Conversion factors for ultraviolet radiation dosage with different types of ultraviolet therapy.

Erythema thresholds were determined using 4 different UV units before and after photosensitization with psoralens or coal tar in healthy volunteers. Conversion factors for equivalent erythema-producing UV doses with the different types of treatment were calculated on the basis of the erythema thresholds. A simple mode of transfer of patients between the different treatments with good precision of UV dosage is thus offered.