infrared reflectography (IR) and reflectogram

   Infrared reflectography (IR) is used to examine the surface of the ground layer of any painting, even when it is covered by one or many layers of paint. The result of the IR technique is a black and white photograph called a reflectogram. Such images permit art historians to read the underdrawings beneath the paint. This is possible because light in the near-infrared region of the spectrum (wavelengths from about 1 up to 2 microns) easily passes through the paint, to be reflected by the marks used to make the underdrawing. Reflectograms allow a researcher to obtain information about the painter's drawing media and techniques, and even to find and read hidden writings. These might include signatures and dates sometimes covered by earlier restorations. Reflectograms occasionally reveal artists' earlier versions of their compositions (pentimenti), and even sketches of things not visible in the painting as it is seen today. Reflectography is often used by restorers and conservators. In many cases it improves analysis of the creative development of the artwork and reveals pieces of previous restorations. Reflectograms were first made in the early 1930s, when the technique was called "IR photography." Various improvements to IR since then have led to a digital version of the technique — an infrared scanner is connected to a high-powered computer. This advance has made possible far more useful images than ever before, because more gradations of value are now achieveable.