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Ultraviolet Light

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Acronym: UV light

Definition: invisible light with wavelengths shorter than ≈ 400 nm

German: ultraviolettes Licht

Category: general optics

How to cite the article; suggest additional literature

Ultraviolet light is light with a wavelength shorter than ≈ 400 nm, the lower limit of the visible wavelength range.

Different definitions are used for distinguishing different spectral regions:

However, the precise definitions of these spectral regions vary in the literature.

UV light finds a wide range of applications, including UV disinfection of water and tools, UV curing of adhesives, quality control for many materials and exciting fluorescence for analytical purposes.

Essential Properties of Ultraviolet Light

Compared with visible light, ultraviolet light is different in essentially two different respects:

Generation of Ultraviolet Light

The technology of lasers for the generation of ultraviolet light faces various challenges; nevertheless, there are a few kinds of ultraviolet lasers which can directly generate UV light: some bulk lasers (e.g. based on cerium-doped crystals such as Ce:LiCAF), fiber lasers, laser diodes (mostly GaN-based), dye lasers, excimer lasers, and free electron lasers. Another way of generating ultraviolet light is by nonlinear frequency conversion of the outputs of near-infrared lasers. The article on ultraviolet lasers gives more details.

Particularly for the EUV region, gas discharges (e.g. with xenon or with tin vapor) or laser-induced plasmas are used for generating UV radiation with high powers of multiple watts or even dozens of watts. However, such sources do not emit coherent radiation.

In many cases, UV radiation is generated with devices other than lasers. Of particular importance are gas discharge lamps (e.g. mercury tubes), but light-emitting diodes (UV LEDs) are also attracting interest for a range of applications.

UV Optics

For handling UV light, special UV optics are required. Important material parameters for UV applications are a low bubble and inclusion content, good homogeneity of the refractive index, a small birefringence, and the potential for polishing surfaces with very small roughness. Particularly for applications with intense UV lasers, the long-term resistance against UV light is also important. UV optics are often made from highly purified calcium fluoride (CaF2), which has a very low UV absorption, high homogeneity, low birefringence, relatively high hardness (compared with other fluoride materials), high physical stability, and high optical damage threshold. It can be used down to ≈ 160 nm and is thus suitable for use, e.g., with argon fluoride (ArF) excimer lasers. However, it is brittle, naturally anisotropic, and hygroscopic. As an alternative, UV-grade fused silica can be used for wavelengths down to ≈ 200 nm, whereas the cheaper standard-grade fused silica has significant attenuation below 260 nm. Another possible material choice is diamond, which is transparent down to ≈ 230 nm and very robust, but expensive.

Some optical fibers can be used in the near-ultraviolet spectral region, although with relatively high propagation losses. Fiber delivery of ultraviolet light is usually not feasible for shorter wavelengths and/or high optical powers.

In the EUV region, basically all solid materials are relatively strongly absorbing, and even air causes strong attenuation below ≈ 200 nm, so that e.g. lithography with vacuum UV or EUV light has to be performed in vacuum. Bragg mirrors can still be made for the EUV region, e.g. with molybdenum/silicon (Mo/Si) structures, which allow, e.g., ≈ 70% reflectivity at 12 nm wavelength to be reached. Due to this limited reflectivity, EUV optics have to be designed with the smallest possible number of reflecting surfaces.

Safety Hazards

Ultraviolet light is dangerous for the eyes (particularly for wavelengths in the range 250–300 nm) and for the skin (particularly for 280–315 nm), because it can cause cataracts or photokeratitis of the eye's lens and skin cancer, apart from hyperpigmentation and erythema. Lower doses, not yet causing acute effects, can accelerate aging of the skin. Therefore, work with UV light sources, in particular with UV lasers, demands special precautions for laser safety. For example, UV beams in open optical setups usually have to be enclosed with metal tubes.

See also: nonlinear frequency conversion, frequency doubling, excimer lasers, laser safety
and other articles in the category general optics

In the RP Photonics Buyer's Guide, 15 suppliers for ultraviolet light sources are listed.

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