Wood's glass

Wood's glass is an optical filter glass invented in 1903 by American physicist Robert Williams Wood (1868–1955) which allows ultraviolet and infrared light to pass through while blocking most visible light. It was developed as a light filter used in communications during World War I. His "invisible radiation" technique worked both in infrared daylight communication and ultraviolet night communications. His glass filter removed the visible components of a light beam, leaving only the "invisible radiation" as a signal beam. Wood's glass has been commonly used to form the envelope for fluorescent and incandescent ultraviolet bulbs ("black lights"). In recent years, due to its disadvantages, other filter materials have largely replaced it (see below).