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The set-up for the RTI method includes a fixed position for the camera and object being imaged. [Photo by Jeff Speakman, MCI]
Reflectance Transformation Imaging (RTI) is a new imaging technique that creates hyper-realistic digital surrogates that are interactively controlled by the viewer. This new method is based upon the synthesis of multiple digital images of a subject in a fixed position collected from a fixed camera position.
RTI was invented by Hewlett Packard Labs researcher, Tom Malzbender, in 2001, taking the initial form of Polynomial Texture Mapping (PTM). This open-source software has been adopted and developed by Cultural Heritage Imaging (CHI) and has found its niche with cultural heritage and natural history objects. The technique involves the capture of multiple images with the subject and camera position fixed and the light source varying. The series of images are lit from a point source of light that is at a constant radius from the subject, but relocated through a virtual hemisphere of positions. The software processes the multiple images into a single file that derives all possible light positions within the virtual hemisphere of light. The final image looks like a 2D photograph, but is actually the documentation of the subject’s surface interaction with the light positions, at the individual pixel level. By moving a mouse (or other pointing device), the viewer can control the light direction, zoom in and out, and select data enhancement options that increase sharpness and contrast and even change surface properties. For instance, non-reflective surfaces such as textiles can be given a metallic surface quality to increase legibility and remove color and stains.
RTI can be used to display and conduct research on a variety of object sizes and types, some of which are difficult or impossible to image with any other method. The technique can produce surrogates of minute objects, smaller than buttons and coins, as well as larger ones, even larger than easel paintings. For sensitive and fragile collection objects, RTI is a non-destructive method that produces web-ready surrogates easily accessible to researchers and the general public.
Left to Right: Normal View of paper squeezes. Raking Light View of paper squeezes. Specular Enhancement View of paper squeezes.
Objects that have been imaged in the MCI Imaging Studio using RTI include paper "squeezes" or molds made from the impressions of ancient archeological sights created from paper pulp. In some cases these "squeezes" are primary resources containing rare intellectual and physical information from sites that no longer exist. The fragility of the paper minimizes accessibility of these objects to researchers and scholars making them great candidates for the non-destructive documentation of the 3-D characteristics with the RTI method. This imaging technique increases legibility through the combination of raking light features and the specular enhancement option while also creating a surrogate that is more accessible to researchers and scholars.
In 2010 the MCI Imaging Studio received a Collections Care and Preservation Fund (CCPF) grant in collaboration with Smithsonian Freer and Sackler Galleries to imaging paper squeezes using RTI. For more information click here.
A video walking through the viewing of the RTI of a daguerreotype.