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How does ultraviolet light reveal later additions or changes to a painting?

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kouros — Art fraud Kouros of uncertain provenance, formerly on display with the label “about 530 bc or modern forgery,” dolomitic marble; in the J. Paul Getty Museum, Villa Collection, Malibu, California.

Ultraviolet light is a valuable tool in the technical analysis of a painting because it can reveal later additions or alterations that are not visible to the naked eye. This process relies on the phenomenon of fluorescence, where certain substances emit visible light when irradiated by ultraviolet rays.12

When a painting is examined under ultraviolet light, the existing varnish layers and various paint layers fluoresce in different colours. Because these materials react uniquely to ultraviolet radiation, any areas that have been retouched, overpainted, or modified will typically display a different fluorescent response than the original, aged surface. By observing these inconsistencies in fluorescence, conservators and art historians can identify where the original composition has been tampered with or restored, helping to distinguish between authentic historical work and later, potentially fraudulent, additions.13