Reprinted from LinkedIn.
The use of longwave ultraviolet light is a valuable aid in the detection of overpainting, repairs and forged signature on artwork. Many materials on the face of a painting absorb invisible ultraviolet energy and transforms this energy to visible colored light, easily distinguished by the human eye. In some cases, the UV light is absorbed without emission, making these areas appear quite dark in contrast to the fluorescent areas. This makes the use of UV light a valuable tool in checking restoration work as it progresses. A chart at the end of this article gives several clues in the detection of altered artwork.
Longwave ultraviolet is approximately 340 to 380nm and is harmless to the eyes and skin of the average healthy human and is often referred to as "Woods Light". It is the most important wavelength in the examination of art and artifacts.
The longwave ultraviolet lamp, used in art conservation, should be a professional quality lamp with most or all visible light filtered out. Blak-Ray® lamps, manufactured by UVP/Analytik Jena US, are ideal longwave UV lamps for this purpose. These lamps are available as plug-in units (UVL-21 and UVL-56), as portable battery-operated units (ML- 49), powerful 100-watt lamps (B-100A), and wide range of UV fixtures for permanent or studio use.
Longwave ultraviolet light is invaluable in viewing artwork before appraisals or to inspect damaged paintings. An alert restorer or framer will find that it pays to examine all artwork for hidden or concealed damage that he might be blamed for at a future date. The slightest erasure or alteration on a graphic or watercolor will stand out with extreme clarity. Many forgers of check and legal documents have been apprehended by the "invisible light".
Protective eyewear (UVC-303) increases the contrast and filters out the "blue haze" interference from ultraviolet light. It fits comfortably over regular eyeglasses.