Brightness of different hues is a single psychophysical ratio scale of intensity.

Recent studies based on testable behavioral axioms have concluded that psychological scales of subjective intensive attributes involving the ears and eyes form ratio scales. These studies have shown that a certain commutativity of proportion property must hold under either successive increases or successive decreases, with all other independent dimensions fixed. However, until recently limited attention has been paid to whether such subjective intensity scales differ when a dimension independent of intensity, such as frequency or wavelength (e.g., pitch in audition, hue in vision) is varied. Using a simple and favorably tested theoretical model for global psychophysics, Luce, Steingrimsson, and Narens (2010) arrived at a necessary and sufficient cross-frequency, commutativity condition for there to exist a common intensity ratio scale. Here we show that brightness--already established to be a ratio-scalable dimension--and hue satisfy the same conditions.