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. Because fluorescent tubes are not "thermal radiators", they do not produce a continuous spectrum like those of the sun and incandescent bulbs. Instead, they emit a sharply defined line spectrum that has high intensity spikes in the green region. Our eyes barely perceive this special green, but most color films are especially sensitive to those wavelengths and they react with a strong pronounced cast. For hobby photographers who do not wish to make a large investment in special instruments and who do not care to carry along a bunch of filters, the B+W F-Day Filter is far less expensive and much more practical solution. Its filter factor is approximately 2.