Estimating Individual Cone Fundamentals:A Fitting Exploration Using Heterochromatic Flicker Photometry

Previous studies indicate that spatial color models fail to accurately predict the perception of temporally modulated colored light, which can lead to unpleasant perceptual effects like flicker or jerkiness. Additionally, color perception varies substantially across individuals, even those with normal color vision. While most models rely on a standard observer, recent efforts (e.g., by the International Commission on Illumination) have begun modeling individual observers. This study gathered foundational data for the purpose of developing a temporal uniform color space accounting for individual differences in cone spectral sensitivities. Two psychophysical experiments were conducted. In Experiment 1, we measured isoluminance ratios for chromatic temporal modulations across 15 base colors and 4 modulation directions for 15 participants. For each participant, adjustments between two chromatic extremes identified the point of minimal perceived flicker, yielding 60 isoluminance ratios (i.e., 15 colors × 4 directions). These ratios estimated individual cone spectral sensitivities using an adapted Individual Colorimetric Observer (ICO) model. The ICO model outperformed the standard observer in predicting perceived luminance. Building on these results, we assessed temporal contrast sensitivity at 15 base colors, 4 modulation directions, and 3 temporal frequencies for 3 participants (selected from Experiment 1 based on divergent isoluminance ratios) in Experiment 2. The results provided initial insights into the extent of individual differences in sensitivity to chromatic temporal modulations. We introduce two metrics—circularity and homogeneity—to quantify local and global uniformity in color spaces. The Derrington-Krauskopf-Lennie (DKL) space showed greater uniformity than the cone-based LMS space, and ICO-based fundamentals enhanced temporal uniformity. Surprisingly, the CIE 1976 UCS diagram (10°) demonstrated the highest circularity and homogeneity overall. We discuss these findings and propose future research to design perceptually uniform temporal color spaces.