Although digital multispectral imaging-particularly
ultraviolet-induced fluorescence imaging-is a very common
examination tool, its interpretation remains fraught with difficulties. Interpretation is strongly dependent on the capture
methodology, requires an understanding of the physical and chemical characteristics and interactions among materials in
artworks and is affected by data-analysis procedures.
The present research, which began with imaging of paint materials of known composition and proceeded to a range of
representative case studies, confirmed that fluorescence emissions by painting materials-such as organic binders or
colorants-are generally severely affected by the presence of absorbing non-fluorescing materials, such as inorganic
pigments. Application of a mathematical model based on the
Kubelka-Munk theory, resulted in the possibility of
distinguishing between real and apparent fluorescence emissions. Real emissions correspond to the presence of materials
which de facto exhibit fluorescent properties (typically organic binders and colorants), while apparent emissions relate to
the optical interactions among fluorescent materials and surrounding non-fluorescent materials (typically inorganic
pigments). Correction for the 'pigment-binder interaction' can also provide useful information on the presence of
materials whose fluorescence is almost obliterated by absorbing pigmented particles. Therefore, this image-processing
methodology can be used to characterise and reveal emissions that are dimmed or altered by re-absorption. This capacity
to reveal the presence of weakly fluorescing emitters has important conservation implications and informs the sampling
strategy for further analytical investigations.
Examples of the application of this data analysis to images made at the Grotto Site in Dunhuang, China, and at the
British Museum are presented.