| Definition: | factor describing the protective quality of a sunscreen against solar ultraviolet radiation, normally determined by in vivo testing Note 1 to entry: The sun protection factor is usually determined by comparing the dose of ultraviolet radiation from a solar simulator (often a xenon lamp) required to produce just-perceptible actinic erythema approximately 24 h after exposure in ultraviolet-irradiated human volunteers, with and without a standard amount of the sunscreen being present (FMED,f and FMED,i, respectively) and is calculated as follows: FSP = FMED,f / FMED,i. Note 2 to entry: The sun protection factor can also be determined by equivalent in vitro methods. Note 3 to entry: The sun protection factor has unit one.
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