
D65 plays a critical role in color reproduction, specification, and characterization. Displays commonly use D65 chromaticities as the white point, but many studies shown that a stimulus produced by displays having a slightly higher CCT was perceived to produce neutral white appearance. Moreover, our two recent studies found that LCD/OLED displays need to produce a stimulus having a slightly higher CCT to match the appearance of a high quality D65 illuminant that was produced by 14 channel spectrally tunable LED device. We hypothesize that a higher CCT required by displays to produce a neutral white appearance was caused by the CIE 1931 Color Matching Functions and a D65 illuminant can indeed produce a neutral white appearance. In this study, we use the 14 channel LED device to produce a series high quality daylight simulators. The observers generally selected the stimulus having a CCT between 6346 and 6906 K to have a neutral white appearance. When using an LCD display to match the color appearance of the selected stimulus, the average CCT was between 7209 and 7943 K. Such a finding supports our hypothesis and suggests the importance of color matching functions in display specification and calibration.

Various uniform color spaces and color appearance models were mainly developed for characterizing stimuli under a low dynamic range condition. Real scenes in daily life, however, are commonly high dynamic range (HDR), containing highlights with luminance beyond the diffuse white, whose color appearance characterization was never investigated in the past. This study was carefully designed to investigate the color appearance characterization of highlights in HDR scenes, covering extremely wide ranges of diffuse white luminance (up to 11000 cd/m<sup>2</sup>), stimulus luminance (up to 49000 cd/m<sup>2</sup>), stimulus chromaticities (reach Rec. 2020 gamut), and scene luminance contrast (up to 72045). The observers viewed two stimuli, including one highlight and one dark stimulus, in a viewing booth, and were asked to adjust the color appearance of another stimulus, so that the color differences between the adjusted stimulus to each of the other two stimuli appeared the same. The results clearly showed that none of the existing models, including the one (i.e., IC<sub>t</sub>C<sub>p</sub>) that was recently designed for HDR scenes, has a good performance. The models using a power function to characterize the non-linear compressive responses of the human visual system (i.e., CIELAB and IPT) had a slightly better performance. The findings provided some guidance for performing tone mapping and chroma/saturation adjustments, and clearly suggest the necessity to carry out further work to develop a better model for HDR scenes.