定义：波长小于约400nm的不可见光。

紫外光是波长小于约400 nm（可见光波长的下限）的光。 
区分不同光谱区域有几种不同的定义： 

1. 近UV光谱区域从400nm-300nm。中UV光从300nm-200nm，而200nm-10nm则属于远UV区域。更短的波长属于极紫外光（EUV）。 
2. 真空UV（约小于200nm）是指真空装置通常采用的波长范围，因为该波长的光能被空气强烈吸收。真空UV包括远UV和极紫外光。 
3. UVA代表波长范围为320-400nm，UVB为280-320nm，UVC为200-280nm。 

UV光具有很多的用途，例如UV消毒水和工具，UV固化胶黏剂，控制许多材料质量和激发荧光。 

紫外光的主要性质 
紫外光在下面两个方面与可见光不同： 

1. 其短波长可以准确聚焦并且产生非常精细结构（假如采用很高空间相干性的光源）。这可以应用到UV光刻技术中，用来制备微电子装置，例如，微处理器和芯片。未来微处理器需要更精细的结构，需要EUV区域的光刻技术。目前正在研发EUV光源和其对应的光阻剂。 
2. 其光子能量比很多物体的带隙能量高。因此，紫外光可以被很多物质吸收，产生的激发过程能引起物质化学结构发生变化（例如，化学键断裂）。这可以用到激光材料加工中（例如，激光刻蚀，脉冲激光沉积，制备光纤布拉格光栅），对水或医学器件消毒杀菌。UV光会损害人类的皮肤，尤其是UVC光具有杀菌作用。当紫外光与空气中的微量烃发生相互作用时可以将有机薄层沉积在附近的表面上；这种光污染会降低UV激光光源中非线性晶体的质量。 

产生紫外光 
激光器产生紫外光面临很多问题，但是还是有一些紫外激光器可以直接产生UV光：一些体激光器（例如，采用掺铈晶体， Ce:LiCAF），光纤激光器，激光二极管（大多数采用GaN），染料激光器，准分子激光器和自由电子激光器。

另一种产生紫外光的方式是将近红外激光器的输出光进行非线性频率转换。参阅紫外激光器得到更多细节。 

尤其是在EUV区域，通常采用气体放电（例如，氙气或锡蒸汽）或激光诱导等离子体来产生几瓦特甚至几十瓦特的高功率的UV辐射。但是，这种光源不是相干的。 

有时紫外光不是由激光器产生的。尤其重要的是气体放电灯（例如，水银管），另外发光二极管（UV LEDs）也应用很广泛。 

UV光学 
对待UV光时，需要特殊的UV光学理论。UV应用中重要的材料参数是低泡和夹杂物含量，折射率很好的均匀性，双折射很小，表面很光滑。尤其是应用强UV激光器时，长期抗紫外线强度也很重要。

在纯的氟化钙中需要用到UV光学，该材料具有很低的UV吸收，很高的均匀性，低双折射，高硬度（与其它氟化物材料相比），高稳定性和高损伤阈值。可以在低于160nm时使用，因此可用于氟化氩准分子激光器。

但是它是易碎的，非各向同性的，并且吸湿。它的替代物是UV级的熔融二氧化硅，可以用于波长小于200nm时，而便宜的标准的熔融二氧化硅在小于260nm时具有很大的损耗。另一个可用的材料是钻石，它在小于230nm时是透明的，但是非常昂贵。 

有些光纤可以用于近紫外光谱区域，但是传播损耗相对比较高。用光纤传输紫外光在波长较短或者功率更高的情况下都是不可行的。 

在EUV区域，几乎所有的固体材料都有强烈的吸收，空气中在小于200nm时也会产生很强的衰减，因此真空UV或EUV用于光刻时需要在真空条件下。

布拉格反射镜可以在EUV区域，采用钼/硅（Mo/Si）结构制备，在12nm处可以得到约70%的反射率。由于其反射率有限，因此需要改变EUV光学结构设计得到最小数目的反射表面。 

安全隐患 
紫外光对眼睛（尤其是在250-300nm）和皮肤（尤其在280-315nm）都是有伤害的，它会引起白内障或角膜炎，除了引起色素沉积和红斑外，还会引发皮肤癌。
而小剂量不足以引起急性反应的，也会加速皮肤的老化。因此，如果采用UV光工作，尤其是UV激光器，需要特殊的激光防护措施。例如，开放光学装置中的UV光束需要采用一些金属管封闭。

Acronym: UV light

Definition: invisible light with wavelengths shorter than ≈ 400 nm

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:

• The near-UV spectral region ranges from 400 nm down to 300 nm. The middle-UV region ranges from 300 to 200 nm, and shorter wavelengths from 200 nm down to 10 nm belong to the far-UV region. Still shorter wavelengths belong to the extreme UV (EUV).
• The term vacuum UV (below ≈ 200 nm) refers to the wavelength range where a vacuum apparatus is often used, because the light is strongly absorbed in air. The vacuum UV includes the far and extreme UV.
• UVA stands for the range from 320 to 400 nm, UVB for 280–320 nm, and UVC for 200–280 nm.

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. In the Covid-19 crisis, the ability of UV light to deactivate viruses has received increased attention.

Essential Properties of Ultraviolet Light

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

• The short wavelength allows precise focusing and the generation of very fine structures (provided that a light source with high spatial coherence is used). This is utilized in UV photolithography, as used e.g. for the fabrication of microelectronic devices such as microprocessors and memory chips. Future generations of microprocessors will have even finer structures and will require photolithography in the EUV region. Powerful EUV sources and the corresponding photoresists are currently being developed.
• The photon energy is higher than the band gap energy of many substances. As a consequence, ultraviolet light is strongly absorbed by many substances (e.g. in optical glasses, being transparent for visible light), and the induced excitation can lead to changes in the chemical structure (e.g. breaking of bonds). This is important for laser material processing (e.g. for laser ablation, pulsed laser deposition, and for the fabrication of fiber Bragg gratings), and for sterilization of water or medical instruments. UV light can also damage the human skin (see below), and particularly UVC light has germicidal effects. When ultraviolet light interacts with trace hydrocarbons in air, it can lead to the deposition of organic films on nearby surfaces; such kind of photocontamination can e.g. degrade the quality of nonlinear crystals in UV laser sources.

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.

There are also various gas discharge lamps, e.g. xenon lamps and xenon/mercury lamps, which can be used for specific UV spectral lines or as broadband UV sources. Besides, there are excimer lamps, used as quasi-monochromatic UV sources in pulsed or continuous-wave mode. 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. All such sources do not emit highly coherent radiation.

Light-emitting diodes (UV LEDs) are also attracting interest for a range of applications, e.g. for water disinfection.

UV Optics

For handling UV light, special ultraviolet optics are required. A very high material quality in various respects is essential to avoid problems with parasitic absorption, scattering, degradation and others. See the article on ultraviolet optics for more details.

There are also optical fibers which can guide ultraviolet light while avoiding substantial degradation (“solarization”). They are called solarization-resistant fibers.

Detection of Ultraviolet Light

Various kinds of photodetectors can be used for the detection of ultraviolet light. These include devices based on the internal photoelectric effect, such as photodiodes, and others based on the external photoelectric effect, for example phototubes and photomultiplier tubes. Some of those are insensitive to visible and infrared light; they are called solar-blind photodetectors.

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 optical setups usually have to be enclosed with metal tubes.

When working with ultraviolet light sources, one may require protective eyewear, clothing and gloves.

For wavelengths below about 260 nm, there is also the problem that ozone is generated in air. It may thus be necessary to remove the ozone with suitable additional devices or to avoid its generation by avoiding the presence of oxygen.