UV transmission of glass used for behind glass exposure tests is highly variable and is affected by the source of the glass and the spectrophotometer used for transmission measurement. The spectrophotometer is the most significant source of variability for wavelengths up to 300 nm. For wavelengths of 310 nm or longer, the source of the glass is the largest source of variability. UV transmission is related to iron and sulfur content of the glass, but the relationship is not strong enough to establish a meaningful specification. After three months exposure, UV transmission of window glass drops by 30–60% at 310 nm and by 25–50% at 320 nm. There is little change in UV transmission between three and thirty months, but the differences between glasses are still very significant. Variations in UV transmission cause tremendous differences in the short wavelength UV energy received by specimens being exposed behind different glasses. These differences can cause differences in degradation rate of 100–300%, depending on the relationship between intensity of UV radiation and rate of photochemical degradation.