Heliopan 24mm IR BG (39) Filter (702473)Heliopan IR (Infrared) BG (BG3 Blue Glass) is a type of optical filter, often made from blue tinted Schott glass, that blocks most visible light while allowing infrared (IR) wavelengths to pass, used primarily in scientific imaging or specialized infrared photography to create dramatic effects, though sometimes "IR filter" refers to the IR cut filter in cameras that does the oppositeblocking IR to keep colors accurate.