Infrared imaging has long been used to assist with art history and restoration, often being used to determine the authenticity of a piece of art or establish the period of an anonymous work.
Infrared reflectography (IRR) is a means of studying a painting by looking beneath the visible layers of paint, and was first developed in the late 1960s by Dutch physicist J. R. J. van Asperen De Boer.
Only radiation from the near infrared region of the spectrum is used, which has a slightly longer wavelength than visible light.
Infrared light has too long a wavelength to see, but it can however be photographed. When the infrared radiation penetrates the paint layers, the upper layers appear transparent.
The infrared-sensitive Osiris camera allows us to see differences in the absorption of infrared light upon the underlying layers, thereby uncovering the initial stages of a composition (as shown below).
The degree of penetration depends on the thickness of the paint, the type of paint used, and the wavelength of infrared radiation.
The longer the wavelength of the infrared and the thinner the paint layers, the easier it is to penetrate to the layers beneath. Infrared reflectography is especially valuable for studying underdrawing, cracks, pentimenti, hidden signatures, etc or the initial laying out of a composition with charcoal or graphite.
Underdrawings executed in infrared-absorbing materials, such as black chalk or bone black, will appear dark on the screen, because they do not reflect infrared light.
The resulting image, known as an infrared reflectogram, is converted digitally by the Osiris software, producing a black and white image on the computer monitor.