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Ultraviolet fluorescence

When exposed to ultraviolet radiation, some materials used in paintings become fluorescent, giving off visible light.
In particular, traditional natural resin varnishes (such as mastic and dammar) give off a characteristic greenish yellow colour. By looking at a picture under UV to see how much light it gives off, it is therefore possible to monitor the removal of old varnishes.

Retouchings applied over a layer of varnish also show up clearly in UV. Since they prevent the UV light from reaching the varnish, they show as dark patches in an otherwise fluorescent area. 

While most paints emit very little fluorescence, some pigments and dyes used in paint do give off a characteristic colour. This can be useful to identify the presence of a particular pigment. For example, the dyestuffs in different red lake pigments give off different colours, from bright orange to pink.