Ultraviolet Fluorescence Photography
Ultraviolet fluorescence photography records the visible light emitted by an object which is irradiated with ultraviolet radiation--electromagnetic radiation of wavelength just below the short wavelength end of the visible range (below 400 nm). Fluorescence can also be produced by short wavelength (violet or blue) visible light. Overpaints, partially cleaned areas, damages, and other discontinuities often differ markedly from original paints in their fluorescence which allows them to be clearly distinguished. For this reason, a fluorescence photograph is frequently an essential guide to choosing locations to remove samples for analysis.
Photographs are recorded on black-and-white or colour film. Illumination is from a complement of four Balcar electronic flash units with four bi-tube flash heads. A Corning ultraviolet transmitting glass exciter filter is placed on each bare tube head. These are large glass filters which transmit ultraviolet radiation in the range 320-400 nm. A Kodak Wratten No. 2E barrier filter is placed on the camera lens. This combination permits ultraviolet radiation to strike the object but only visible light of wavelength greater than 415 nm to reach the film.
In addition to the 2E barrier filter, a light yellow filter (Kodak Wratten No. 3), and Kodak Wratten colour compensating filters CC10C (cyan) and CC20M (magenta) can be placed in front of the camera lens. This combination attenuates unwanted blue light and produces excellent results with Kodak Ektachrome Film 6176.
It should be emphasized that the purpose of fluorescence photography, as well as other methods described here, is primarily to provide qualitative differentiation of areas within a painting. Comparisons between paintings in questions of provenance and art history can also be accomplished. It is therefore necessary to be aware that differences in colour rendition and surface detail can arise as a result of the use of different sources of illumination, camera lenses, filters, films, and film processing.
