Light with wavelengths shorter than those of visible light.

Ultraviolet Light is light with a Wavelength shorter than `400 nm, the lower limit of the visible wavelength range. The near UV spectral region ranges from 400 NM (or 380 nm) down to 200 nm. Shorter wavelengths from 200 nm down to 10 nm belong to the vacuum UV (or FAR UV), and still shorter wavelengths to the extreme UV (EUV). However, the precise definitions of these spectral regions vary in the literature.

Compared with visible light, Ultraviolet light is different essentially in 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 extreme ultraviolet (EUV) region. Powerful EUV sources as well as the corresponding photoresists are currently developed.
• The photon Energy is higher than the bandgap of many substances. As a consequence, ultraviolet light is strongly absorbed by many substances, and the induced excitation can lead to changes of 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).