Amendment of CIECAM02 with a technical extension to compensate Helmholtz-Kohlrausch effect for chromatic characterization of display devices☆
Introduction
CIECAM02 is the color appearance model published in 2002 by the International Commission on Illumination (CIE) Technical Committee 8-01 (Color Appearance Modeling for Color Management Systems) [1]. Among the various color appearance models, CIECAM02 has been recommended as an international standard in both self-luminous and surface colors because of its simplicity and reversibility [2], [3], [4], [5]. Despite being able to predict most color appearance phenomena, CIECAM02 does not include the color appearance phenomena associated with the influence of rod photoreceptors such as Helmholtz-Kohlrausch (H-K) and Helson-Judd effects [6].
Helmholtz-Kohlrausch (H-K) effect refers to the color appearance phenomenon that colored object (or light) appears brighter than achromatic object (light) of the same luminance [7], [8], [9], [10], [11]. This effect is best illustrated by examining the contours of constant brightness-to-luminance ratio as shown in Fig. 1 [12]. The contours represent chromaticity loci of a certain perceived brightness at a constant luminance in CIE 1931 x-y plane. The labels on the contours indicate the brightness of the colors relative to the white. These contours show that perceived brightness increases as the saturation increases despite the same luminance. In addition, a hue dependency of the H-K effect was also represented.
The effectiveness of CIECAM02 had been mostly validated in the early 2000s. At that time, the lack of the prediction of the H-K effect on CIECAM02 was not a big issue because it was concluded that CIECAM02 worked well for display devices in most studies [13], [14], [15], [16]. The display devices in those days had color gamut of 40–70% of sRGB standard. In other words, the saturation of the color represented by the display devices could not be inherently high. Since the H-K effect is a color appearance phenomenon observed in colors with high saturation, the H-K effect might not be observed in display devices with small color gamut. However, we found out that the H-K effect was prominently observed when the psychophysical experiments were performed using a liquid crystal display (LCD) with color gamut close to sRGB standard. In addition, the saturation of colors represented by display devices is getting higher owing to development of active matrix organic light emitting diode (AMOLED) displays and quantum dot LCD (QD-LCD), etc. Therefore, in order to extend the universal applicability of CIECAM02 as an international standard for characterizing display devices, compensation of the H-K effect becomes an essential issue and deserves further consideration.
Section snippets
Hardware setup
A 15-in. active-matrix LCD (AMLCD) monitor panel was used for this study; its specifications are listed in Table 1. Color gamut of the AMLCD panel was measured by a Minolta CS1000 tele-spectroradiometer. CS1000 measures spectral power distribution from 380 nm to 780 nm in 1 nm step. Fig. 2 shows the measured color gamut of the AMLCD panel in CIE 1931 x-y plane. As shown in Table 2, the color gamut of the AMLCD panel was 98% of the gamut size of the sRGB standard.
A viewing cabinet was used in
Conventional CIECAM02
Fig. 5 shows the block diagram of CIECAM02. By putting the tri-stimulus XYZ of the stimulus and several additional inputs into the CIECAM02, a series of processes are performed to predict various color appearance attributes including lightness [1] as shown in Fig. 5.
The tri-stimulus XYZ of the 28 test colors and the reference white (XWYWZW) of the AMLCD panel were measured by CS1000 as shown in Table 3. Furthermore, the luminance of the background (Yb) and the adapting field luminance (LA) were
Results and discussion
CIECAM02 is a much more advanced color appearance model compared to the CIELAB. Although CIECAM02 can predict most color appearance phenomena, it does not include the H-K effect. It is because it does not allow for predictions on the influence of rod photoreceptors on color appearance such as the Helson-Judd and H-K effects [6]. From the results of the psychophysical experiments, it was found out that the compensation of the H-K effect is necessary for CIECAM02 to be universally applied to the
Conclusion
CIECAM02 has been increasingly becoming the international standard of color appearance model, and its use is encouraged. However, even though it is able to predict most color appearance phenomena, it has the limitation that it cannot predict the lightness when the Helmholtz-Kohlrausch (H-K) effect occurs. H-K effect refers to the color appearance phenomenon that colored light appears brighter than achromatic light of the same luminance. In order to extend the universal applicability of CIECAM02
Conflict of interest
The authors declared that there is no conflict of interest.
Acknowledgement
This work was supported in part by the Basic Science Research Program through the National Research Foundation of Korea (NRF), funded by the Ministry of Education (NRF-2016R1D1A1B03935421) and in part by the BK21 Plus Program (Future-oriented innovative brain raising type, 21A20130000018), funded by the Ministry of Education (MOE, Korea) and the NRF.
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This paper was recommended for publication by Pen-Cheng Wang.