The effects of various wavelength ranges of ultraviolet radiation (UVA, 320 to 400 nm; UVB, 290 to 320 nm; and UVC, 200 to 290 nm) on psoriatic plaques were studied. UVA, UVB, and UVC individually induced no clinical or histologic response. However, using a combination of UV (A and B) in a proportion mimicking summer sunlight, a beneficial effect was noted using clinical and histologic criteria. Clinically, one sees an area of clearing in a psoriatic plaque corresponding to the site of UV irradiation. Histologic improvement is observed as diminished acanthosis and parakeratosis and a disappearance of Munro microabscesses, No beneficial response was elicited in patients whose disease was exacerbated during the summer months.
Stern WK, Kihiczak G. Photobiology of PsoriasisWavelength Dependence. Arch Dermatol. 1974;109(4):502-505. doi:10.1001/archderm.1974.01630040012002