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How does ultraviolet light help investigators find alterations in paintings?

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Ultraviolet (UV) light is a valuable tool for investigators because it can reveal additions or alterations to a painting that are not visible to the naked eye.1

When a painting is exposed to ultraviolet rays, the various materials on its surface—such as varnish layers and specific paint layers—fluoresce, or emit visible light, in different colors. Because these materials react uniquely to UV radiation, any areas that have been retouched, repaired, or altered will often stand out against the original paint, as the newer materials will fluoresce differently than the older, original ones.1

This technique is part of a broader technical analysis used to verify the authenticity of art. While UV light is highly effective for identifying surface-level modifications, investigators often use it alongside other methods, such as infrared rays to reveal underpaintings or X rays to examine internal structures, to build a comprehensive assessment of a work's condition and history.1