Phase shift in red and green counterphase flicker at high frequencies.

When balanced red and green lights are alternated more than 20 times per second, the perceived flicker can be reduced by advancing the green flicker about 10 degrees of the red-green cycle. The required advance for least flicker is greatest at retinal illuminances around 1000 td and frequencies between 30 and 35 Hz. A model that predicts tuning at this frequency exists, but the tuning curve that is predicted is broader than that observed. A modified model is left for future publication. Meanwhile, other empirical properties of the advance required by green over red are described. In addition to the intensity dependence of this phase shift, we describe its dependence on intensity balance between red and green. Also, the intensity balance turns out to depend on the frequency being used, in contrast to the independence expected by Ives, the inventor of heterochromatic flicker photometry.