The response of the skin to increasing doses of ultraviolet light (UVL) (simulating the ultraviolet component of sunlight) was evaluated in a series of 115 individuals, 53 of whom were considered to have some disease process associated with UVL. The clinically normal response consisted of erythema of the skin produced by one of a geometrically increasing series of doses of UVL. Induration, papules, vesicles, or urticaria were considered to be abnormal responses. These abnormal responses reproduced histopathologically the clinical disease state of the patient. The dose of UVL required to produce either erythema or an abnormal skin response was significantly different in normal patients than in those with light associated disease.
Stern WK, Urbach F. The Diagnostic Significance of the Minimal Erythema Dose. Arch Dermatol. 1972;105(3):387-393. doi:10.1001/archderm.1972.01620060029005