Optical coherence tomography (OCT) is an imaging technique that uses low intensity near-infrared light to make cross-sectional pictures. It is commonly used to take pictures of the retina and other tissues.
Normally, most of the light shining on an object ends up scattering off in all directions. Only a fraction of the light reflects off structures beneath the surface of the object. OCT filters out the scattered glare, detecting only the reflected light. The technique, which can be likened to an optical ultrasound, can be used to build up a layered picture of biological tissue up to 2 mm deep.
OCT has also been used to exam art objects such as oil paintings. For those of you who know as little about art as I do, oil paintings are made up of many separate layers. Today, gesso is usually applied first as a primer, in earlier times glue made from animal hides was used. On top of that, the artist may use a variety of layers including sketches or outlines, multiple coats of paint, glazes and varnish. The artist will usually add a signature or other inscription as one of the last touches.Piotr Targowski and his team from Nicolaus Copernicus University, Poland, have used OCT to examine two oil paintings, putatively from the 18th and 19th centuries. The researchers determined in which order various inscriptions and paint layers had been laid down on the canvases. With this information, they could more accurately determine the first painting’s date of creation. Perhaps more importantly, the timing of the layers strongly suggested that the artist’s signature on the second painting was a forgery.