In preparations (sunscreens) for protecting the skin from the sun, the ratio of the amount of exposure needed to produce a minimal erythema response with the sunscreen in place divided by the amount of exposure required to produce the same reaction without the sunscreen. This index assesses the ability of sunscreens to block (short-wavelength) ultraviolet B rays hut does not measure the protective effect of sunscreens against (long-wavelength) ultraviolet A radiation.