Erythemal irradiance is frequently expressed in terms of the UV index = 25 mW per m2 = 2.5 ^W per cm2. The index is an arbitrary unit such that very high values reported by weather services have a UV index of 10 (25 ^W per cm ). Erythemal exposure or dose is a time-integrated quantity normally expressed in kJ per m2. Many high altitude locations with extreme UV amounts can exceed 10 on clear days. The weather service for the populated region near Cuzco, Peru, frequently reports a UV index of 10 to 11 for extended periods and occasionally has a UV index that reaches 25, the latter based on TOMS ozone data analysis (Liley and McKenzie, 2006). Spectral irradiance is expressed in mW per m per nm. The solar irradiance has a peak value of about 2,100 mW m nm at about 450 nm. Sky radiance is expressed in mW m-2 nm-1 per steradian. Zenith sky irradiance for 0.5° FOV is of the order of 10-5 of the solar irradiance in the vicinity of 400 nm, but the ratio increases with decreasing wavelength because of increasing Rayleigh scattering in the UV wavelengths.
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