Infrared Photography (Reflected and Transmitted)
The region of the electromagnetic spectrum which is visible to the human eye (i.e. light) lies in the wavelength range of about 400 to 700 nanometers (abbreviated nm). Infrared radiation is electromagnetic radiation with wavelength longer than 700 nm and infrared films are available which are sensitive in this region of the spectrum up to about 900 nm. Most commercially available infrared films have a maximum response at approximately 850 nm. Infrared photography is widely utilized in such fields as medicine, forensic science, and in remote sensing in the earth sciences and archaeology.
Infrared photography can often reveal or clarify obscured or faded painting, overpaints, and underdrawings executed with charcoal or graphite. Under favourable conditions, carbon in the black pigment of an underdrawing (charcoal black, lamp black, graphite etc.) absorbs infrared preferentially, resulting in an enhancement of the drawing. This effect can be obscured by pigments such as azurite and malachite. On the other hand, lead white and cadmium red, which are radiopaque (see below under X-Radiography), are very transparent in the infrared. Consequently, infrared can reveal information which is obscured by lead white grounds in x-radiographs. Damages and other discontinuities are often revealed by infrared photography.
Infrared photographs are exposed on Kodak High Speed Infrared Film 4143 with a Kodak Wratten Filter No. 87 over the camera lens to filter out unwanted visible light from the tungsten-halogen lamps or flash Photographs can be taken with the camera either in front of the painting (reflected) or, where applicable, behind the painting (transmitted), with the positions of the lamps changed accordingly.
