Wood's glass was developed by Robert Williams Wood (1868–1955) 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 is commonly used to form the envelope for fluorescent and incandescent ultraviolet bulbs ("black lights").
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.
A photographic filter, Kodak Wratten 18A, is based on Wood's glass.[1]
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. The nickel and barium oxides are also chemically reactive, with tendency to slowly form a layer of hydroxides and carbonates in contact with atmospheric moisture and carbon dioxide. The susceptibility to thermal shock makes manufacture of hermetically sealed glass bulbs difficult and costly, therefore most contemporary "Wood's glass" bulbs are made of structurally more suitable glass with only a layer of an UV filtering enamel on its surface; such bulbs however pass much more visible light, appearing brighter to the eye. Due to manufacturing difficulties, Wood's glass is now more commonly used in standalone flat or dome-shaped filters, instead of being the material of the light bulb. Bulbs made of Wood's glass are potentially hazardous in comparison with the ones made of enameled glass, as due to reduced visible light throughput it is easy for the observer to expose himself to unsafe levels of UV light as the source looks less bright. With prolonged exposure to ultraviolet radiation, Wood's glass undergoes solarization, gradually losing transparency for UV.
