Characterization of Color Appearance of Self-Luminous Highlights Under HDR Scenes, Part I: Brightness for Neutral Stimuli

Color appearance characterization of stimuli is important in color science, with great efforts made to develop uniform color spaces (UCSs) and color appearance models (CAMs). These UCSs and CAMs, however, were mainly developed for stimuli with luminance below the diffuse white in a standard dynamic range (SDR) scene. This study was carefully designed to investigate the color appearance of self-luminous highlight stimuli, in terms of relative brightness, in a high dynamic range (HDR) scene, and to see whether a new model for characterizing the brightness of highlights is needed. The observer viewed a dark stimulus (Stimulus 0) and a self-luminous highlight (Stimulus 2) simultaneously under a certain diffuse white luminance level, which was an HDR scene, and adjusted the luminance of the other self-luminous highlight (Stimulus 1) so that its brightness appeared in the middle of Stimuli 0 and 2. The luminance of Stimulus 2 was designed to as high as 49 000 cd/m², the luminance contrast between Stimulus 2 and Stimulus 0 was as high as 72 045:1, and the diffuse white luminance level was designed to between 10 and 11 000 cd/m². The results suggested that the UCSs and CAMs adopting a power function to characterize the non-linear relationship between the relative luminance and perceived lightness (e.g., IPT, CIECAM02, and CIELAB) had much better performance to characterize the relative relationship of the lightness among the stimuli. Significant variations, however, were found among these UCSs and CAMs in characterizing the absolute magnitude of lightness and chroma, which merits further investigations.