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RTI Overview

Reflectance Transformation Imaging (RTI)

Reflectance Transformation Imaging

Different views of squeeze C_018 from Reflectance Transformation Imaging (RTI). Left to Right: "normal" view, raking light view, and specular enhancement view.

What is RTI?

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. The technique involves the capture of multiple images with the subject and camera position fixed and the light source position 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 paper can be given a metallic surface quality to increase legibility and remove color and stains.

Set-up for RTI of squeezes

Left: Setup used for imaging the paper squeezes. Right: E. Keats Webb using RTI to image paper squeezes.

History of RTI

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 applications within cultural heritage and natural history objects.

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.

Squeeze Imaging Project

The image acquisition of the Herzfeld squeezes was completed by imaging specialist contractor, E. Keats Webb, under the supervision of Senior Conservator, Melvin Wachowiak. The imaging was done in the Imaging Studio of the Museum Conservation Institute (MCI) at the Smithsonian Institution. The funding to complete the project was from the 2010 Collections Care and Preservation Fund (CCPF).