer. © 2006 Wiley Periodicals, Inc. Col Res Appl, 32, 22–24, 2007; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/col.20283