A modern examination technique for studying underdrawing and underlayers in paintings that makes use of a camera with a detector that is sensitive to infrared radiation. An image is created from the differences in absorption of infrared radiation by the materials in the painting.
Some underdrawing materials (suc as charcoal or graphite) reflect light in the infrared range. When a painting is studied with an infrared camera, areas of the paint layer that do not reflect light in the infrared range become transparent, and the camera reveals the infrared reflective underdrawing; the image can be transferred to a television monitor or computer screen for viewing. Individual details can be assembled digitally, and the resultant image is called a reflectogram.
Before the advent of infrared reflectography, paintings were photographed with infrared-sensitive film in order to study underdrawings. However, this technique did not yield as much information as the image associated with the more sophisticated infrared-sensitive detector.
Studies of underdrawing not only can tell us a great deal about an artist’s technique but also can play a role in solving problems of attribution.
Source: Looking at Paintings: A Guide to Technical Terms / eds. Tiarna Doherty and Anne T Woollett. (Los Angeles: J. Paul Getty Museum, 2009), p. 43.