the world > matter > physics > electromagnetic radiation > light > [noun] > state of equilibrium (3)
a state of equilibrium in regard to the effect of incident light, e.g. in a photochemical reaction.
the apparent steadiness of a regularly varying source of light when the frequency of the variation is sufficiently great.
the lowest rate of variation at which flicker is not perceptible; also called critical flicker (also fusion) frequency.