Conservation of Devout Men Taking the Body of Saint Stephen, dated 1776, by Benjamin West (American, 1738–1820):
Ultraviolet Fluorescence (UV)

How does it work?
Ultraviolet (UV) wavelengths begin just outside the visible portion of the electromagnetic spectrum at the violet end. These wavelengths are shorter and have higher energy than visible light. While UV rays are invisible to the human eye (some insects and birds can see them), they react with the materials on the surface of a painting and are reflected back as low energy radiation, a phenomenon known as fluorescence. Using UV lamps for illumination, a painting’s UV fluorescence can be documented using normal cameras, sometimes employing special filters.

What do we see?
Not all materials fluoresce (and this can be an important observation in itself). Those that do may respond differently from one another and have a characteristic color and/or intensity. These differences in fluorescence give conservators information about the surface condition of a painting. Most commonly, UV can provide information about the varnish layer(s) present and reveal areas of previous restoration. Restorations are typically visible as dark purple patches because the materials used differ from the painter’s original materials and because they are more recently applied (less aged).

So what do you see?
UV fluorescence shows previous restorations, but more importantly can reveal the extent of these restorations.

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The details above, taken of the seam in the center of the painting (with visible light at left and UV fluorescence at right), reveal the presence of at least two campaigns of repaint. In visible light, retouching over the seam is noticeable only by small differences in color, paint texture, and form. Under UV, one restoration has a dark blue fluorescence (red arrows), while a second has a slightly lighter fluorescence (green arrows). The repaint indicated by the green arrows was likely applied to obscure damages to the paint layer along the seam. The repaint indicated by the red arrows appears to reinforce contours and strengthen shadows and forms; these areas may have become difficult to discern.