Fluorescence: Reflected glories

Since as early as the mid-20th century, historical art objects such as stringed instruments have been examined and appraised not only in visible light (VIS), but also with light in other spectral wavelength ranges. This has nowadays become a significant part of detailed examinations, as it makes visible several features that would otherwise go undetected, to visualise and interpret the condition of an instrument more fully. Information gained in this way can be helpful in determining the instrument’s condition, both for the purposes of documentation and also as an aid to restoration work.

The most common technique, which has been used almost exclusively until now, is examination with ultraviolet light (UV). It lies in the range that is invisible to the human eye, and in the commercial lamps we use is usually restricted to the ‘UVA’ wavelength range: 320–400nm. When using this light for analysis, what we can see is essentially the fluorescence induced by the light, which in this wavelength range is well differentiated and rich in contrast. An experienced observer will be able to differentiate between original varnish and retouching, damage, or the addition of new material.