The inflammatory response of the skin to UV irradiation is the vasodilation of cutaneous blood vessels resulting in erythema, or reddening of the skin.
The CIE erythemal action spectrum, published in CIE S 007/E-1998 (ISO 17166:1999) describes the efficiency of UV wavelengths to induce erythema.
This action spectrum is the basis of the standard erythemal dose unit, the minimum erythemal dose and the UV index used for public information on solar UV exposure.
Link to - UV INDEX
The complex range of interactions between tissue and UV light are largely photochemical in nature, following the Bunson-Roscoe law of reciprocity which states that the biological response is proportional to dose and not on the profile of exposure.
Since the skin can be exposed to light from the entire hemisphere above its surface, the measurement quantity to consider is that of spectral irradiance. Erythemal irradiance is computed from integration of the product of spectral irradiance and the erythema action curve. The standard erythemal dose is defined as 100 J.m-2 erythemal irradiance.
The minimum erythemal dose, to induce just-perceptible erythema can be determined through exposing skin to various erythemal doses followed by evaluation of the exposure sites, or estimated according to skin type.
|I||Fair skinned Caucasians who burn very easily and never tan||2|
|II||Fair skinned Caucasians who burn easily and tan slowly and with difficulty||2.5|
|III||Medium skinned Caucasians who burn rarely and tan relatively easily||3|
|IV||Darker skinned Caucasians who virtually never burn and tan readily, e.g. some individuals with Mediterranean ancestry||4.5|
|V||Asian or Indian skin||6|
|VI||Afro-Caribbean or Black skin||10|