In the infrared portion of the spectrum the wavelengths are greater than 700 nanometers with photon energies less than 1.8 electron volts 76 (eV). These infrared photon energies, typically around 1 eV, are so low that the photons are not absorbed by most pigments. The paint layers are therefore relatively transparent to infrared radiation. Infrared radiation penetrates the upper paint layers but is absorbed by dark preliminary drawings that reside beneath them. The remaining radiation is reflected by white or light-colored grounds. With the aid of infrared-sensitive photographic equipment, the underdrawing can be seen be-
cause of the difference between absorbed and reflected radiation.
A particularly convincing demonstration of the transparency of pigments to the infrared is provided in Figure 7.1. First, an underdrawing is made using charcoal on a white ground. A layer of paint consisting of different colors is then applied to the drawing. Finally, the composite drawing/painting is viewed with an infrared-sensitive video camera, clearly revealing the image of the underdrawing in the infrared display.
Different pigments have different reflection and absorption properties at different wavelengths. When viewed in the visible range of the spectrum each pigment has a certain thickness that is required to cover up or hide an underlying pigment. To make the underdrawings observable, the hiding ability of the paint has to be minimized. The paint layer thickness cannot be modified, and therefore infrared radiation is
Fig. 7.2a,b,c. (a) Francisco Goya, Doña Isabel de Porcel. Oil on canvas, 32 1/4 X 21 1/2 inches, National Gallery, London. (b) Infrared reflectogram detail showing the presence of an eye beneath the portrait of Doña Isabel. (c) X-radiograph showing a more complete image of the overpainted portrait.
7.2 Pigment Response to X-rays: Absorption
used because absorption and scattering are reduced in that region of the spectrum.
As the wavelength increases into the infrared region, the amount of scattering decreases for pigment particles embedded in a binding medium. The decrease in scattering leads to less reflection by the pigments. Because the photons are not absorbed they continue to penetrate through the paint layers. At the ground layer of a painting the photons are strongly absorbed by the underdrawing (charcoal or dark-colored ink) and universally reflected by the white pigments in a typical ground. Differences between areas of reflection and absorption are therefore detected.
Doña Isabel de Porcel, by Francisco Goya, provides an example of the penetration of infrared radiation through a paint film. The image seen in visible light (Figure 7.2a) gives no indication of the existence beneath it of an unrelated portrait. The infrared image (Figure 7.2b) shows the eye and eyebrow of the hidden portrait. We are able to detect this image because pigments used to paint the eye absorb photons in the infrared region, while surrounding light-pigmented areas reflect the photons. A more complete image of the overpainted portrait is provided by the x-radiograph shown in Figure 7.2c.
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