Colorimetric purity discrimination: data and theory.

Colorimetric purity, measured as the first step from white toward the spectrum has a V-shaped function. Purity discrimination is best near 400 nm, least at 570 nm and intermediate at mid-spectrum and long wavelengths. A much flatter function occurs when colorimetric purity is measured as the first step from the spectrum toward white. In this study, we applied the formulation of chromatic discrimination thresholds measured along the S-cone and M/L-cone axis to account for chromatic discrimination in the equiluminant plane. The modeling results show that the purity step from white has a 1.6 log unit calculated range, similar to the classical data. The purity step from the spectrum is much flatter. The predicted range is dependent on the individual variance in chromatic discrimination thresholds and the luminance level. We then used psychophysical procedures to test the model's predictions. The resulting purity discrimination functions were generally in agreement with the model. Our modeling indicates that discrepant data of colorimetric purity can be explained with the context of discrimination models.