Wood's glass is an optical filter glass invented by American physicist Robert Williams Wood (1868–1955) which allows ultraviolet and infrared light  to pass through while blocking most visible light. It was developed as a light filter used in communications during World War I

World War I

World War I , which was predominantly called the World War or the Great War from its occurrence until 1939, and the First World War or World War I thereafter, was a major war centred in Europe that began on 28 July 1914 and lasted until 11 November 1918...

. His "invisible radiation" technique worked both in infrared

Infrared

Infrared light is electromagnetic radiation with a wavelength longer than that of visible light, measured from the nominal edge of visible red light at 0.74 micrometres , and extending conventionally to 300 µm...

 daylight communication and ultraviolet

Ultraviolet

Ultraviolet light is electromagnetic radiation with a wavelength shorter than that of visible light, but longer than X-rays, in the range 10 nm to 400 nm, and energies from 3 eV to 124 eV...

 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 has been commonly used to form the envelope for fluorescent and incandescent ultraviolet bulbs ("black light

Black light

A black light, also referred to as a UV light, ultraviolet light, or Wood's lamp, is a lamp that emits ultraviolet radiation in the long-wave range, and little visible light...

s"), although due to its disadvantages in recent years other filter materials have largely replaced it (see below).

Composition

Wood's glass is special barium

Barium

Barium is a chemical element with the symbol Ba and atomic number 56. It is the fifth element in Group 2, a soft silvery metallic alkaline earth metal. Barium is never found in nature in its pure form due to its reactivity with air. Its oxide is historically known as baryta but it reacts with...

-sodium

Sodium

Sodium is a chemical element with the symbol Na and atomic number 11. It is a soft, silvery-white, highly reactive metal and is a member of the alkali metals; its only stable isotope is 23Na. It is an abundant element that exists in numerous minerals, most commonly as sodium chloride...

-silicate glass incorporating about 9% nickel oxide

Nickel oxide

Nickel oxide may refer to:* Nickel oxide, NiO, green, well-characterised oxide* Nickel oxide, Ni2O3, black, not well-characterised 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 nanometre

Nanometre

A nanometre is a unit of length in the metric system, equal to one billionth of a metre. The name combines the SI prefix nano- with the parent unit name metre .The nanometre is often used to express dimensions on the atomic scale: the diameter...

s 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

Cobalt(II) oxide

Cobalt oxide or cobalt monoxide is an inorganic compound that appears as olive-green to red crystals, or as a greyish or black powder...

 in Wood's glass.

Properties and uses

Wood's glass has lower mechanical strength and higher thermal expansion

Thermal expansion

Thermal expansion is the tendency of matter to change in volume in response to a change in temperature.When a substance is heated, its particles begin moving more and thus usually maintain a greater average separation. Materials which contract with increasing temperature are rare; this effect is...

 than commonly used glasses, making it more vulnerable to thermal shock

Thermal shock

Thermal shock is the name given to cracking as a result of rapid temperature change. Glass and ceramic objects are particularly vulnerable to this form of failure, due to their low toughness, low thermal conductivity, and high thermal expansion coefficients...

s and mechanical damage. The nickel and barium oxides are also chemically reactive, with tendency to slowly form a layer of hydroxide

Hydroxide

Hydroxide is a diatomic anion with chemical formula OH−. It consists of an oxygen and a hydrogen atom held together by a covalent bond, and carrying a negative electric charge. It is an important but usually minor constituent of water. It functions as a base, as a ligand, a nucleophile, and a...

s and carbonate

Carbonate

In chemistry, a carbonate is a salt of carbonic acid, characterized by the presence of the carbonate ion, . The name may also mean an ester of carbonic acid, an organic compound containing the carbonate group C2....

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 "Wood's glass" bulbs are made of structurally more suitable glass with only a layer of an UV filtering enamel

Vitreous enamel

Vitreous enamel, also porcelain enamel in U.S. English, is a material made by fusing powdered glass to a substrate by firing, usually between 750 and 850 °C...

 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 be exposed 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.

Photographic filters for Ultraviolet photography

Ultraviolet photography

Ultraviolet photography is a photographic process of recording images by using light from the ultraviolet spectrum only.-Overview:Light which is visible to the human eye covers the spectral region from about 400 to 750 nanometers. This is the radiation spectrum used in normal photography...

, notably the Kodak Wratten

Wratten number

Wratten numbers are a labeling system for optical filters, usually for photographic use comprising a number sometimes followed by a letter. The number denotes the color of the filter, but is arbitrary and does not encode any information ; letters increase with increasing strength.They are named for...

18A and 18B, are based on Wood's glass.