Infrared light can reveal the 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. 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.