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.

Wood's glass was developed as a light filter used in communications during World War I. The glass filter worked both in infrared daylight communication and ultraviolet night communications by removing the visible components of a light beam, leaving only the "invisible radiation" as a signal beam. Wood's glass was 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).

Wood's glass is special barium-sodium-silicate glass incorporating about 9% nickel oxide. It is a very deep violet-blue glass, opaque to all visible light rays except longest red and shortest violet. It is quite transparent in the violet/ultraviolet in a band between 320 and 400 nanometres with a peak at 365 nanometres, and a fairly broad range of infrared and the longest, least visible red wavelengths.

Some sources erroneously state presence of cobalt(II) oxide in Wood's glass.

Wood's glass has lower mechanical strength and higher thermal expansion than commonly used glasses, making it more vulnerable to thermal shocks and mechanical damage.

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