3. IRR explanation

What is IRR?

IRR (infrared reflectography) is an imaging technique that was used to study preparatory drawings beneath the paint layers. These underdrawings served as a compositional guide for the artist or his assistants in the painting process. IRR’s can provide important information for art historians since changes in composition can be detected during the different phases of a painting’s execution. Stylistic analyses of underdrawings can also be used to consider attributions. Lastly IRR is an important aid for art conservators, since it can detect paint losses and retouchings, sometimes invisible to the naked eye.

How does IRR work?

IRR registers patterns of absorption and reflection of infrared light, which can penetrate through most thinly painted oil paints, except black. Only wavelengths in the so-called near-infrared region of the electromagnetic spectrum are utilised (up to around 2000 nanometers), and the technique is thus unrelated to the detection of heat, which uses longer wavelengths of infrared light. Since IRR detects black materials it is a perfect complement to X-radiography, which typically registers lighter materials.

IRR and the Ghent Altarpiece

For the occasion of the recent study campaign, the entire altarpiece was documented using a digital IRR video camera with a platinum silicide detector. These cameras typically have a relatively low resolution that requires close-up documentation of numerous small details, of in this case 4 x 4 cm. This resulted in literally tens of thousands of individual infrared reflectograms, which were digitally assembled in larger images.