Recent lines of research show that light is important not only to human vision but also to human circadian regulation. The photometric quantities based on the relative photopic luminous efficiency function are used to express lighting condition. However it has been shown that the spectra sensitivity curve for circadian regulation is different from the spectra sensitivity curve for the photopic visual system. In order to support lighting design considering circadian regulation, in this paper, a method to evaluate light based on melatonin suppression, one of circadian rhythms, was proposed. A formula, using spectrum distribution of radiant flux and action spectrum of melatonin suppression shown in a previous study, was developed to calculate the effect of light on human circadian regulation. Also a simplified formula was proposed in which illuminance and color temperature were used. The applicability of the simplified formula to actual spaces was tested by measuring light from lamps, the sun, and computer display and light in houses.
