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CHARISM A I 7 a The extent to which this radiation will penetrate the object under investigation will be dependent on its wavelength and on the absorbance of the materials which compose the object, with longer wavelengths of radiation generally penetrating further into the piece. For example, when examining a painting, shorter wavelengths (such as UV) are often readily absorbed by the outer layers (usually varnishes), while longer wavelengths can pass through the varnish and interact with the pictorial film and the under drawing.” The radiation reaching any particular point in the object can be: (i) absorbed; (ii) reflected; and/or (ili) absorbed and re-emitted as luminescence at longer wavelengths.° Each outcome produces an image set which yields information specific to that point. Thus by selecting particular combinations of illumination and detection ranges, it is possible to gain insight about the distribution of materials in the object under study. a. Image sets The image sets possible under each of the illumination conditions discussed are summarised in Figure 1-2. Thus; 1) UV illumination can produce ultraviolet-reflected images (UVR) and ultraviolet-induced luminescence images (UVL)’®, where the emitted radiation is in the ultraviolet, visible or infrared range. 2) Visible illumination can give rise to visible-reflected images (VIS) and visible-induced luminescence images, where the emitted radiation is in the visible range (VIVL)" or in the infrared range (VIL) 23 3) Infrared illumination yields infrared-reflected images only,’ as so far there are few known cases of infrared-induced luminescence in cultural heritage applications." However, rules similar to those illustrated for UVL, VIVL and VIL would apply for materials that absorb and re-emit radiation in the infrared range. The images produced therefore fall into two categories: e Reflected radiation images e Emitted radiation or photo-induced luminescence images (Figure 1-2). These are discussed in more detail in the following sections. Version No. 1.0 Date : 14/10/2013
