Besides identifying pigments, hyperspectral imaging generates an accurate digital record for art conservation. This may be used for monitoring change or damage to paintings, as digital documentation resists deterioration better than photographs (Casini et al., 2002a). Digital imaging may also assist in the restoration of artwork. It has already been applied to assessing damage from laser cleaning (Balas et al., 2003), where computer- aided comparison of before and after images have provided superior results. Finally, digital imaging assists in discovering the history of a piece of art through revealing underdrawings and retouchings (Day, 2003a). Thus, multiple analysis objectives may be achieved simultaneously, which previously had required several different instrumental techniques.
2.4.1. Identification for retouching
Damaged paintings frequently require retouching to provide visual compensation for losses in the paint film, but using the correct pigment for retouching damaged artworks is complicated by the possibility of metamerism. This occurs when two colours that appear similar under one light source look different under another. To make losses indistinguish- able from the surrounding undamaged areas, identical reflectance properties in both retouching paint and the original must therefore be achieved. Colour matching, therefore, involves mixing pigments to match a given standard, assuming normal colour vision and a standard source of illumination (Day, 2003a). A complete spectral match is obtained when the same colour is perceived in all light temperatures, which is only achieved when either the same pigments or those with the same reflectance pattern are used (Morgans, 1982; Imai et al., 2000). In this manner, pigment identification plays an important role in conservation.
2.4.2. UV imaging
Photographic documentation of fluorescence emitted under UV exposure has been an important diagnostic method for studying historical and artistic objects since the 1920s (Casini et al., 2002b; Pelagotti et al., 2005). This simple diagnostic method has the capacity to reveal information about an object that is otherwise not visible, such as overpainting and varnishes (Eastman Kodak, 1972). Since many inorganic and organic substances exhibit a characteristic emitted fluorescence under UV illumination, fluorescence spectroscopy theoretically can also be applied to differentiate materials such as resins and pigments.
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decades (Mansfield et al., 2002). Some inks and pigments that are visually identical are frequently different under NIR (780–2500 nm). Near-IR has less energy than visible light and usually excites vibrational overtones rather than electronic transitions (Mairinger, 2000b). The ability of NIR to penetrate through some pigments has allowed the study of underdrawings. The technique of using NIR photography for detection of underdrawings was already in use by the late 1930s, when Ian Rawlins used an NIR camera to improve visual assessment of paintings (Roselli and Testa, 2005). This is due to the low absorption by some pigments in the NIR range (Gargano et al., 2005), and it is thus possible to divide between carbon-based (e.g. carbon black), iron-oxide-based (Mars Black), and organic- based compounds (sepia, bistre, and iron gall ink) (Mairinger, 2000b; Attas et al., 2003), as is increasing the legibility of texts obscured by dirt, deterioration, bleaching, or mechan- ical erasure. Generally, a paint layer becomes more transparent with greater wavelength of the incident radiation, smaller thickness of the paint layer, smaller number of particles in the layer, and lesser refractive index difference between pigment and medium (Mairinger, 2000b). Some work has already been performed with the aid of CCD technology; some pigments were found to become transparent between 800–1100, others in all the NIR region, and others only beyond 1000 nm (Gargano et al., 2005). Maximum transmittance for many pigments occurs between 1800 and 2200 nm, while >2000 nm is required to penetrate blue and green layers (Mairinger, 2000b). The NIR region is also applied for materials identification (Attas et al., 2003). Mansfield et al. were able to differentiate between pigments using digital images taken between 650 and 1100 nm, finding sufficient specificity in this region (Mansfield et al., 2002). Clarke also chose to use NIR imaging in favour of UV–Vis spectroscopy for pigment identification. He concluded that imaging between 700 and 950 nm was useful for certain pigments, particularly blues found on early mediaeval manuscripts (Clarke, 2004). The entire visible and NIR region cannot be covered by a single camera (Gargano et al., 2005), so a combination of two systems is recommended.
Gargano et al. (2005) found that a good Si CCD camera facilitated adequate detection work up to 1000 nm. For infrared imaging beyond this wavelength, NIR-sensitive diode arrays are required, such as an InGaAs (Geladi et al., 2004), PbS (Baronti et al., 1997), or PtSi camera (Gargano et al., 2005).
2.4.4. Colorimetry
“Colour is a subject that ought to give intellectuals a headache. Its definition is so completely intangible”.
Sidney Nolan, writing to Sunday Reed, April 6, 1943 (Nolan, 1943a).
Measuring the spectral reflectance of an object surface objectively provides a description of the surface’s inherent physical characteristics, while colour perception depends on many factors, such as the illumination, the observer, and the surrounding conditions (Barnes, 1939b; Nassau, 2001; Zhao et al., 2004). Colour is determined only by absorbance in the visible spectrum, and conversely, colour assignment can, easily be calculated from spec- tral reflectance. The three subjective natures of colour are hue (wavelength), saturation (purity), and luminosity (intensity of reflected light). These primaries are assigned the
tristimulus values X, Y, and Z, respectively, obtained by measurement of the object reflectance and source emission spectra, and may be used to quantitatively describe colour according to Commission Internationale de l’Eclairage (CIE) conventions (McLaren, 1983). CIE Y value is a measure of the perceived luminosity of the light source, whereas Xand Zcompo- nents give the colour or chromaticity of the spectrum. Currently, the CIELAB 1976 system is the most widely used for describing colour changes. This definition starts from the tristimulus values defined, and defines three other quantities, L*, a*, and b*, where:
L* =Lightness (0 =black, 100 =white)
a* =Magenta–Green (-128 =green, +127 =magenta) b* =Yellow–Blue (-128 =blue, +127 =yellow)
CIE L*a*b* measurements provide a permanent colour record (Morgans, 1982) and are calculated to linearise the perceptibility of colour differences. Each colour can thus be represented in a 2D or 3D chromaticity diagram that depends on the adopted CIE conven- tion. In the 2D diagram, the area on the graph surrounded by the triangle is known as the spectrum locus and covers the visible wavelengths 400–700 nm (Fig. 2).
Hyperspectral Imaging: A New Technique for the Non-Invasive Study of Artworks 207
Fig. 2. Two-dimensional CIE Lab diagram (Frei and MacNeil, 1973) allowing the plotting of colour in functions of X, Y, and wavelength.
M.Kubik
Fig. 3. Technical drawing of CCD setup with filter carrousel and lens.