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Reflectance transformation imaging (RTI) is a highly useful imaging technique for recording the topography, surface texture, and colour of a work of art. Using a stationary, high-resolution digital camera, multiple images are taken of a painting using light from a strobe lamp that is aimed at the surface from various directions. The images, which record the shadows and highlights produced by the strobe, are then processed with sophisticated computer software to produce a model of the painting’s surface. By contrast, raking light images, which capture surface features with light projected from only one direction, produce images where large areas of the paint surface are in deep shadow, resulting in a loss of information. RTI resolves this issue by enabling the researcher to examine the surface from an infinite number of lighting angles so that all the topographical features of a work of art are recorded and visible. When viewed in the RTI software on a computer, the researcher can interactively view the painting surface by manipulating the “virtual” light around and across the image. Other rendering models allow the viewer to examine the surface in different modes. For example, the colour information on the image can be reduced or completely eliminated, thereby enhancing the surface topography to reveal features that are otherwise not visible with conventional viewing methods.

Using RTI, conservators, curators, and conservation scientists can detect the character of brush strokes, areas of impasto, and other minute surface features of a work of art. RTI can even be combined with high-resolution macro photography to detect surface features at very high magnifications. In addition to recording the artistic characteristics of a painting, the condition and stability of the surface can be recorded and monitored. For example, physical degradation of the painting, such as detachment of the paint layers, that might escape identification through normal visual inspection is easily rendered and enhanced with RTI.

http://culturalheritageimaging.org/Technologies/RTI/

http://www.hpl.hp.com/research/ptm/se.html

http://www.hpl.hp.com/research/ptm/ri.html