Infrared reflectography
underdrawing that lies below the paint
surface. Only radiation from the near
infrared region of the spectrum is used,
which has only a slightly longer
wavelength than visible light. It is
distinct from thermal infrared, the region
of the spectrum used for night surveillance
and medical imaging. Paintings can be
examined with infrared photography,
using film sensitive up to 900 nm.
A superior method is infrared reflectography (IRR), which detects wavelengths of up to 2,000 nm.IRR is set up as a closed-circuit television system. A light source is directed at the painting, and the camera detects reflected infrared radiation. This signal is converted into a black and white image on a television monitor. 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.
In the image above, Portrait of a Man is being examined with infrared reflectography (IRR).
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