Linearity law reexamined for flicker photometry by the summation-index method.

The summation-index technique was applied to flicker photometry to investigate the linearity law. Two test stimuli of different wavelengths were mixed, and the luminous efficiency of the mixture was measured under three different adapting conditions: no adaptation, 507-nm adaptation, and 650-nm adaptation. Most of the wavelength combinations and adapting conditions gave a summation index of 0.30, which is the linearity in the visual system that is responsible for the flicker perception, namely, the achromatic channel. An exception was the red and green combination under the no-adaptation condition, which gave a slightly smaller summation index than 0.30, indicating that these two stimuli do not add linearly. The subsummation was interpreted as cancellation of red and green responses in the red-versus-green opponent channel. The subsummation disappeared when the flicker frequency was increased from 5 to 12 Hz, confirming the interpretation.