is an analytical method that renders invisible infrared radiation visible to the naked eye. More specifically, infrared reflectography is a photographic or digital imaging technique that uses a specially designed heat-sensitive film or detector to capture the reflected and absorption/emission characteristics of reflected infrared radiation. Some materials that are opaque to visible light are transparent to infrared radiation. When an object is illuminated with natural or incandescent light, infrared radiation will not only interact with the surface but will also penetrate and interact with underlying layers so that things below as well as on the surface can be imaged.
Conservators and art historians have used infrared reflectography to study works of art since the 1960's. The technique and has seen its greatest application in the study of paint layers and underdrawings in paintings. Since the absorption/transmission and reflection of IR radiation varies for different materials (i.e., pigments), the resulting images can provide information about an artist's working methods and materials used in a painting or underdrawing.
The Infrared spectrum can be divided into near middle and far IR; the boundaries between these regions are not precisely agreed upon. The near Infrared spectral range is that portion of the electromagnetic spectrum (roughly) between .7 (just beyond the red of the visible spectrum) and 5 microns. A micron is a unit used to measure wavelength; 1 micron equals one millionth of a meter.
Typically, for examining works of art IR reflectography has used that part of the near IR range between .7 to 2.5 microns, this is also known as the non-heat portion of the IR spectrum.
Thermal IR, the middle or heat range of the IR spectrum, uses longer wavelengths of IR radiation; the range of thermal IR utilized is in this application is 3.0 to 5.0 microns. Like near infrared reflectography, thermal IR requires a specialized camera with heat sensitivity extending further in into the infrared range, in this case a Mitsubishi Thermal Imager was used. This range is captured using a special lens called a germanium lens which filters all visible and near IR radiation.
For this images used in this entry tungsten-halogen lights were used for both the near and thermal infrared reflectography. The lights were approximately six feet from the artwork. Intensity was adjusted as needed to even out glare and to be able to record a legible image; in general light intensity had to be reduced for the thermal IR captures. (Holly I need to check this for accuracy)