Located just beyond the violet band of visible light, ultraviolet (UV) radiation causes materials to fluoresce and phosphoresce depending on their properties. Invisible to the naked eye, ultraviolet radiation has a shorter wavelength than visible radiation. When examining a painting, UV can be used to determine surface anomalies. Varnishes, for example, fluoresce differently depending on their composition and age. Aged natural resin varnishes appear greenish yellow while newer synthetic varnishes may appear milky white to purple. The varnish on The Old Guitarist appears greenish yellow in UV, suggesting it is an aged natural resin varnish.
In detail 1 of the lower left corner of the painting, we see the greenish yellow fluorescence of the varnish. In places where the varnish is thinner the color appears blue or in some cases blue-purple. In detail 2, along the left edge of the painting, we see an area from which the varnish has been removed in a cleaning test. Once again we see the difference between the fluorescence of the paint and varnish.