Using the paired comparison technique in psoriatic patients, 1% crude coal tar and ultraviolet light (UVL) >320 nm (Wood filter) was less effective than 1% crude coal tar and UVL from 290 to 240 nm. The addition of the Wood filter following 1% crude coal tar and 290 to 400 nm UVL did not enhance the therapeutic response. These studies suggest that long-wave UVL contributes little, if at all, to the Goeckerman treatment, short-wave UVL being the effective radiation.
Marsico AR, Eaglstein WH. Role of Long-Wave Ultraviolet. Arch Dermatol. 1973;108(1):48-49. doi:10.1001/archderm.1973.01620220020004