Wood's glass is an

optical filter An optical filter is a device that selectively transmittance, transmits light of different wavelengths, usually implemented as a glass plane or plastic device in the optical path, which are either dyed in the bulk or have interference (optics), ...

glass invented in 1903 by American physicist Robert Williams Wood (1868–1955), which allows ultraviolet and

infrared light Infrared (IR), sometimes called infrared light, is electromagnetic radiation (EMR) with wavelengths longer than those of Light, visible light. It is therefore invisible to the human eye. IR is generally understood to encompass wavelengths from ...

to pass through, while blocking most visible light.

History

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 Fluorescence is the emission of light by a substance that has absorbed light or other electromagnetic radiation. It is a form of luminescence. In most cases, the emitted light has a longer wavelength, and therefore a lower photon energy, tha ...

and

incandescent Incandescence is the emission of electromagnetic radiation (including visible light) from a hot body as a result of its high temperature. The term derives from the Latin verb ''incandescere,'' to glow white. A common use of incandescence i ...

ultraviolet bulbs ("

black light A blacklight, also called a UV-A light, Wood's lamp, or ultraviolet light, is a lamp that emits long-wave ( UV-A) ultraviolet light and very little visible light. One type of lamp has a violet filter material, either on the bulb or in a sepa ...

s"). In recent years, due to its disadvantages, other filter materials have largely replaced it."...a BLB

lack light bulb Lack may refer to: Places * Lack, County Fermanagh, a townland in Northern Ireland * Lack, Poland * Łąck, Poland * Lack Township, Juniata County, Pennsylvania, US Other uses * Lack (surname) * Lack (manque), a term in Lacan's psychoanalyti ...

has a thin coating of a visible wavelength (VIS) filter generally applied to the inner wall of the bulb" from ''See also the 2009 report'':

Composition

Wood's glass is special

barium Barium is a chemical element with the symbol Ba and atomic number 56. It is the fifth element in group 2 and is a soft, silvery alkaline earth metal. Because of its high chemical reactivity, barium is never found in nature as a free element. The ...

- sodium-

silicate glass Glass is a non-crystalline, often transparent, amorphous solid that has widespread practical, technological, and decorative use in, for example, window panes, tableware, and optics. Glass is most often formed by rapid cooling (quenching) of ...

incorporating about 9%

nickel oxide Nickel oxide may refer to: * Nickel(II) oxide Nickel is a chemical element with symbol Ni and atomic number 28. It is a silvery-white lustrous metal with a slight golden tinge. Nickel is a hard and ductile transition metal. Pure nickel is ...

. 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.

Properties and uses

Wood's glass has lower

mechanical strength The field of strength of materials, also called mechanics of materials, typically refers to various methods of calculating the stresses and strains in structural members, such as beams, columns, and shafts. The methods employed to predict the re ...

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

hydroxide Hydroxide is a diatomic anion with chemical formula OH−. It consists of an oxygen and hydrogen atom held together by a single covalent bond, and carries a negative electric charge. It is an important but usually minor constituent of water. It ...

s and

carbonate A carbonate is a salt of carbonic acid (H2CO3), characterized by the presence of the carbonate ion, a polyatomic ion with the formula . The word ''carbonate'' may also refer to a carbonate ester, an organic compound containing the carbonate g ...

s 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 The Sabatier effect, also known as pseudo-solarization (or pseudo-solarisation) and erroneously referred to as the Sabattier effect, is a phenomenon in photography in which the image recorded on a negative or on a photographic print is wholly o ...

, gradually losing transparency for UV. Photographic filters for

ultraviolet photography Ultraviolet photography is a photographic process of recording images by using radiation from the ultraviolet (UV) spectrum only. Images taken with ultraviolet radiation serve a number of scientific, medical or artistic purposes. Images may rev ...

, notably the Kodak Wratten 18A and 18B, are based on Wood's glass.

Health effects

Bulbs made of Wood's glass are potentially hazardous in comparison with those made of enameled glass, since the reduced visible light output may cause observers to be exposed to unsafe levels of UV, because the source appears dim. The low output of black lights is not considered sufficient to cause DNA damage or cellular mutations, but excessive exposure to UV can cause temporary or permanent damage to the eye.

References

* * * {{Glass makers and brands Optical filters Glass trademarks and brands Glass compositions American inventions