The angular distributions of ultraviolet spectral irradiance at different solar elevation angles under clear sky conditions

To investigate the angular distributions of UVA, UVB, and effective UV for erythema and vitamin D (vitD) synthesis, the UV spectral irradiances were measured at ten inclined angles (from 0A degrees to 90A degrees) and seven azimuths (from 0A degrees to 180A degrees) at solar elevation angle (SEA) that ranged from 18.8A degrees to 80A degrees in Shanghai (31.22A degrees N, 121.55A degrees E) under clear sky and the albedo of ground was 0.1. The results demonstrated that in the mean azimuths and with the back to the sun, the UVA, UVB, and erythemally and vitD-weighted irradiances increased with the inclined angles and an increase in SEA. When facing toward the sun at 0A degrees-60A degrees inclined angles, the UVA first increased and then decreased with an increase in SEA; at other inclined angles, the UVA increased with SEA. At 0A degrees-40A degrees inclined angles, the UVB and erythemally and vitD-weighted irradiances first increased and then decreased with an increase in SEA, and their maximums were achieved at SEA 68.7A degrees; at other inclined angles, the above three irradiances increased with an increase in SEA. The maximum UVA, UVB, and erythemally and vitD-weighted irradiances were achieved at an 80A degrees inclined angle at SEA 80A degrees (the highest in our measurements); the cumulative exposure of the half day achieved the maximum at a 60A degrees inclined angle, but not on the horizontal. This study provides support for the assessment of human skin sun exposure.