Minimum phototoxicity dose (MPD). Comparison of conventional light sources and different lasers.

A pilot study was performed on groups of 5-7 volunteers to evaluate the minimum phototoxicity dose (MPD) of several light sources with different UVA qualities to optimise photochemotherapy. Visual observation was more suitable than reflection photometry in determining the threshold and gradation of erythema reactions; thermometry gave poor results. There were no individual differences between the MPD of a PUVA 4000 box and a continuously emitting argon-ion laser with 40 and 400 mW, respectively. The pulsed radiation of a nitrogen laser was the most effective. The limited validity of the Bunsen-Roscoe law for high peak power could thus be tested for a secondary photochemical reaction of the skin. The nitrogen laser (337.1 nm) proved that a repetition rate of 20 Hz was superior to 10 and 40 Hz. The MPD was intraindividually higher at 25 Hz than at 10 and 16.7 Hz for the dye laser tested (325 and 330 nm, respectively).