The onset of ultraviolet erythema.

The onset of erythema induced by UVB and by UVC radiation was monitored in two subjects using a reflectance instrument. It was shown that vasodilation occurred some time before erythema became visible, even with small exposure doses of UVB or UVC, after which visible erythema was not apparent for several hours. In one subject, vasodilation was detected very shortly after irradiation and may have even begun during irradiation. In the other subject vasodilation was detected later at most exposure doses, but still some time before erythema became visible. We conclude that the so-called 'latent period' between irradiation and appearance of erythema is an artificial concept arising from the insensitivity of the eye.