Wood's glass is an optical filter glass invented in 1903 by American physicist

Robert Williams Wood Robert Williams Wood (May 2, 1868 – August 11, 1955) was an American physicist and inventor who made pivotal contributions to the field of optics. He pioneered infrared photography, infrared and ultraviolet photography. Wood's patents and theo ...

(1868–1955), which allows

ultraviolet Ultraviolet (UV) is a form of electromagnetic radiation with wavelength from 10 nanometer, nm (with a corresponding frequency around 30 Hertz, PHz) to 400 nm (750 Hertz, THz), shorter than that of visible light, but longer than ...

and infrared light 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 World War I (28 July 1914 11 November 1918), often abbreviated as WWI, was one of the deadliest global conflicts in history. Belligerents included much of Europe, the Russian Empire, the United States, and the Ottoman Empire, with fightin ...

. The glass filter worked both in

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

daylight communication and

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 is ...

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 separat ...

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

sodium Sodium is a chemical element with the symbol Na (from Latin ''natrium'') and atomic number 11. It is a soft, silvery-white, highly reactive metal. Sodium is an alkali metal, being in group 1 of the periodic table. Its only stable iso ...

- silicate glass incorporating about 9%

nickel oxide Nickel oxide may refer to: * Nickel(II) oxide, NiO, green, well-characterised oxide * Nickel(III) oxide Nickel (III) oxide is the inorganic compound with the formula Ni2O3. It is not well characterized, and is sometimes referred to as black nic ...

. It is a very deep violet-blue glass, opaque to all

visible light Light or visible light is electromagnetic radiation that can be perceived by the human eye. Visible light is usually defined as having wavelengths in the range of 400–700 nanometres (nm), corresponding to frequencies of 750–420 te ...

rays except longest red and shortest violet. It is quite transparent in the violet/ultraviolet in a band between 320 and 400

nanometre 330px, Different lengths as in respect to the molecular scale. The nanometre (international spelling as used by the International Bureau of Weights and Measures; SI symbol: nm) or nanometer (American and British English spelling differences#-re ...

s 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 Thermal expansion is the tendency of matter to change its shape, area, volume, and density in response to a change in temperature, usually not including phase transitions. Temperature is a monotonic function of the average molecular kinetic ...

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. I ...

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 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 separat ...

" 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 The Eastman Kodak Company (referred to simply as Kodak ) is an American public company that produces various products related to its historic basis in analogue photography. The company is headquartered in Rochester, New York, and is incorpor ...

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