The macroscopic and microscopic reactions of nonpsoriatic and clinically uninvolved psoriatic skin to ultraviolet radiation (UVR) showed no gross differences. The erythema threshold was decreased in subjects whose psoriasis was in an acute, eruptive stage. On the basis of studies of epidermal DNA synthesis, ultraviolet radiation resulted in an injury response at 48 hours in normal subjects and in psoriatic patients in whom the disease was in an acute, eruptive stage. The injury response to UVR was significantly decreased in those subjects with chronic, nonerutive psoriasis and the reason for this is unexplained.
Hodgson C, Hell E. Ultraviolet Radiation and Psoriasis. Arch Dermatol. 1972;106(4):498-502. doi:10.1001/archderm.1972.01620130030007