Among the examination methods for paintings, ultraviolet fluorescence (UV) is certainly one of the most useful and often used. The technique is used principally for the analysis of the deterioration conditions of the work, and in particular, in the verification of the existence and the magnitude of those areas that may not be part of the original image. By projecting a UV band of light on the surface of the painting we can observe how some of the areas become illuminated while others remain dark. This factor is due to the physical phenomenon of ultraviolet fluorescence in the visible field. That is, the property of some substances to glow when they are hit with UV rays. The UV rays are invisible to us, but when they are absorbed by certain substances they are reflected as visible light. The difference in luminosity (fluorescence) can be seen on a painting "lit up" by a UV light only in function of the chemical composition of the various substances that make up the varnish and the pictorial layers. This fluorescence varies however based on the amount of time that has passed since the materials were applied. With this examination it is often easy to differentiate retouching from the original painting: the overlying materials that are not as old show up as being darker.