This B+W filter eliminates the green cast that occurs when daylight-type color film is in fluorescent lighting. Examples are interior photographs made in offices, reception areas, factory halls, and subway stations, as well as night-time photographs of office buildings whose windows would appear green because their interiors are usually illuminated with fluorescent tubes. The abbreviation F-Day stands for Fluorescent Light - Daylight , which reflects the fact that these filters are designed for the most commonly used type of fluorescent tube, which emit light of a color that resembles daylight.