Scaling Lightness and Chroma for Highlight Stimuli in HDR Scenes

Lightness and chroma, which are the key elements in uniform color spaces (UCS) and color appearance models (CAM), play critical roles in color characterization, specification, and reproduction. With the development of HDR display and imaging technologies, the characterization of the perceptual attributes of highlight stimuli with the luminance beyond the diffuse white luminance is becoming important. None of the existing UCS or CAM (i.e., CIELAB, CIECAM02, CAM02-UCS, IPT, ICtCp, and hdr-CIELAB), however, was developed for such purposes. In this study, we carefully designed three experiments to investigate the lightness and chroma scales of highlight stimuli in HDR scenes. The results clearly suggested the poor performance of these models, especially the overestimation of the chroma for highlight stimuli. CIECAM02 can be considered relatively better among the six models for color grading on an HDR display with a diffuse white luminance of 203 cd/m² and a peak luminance of 1000 cd/m².