Measurement and modeling of diffuse ultraviolet radiation: A review

The ultraviolet (UV) radiation environment consists of direct UV and diffuse UV components. The ratios of diffuse-to-direct UV for the UVB, UVA, and the erythema UV wavebands are influenced by clouds, aerosols, albedo, and surface reflectance, Rayleigh scattering, and solar zenith angle. At times, the relative proportion of diffuse UV may be higher than the direct UV, for example, on cloudy days or days with high atmospheric aerosols. Consequently, exposures due to diffuse UV radiation play a significant role in the UV exposure received by human subjects. Diffuse UV radiation contributes to the risk of skin cancer and sun-related eye disorders. Reducing personal exposure to diffuse UV by physical protection, including tree shade, purpose-built shade structures, protection by hats, and eyewear, is more difficult than reducing direct UV exposure due to the diffuse UV being incident from all directions. The UV index reported to the public represents the sum of the direct and the diffuse erythema UV, with no information provided specifically on the diffuse UV component. This paper reviews the factors that contribute to diffuse UV exposures affecting human populations. It examines diffuse UV modeling techniques, and broadband and spectral component measurements.