Figure 2. Light levels in contemporary and natural environments39 54–57 and also in phototherapy for seasonal affective disorder, which is typically 2500 lux for 2 h/day or 10 000 lux for 30 min/day.44 Illuminances are given in units of photopic lux. Photopic lux accurately describe the effectiveness of a particular light exposure for overall cone photoreception, which has a peak sensitivity at 555 nm in the green–yellow part of the spectrum (cf, fig 1). A standard circadian lux unit is needed10 40 but has not been adopted yet for comparing the effectiveness of different light exposures for circadian photoreception, which has peak sensitivity at 460 nm in the blue part of the spectrum (cf, fig 1).
