Image Processing Reference
Figure1.20 Visible image of Natura and two details in visible and near-IR light. (Courtesy
to the details shown in the two bottom images. The dark varnish obscures the
signature of the artist as seen in visible light, but the near-IR image reveals the
artist's name and the date of the painting.
Certain paints, like aged varnish, are transparent to infrared light and opaque
to visible light. Figure 1.21 shows three views (visible, NIR and SWIR) of a
special test panel painted with a variety of colors of oil-based pigment. All of
the pigments shown are quite opaque to visible light. The test panel has various
drawing media such as charcoal and pencil drawn across it underneath the pigment
stripes. Note that as the wavelength increases, the paint stripes become more and
more transparent. This property of infrared light helps art conservators authenticate
paintings by revealing the underdrawing without disturbing the paint layers.