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Ultraviolet light

Ultraviolet (UV) light has a wavelength shorter than that of visible light, but longer than that of X-rays.

The sun emits ultraviolet radiation, so natural daylight has a UV component. Prolonged exposure to light from this part of the electromagnetic spectrum can damage many of the materials used in easel paintings, including pigments and dyes, leading to problems such as discoloration, fading or cracking. 

Some substances, when illuminated with UV light, fluoresce (give off visible light). This allows details in a painting not otherwise visible to become apparent (see UV fluorescence).