Human health effects of solar irradiance are waveband specific. While longer, thermal wavebands influence the thermal comfort of people, the shorter ultraviolet wavebands are associated with increased risk for various skin cancers. Radiation transport theory as well as observations show that the irradiance of these various wavebands is strongly dependent on the atmospheric and surface scattering characteristics. In open canopies, where the spacing between trees is equal to or greater than the individual tree crown, the irradiance is also strongly dependent on the relative positions of the individual trees. In the UV wavebands, atmospheric scattering results in diffuse fractions at mid-latitudes of 50% or more under typical clear skies. Measurement of the UV-A (315-400 nm)and UV-B (280-315 nm) irradiance in open canopies and in the vicinity of single trees has shown that the sky view from a location strongly influences the irradiance at that location, with irradiance in the shade of trees reduced to between 30% and 65% of the irradiance above the canopy. A three dimensional geometric model incorporating anisotropic sky radiance functions was developed to estimate the irradiance in open canopies in multiple wavebands. The model accuracy was evaluated by comparison to measurements in the UV-A, UV-B, PAR (photosynthetically active radiation; 400-700 nm).