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.