Quantitative studies on cutaneous erythema induced by ultraviolet radiation.

A reflectance instrument was used to measure the variation in UVR-induced erythema at different positions on the back. The pre-irradiation erythema index decreased from top to bottom of the back but the increase in index remained constant for a fixed exposure dose. In contrast, the minimal erythema dose was higher at lower sites on the back. The measured erythemal response increased linearly with the logarithm of the radiation dose from approximately the minimal erythema dose up to at least fifteen times this value.