The erythema response of skin has been characterized as to (1) quantity of energy, (2) wavelength, (3) time of observation, and (4) anatomic site of testing. Both the MED [minimal erythema dose] and the color of the erythema response vary with wavelength. Short wavelength erythema is pink, while longer wavelength erythema is darker and redder. Although erythema varies with both time and wavelength, the optimum time for observation is approximately eight hours. The time of day at which the ultraviolet light (TJVL) is administered does not affect the 8-hr MED. The MED not only varies markedly with site but also the relative response to the different wavelengths varies with anatomic location. The trunk is best suited for erythema studies because of greater sensitivity and a more uniform response to UVL [ultraviolet light] at all wavelengths.