Infrared light that falls with the 780-3000 nm range (near infrared) of the electromagnetic spectrum can be particularly useful for revealing preliminary sketches, underdrawings, or compositional changes that lie beneath visible paint layers. The technique is non-destructive and has more recently been coupled with other types of imaging (hyper/multi-spectral imaging) to provide additional information relating to the characterization of pigments and binders.
As early as the 1930s, scientists found that IR photography could be used to assess the condition of a painting (beneath layers of varnish and surface grime) but more importantly it was able to to detect things hidden beneath the paint layers. Early IR film, however, was only sensitive to about 900 nm, making it difficult to effectively pentrate certain pigments like copper-containing blues and greens (azurite, malachite, etc.). Even today’s digital IR camera systems with their improved CCD detectors are only able to collect out to about 1000 nm.
By the 1960s, the Dutch physicist, J.R.J. van Asperen de Boer found a way to harness longer IR wavelengths in order to more effectively penetrate paint layers. His system (which he named infrared reflectography or IRR) was based around the IR-sensitive vidicon tube and was able to collect out to 2000 nm, a vast improvement over IR photography, however image collection still remained somewhat of a challenge. Early IRR images were generally collected by taking pictures of the monitor’s screen, developing the images on film, and then painstakingly piecing them together by hand.
Recent improvements in computer technology and IR detectors have led to incredibly sensitive IRR systems. While many museums still employ vidicon detectors and IR photography (e.g. CCD detectors) newer systems are constantly being created to improve imaging. By the mid 1990s, scientists and conservators began to exploit certain types of infrared semi-conductor sensors, a technology that was initially developed for thermal imaging devices used in the military. These Focal Plane Array (FPA) systems were able to cover a much wider range in the near infrared region and first used a platinum silicide detector (PtSi/1200-2500 nm). Today other detectors are used for for their increased sensitivity and contain indium gallium arsenide (InGaAs/900-1700 nm), indium antimonide (InSb/1000-3000 nm), and cadium mercury telluride (MCT/1000-2500 nm). Scholars should always take note of the type of system that was used to collect an IRR image as many paintings are being re-examined with more modern systems.
An ideal painting for IRR imaging contains a white, reflective ground, thinly applied layers of paint, and carbon-containing materials in the preliminary sketch or underdrawing. Certain pigments can also be difficult for the IR system to penetrate (see above) unless it is able to work beyond the 1000 nm range. Carbon-containing materials (e.g. carbon black) are particularly absorptive in the IR region while a light or white ground reflects the IR light. This creates a contrast within the IR image that allows to viewer to perceive features that lie beneath the paint film.
Certain materials that were occasionally used to execute underdrawings and sketches can be difficult to detect (e.g. iron-gall ink, metalpoint, etc.) if they are not rich in carbon black. Furthermore it the artist chose to execute a preliminary oil sketch in light or red-colored paint or worked on a dark-colored ground, imaging with IRR alone is often unsuccessful. Today IRR systems are being coupled with other imaging technologies (hyper/multi-spectral) to help with improvements in detection and identification.
van Asperen de Boer, Jan R.J. "Reflectography of Paintings using an Infrared Vidicon Television System." Studies in Conservation 14 (1969): 96-118.
Bomford, David, ed. Art in the Making: Underdrawing in Renaissance Painting, National Gallery London Publications; Yale University Press, 2002.
MacBeth, Rhona. "The Technical Examination and Documentation of Easel Paintings." In The Conservation of Easel Paintings, edited by Rebecca Rushfield and Joyce Hill Stoner, 296-300. Routledge: London and New York, 2012.
Recent Developments in the Technical Examination of Early Netherlandish Painting: Methodology, Limitations and Perspectives (M. Faries and R. Spronk, eds), pp. 107–16, Brepols Publishers.
Taft, W. Stanley Jr., and James Mayer. The Science of Paintings. Springer: New York, 2001.