The biologic effectiveness of ultraviolet light

The biologic effects of visible and UV light result from photochemical changes in cell components. The amount of photochemical change induced in a small non-self-shadowing structure is proportional to the number of photons traversing it per unit area normal to the direction of propagation, summed over all component beam directions. Within an optically complex, absorbing, and scattering structure, this quantity is difficult to determine, but for skin it is approximately proportional to the total number of photons per unit area entering its outer surface. The magnitude of some photobiologic effects depends on the total amount of photochemical change induced, whereas others depend on the rate of photoproduct formation or on a more complex relation. The nature of the dependence must be determined before light measurements can be related to the magnitude. The effect of a polychromatic illumination depends on its wavelength distribution, weighted by the effectiveness of each wavelength (the action spectrum) under the conditions employed. Until the latter is known, no dosimetric characterization of the light is possible. The wavelength distribution can be determined by spectroradiometric measurement, with the weighting performed numerically, or (more conveniently, though less accurately) by the use of an analog reaction with an action spectrum like that of the photobiologic effect.