The use of infrared imaging in the study of artists' techniques: Infrared reflectography
Introduction: Light
Light is the agency that produces the sensation called sight. Certain bodies such as the sun, lamps, fires etc are said to emit light or to be self-luminous. Most objects, including people are rendered visible by light reflected by them from self-luminous bodies. The branch of physics relating to light is to do with the properties of light and its physical nature/makeup. The emission of light from self-luminous bodies is an atomic phenomenon.
Infrared light spectrum
1. The Spectrum
A beam of white light can be dispersed by a glass or crystal prism into a spectrum. This we know as the visible spectrum; violet, blue, blue-green, yellow, orange, red and deep red. Colours of the rainbow, a result of the dipersion of white light by raindrops. The colours are separated because they represent light of different wavelengths. These wavelengths become longer as we move from blue to red.
2. Infrared (meaning below red, see detail of spectrum picture), the 'Invisible Spectrum'.
Visible radiation is known as light, while IR is radiation we cannot see and is known as electromagnetic radiation. (In physics 'electromagnetic' is a term used to describe the electrical and magnetic forces or effects produced by an electric current.
The overall range of electromagnetic radiations extends from very short wavelengths (gamma and x-rays) to very long wavelengths which comprise radio bands. As the infrare region extends far beyond the visible region, the wavelength increases. The radiation merges into heat waves, and finally into radar and radio waves.The visible spectrum is a narrow band of radiations (400 - 700 nanometres). Ultraviolet radiation (10 - 400 nanometres) the lower end of the visible spectrum. Infrared radiation (750 - 2000 nanometres).
Infrared light is electromagnetic radiation with a slightly longer wavelength then visible light as opposed to UV which is a slightly shorter wavelength then visible light. Infrared light overlaps with the red area of the spectrum and the microwave region.
3. The Use of IR in the Technical Analysis of Artworks
Infrared light can reveal the underdrawing that lies below the paint surface. This is due to the transparency of certain paint layers 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 nanometres (1 nanometre is equal to one-billionth of a metre) or a more immediate way is IRR (Infrared reflectography) which works in the near infrared range of 750 - 2000 nanometres (.75-2 microns).
4. The Setup
IR is set up as a closed-circuit television system. A light source (halogen lamp with IR-D80A filter, with high transmission in the region of more than 850nm) 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 blackchalk or bone black, will appear dark on the screen, because they do not reflect infrared light.
An early IR setup
5. Mosaics
Because of the low resolution of television systems, only a small area can be studied at a time with IRR. To examine an entire painting, the infrared camera has to be moved across its surface. To document an underdrawing, the individual close-up IRRs are captured by taking stills of the television monitor. These images are known as infrared reflectograms. Overlapping individual images can then be pasted together in a kind of mosaic, the IRR-assembly. The analog signal from the infrared camera can also be captured as digital data, which can then be assembled on the computer using specialized software.