Wood's glass

Two "black light" ultraviolet fluorescent tubes. The glass envelopes illustrate the dark blue color of Wood's glass, although these modern tubes actually use another optical filtering material.

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.^[1]

History

Wood's glass was developed as a light filter used in communications during World War I.^[2][3] 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).^[4]

Composition

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.

Properties and uses

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 "black light" bulbs are made of structurally more suitable glass with only a layer of a 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.

With prolonged exposure to ultraviolet radiation, Wood's glass undergoes solarization, gradually losing transparency for UV.

Photographic filters for ultraviolet photography, notably the Kodak Wratten 18A and 18B, are based on Wood's glass.^[5]

Health effects

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 be exposed to unsafe levels of UV light as the source looks less bright. The weak output of black lights is not considered sufficient to cause DNA damage or cellular mutations, but excessive exposure to UV light can cause temporary or permanent damage to the eye.

See also

• Wood's lamp
• Black light
• Dichroic filter

References

• R.W. Wood. "Secret communications concerning light rays". Journal of Physiology 1919, 5e serie: t IX
• J. Margarot & P. Deveze. "Aspect de quelques dermatoses lumiere ultraparaviolette. Note preliminaire". Bulletin de la Société des sciences médicales et biologiques de Montpellier 1925, 6: 375–378.
• Robin Williams & Gigi Williams. "Pioneers of Invisible Radiation Photography"