A tough transparent plastic (Mylar) has the properties of (1) steep transmission curve, (2) total cutoff in the 3,000 to 3,200 Angstrom range, (3) availability in large sheets, and (4) low cost. However, its percent transmission is decreased by ultraviolet light exposure. For this reason, it must be changed occasionally during prolonged use as an ultraviolet filter if the amount of transmission at any particular wavelength is of critical importance.
Sams WM. The Effect of Ultraviolet Light on a Transparent Plastic. Arch Dermatol. 1967;95(2):225-227. doi:10.1001/archderm.1967.01600320081017