Cultural Heritage Imaging

Flexible Solutions for RTI by marlinlum
June 8, 2010, 6:07 pm
Filed under: Commentary, Guest Blogger

CHI Blog Post—May 2010

By Guest Blogger E. Keats Webb

Senior Conservator, Melvin Wachowiak, and I have continued using Reflectance Transformation Imaging (RTI) on a regular basis at Smithsonian’s Museum Conservation Institute aiding in the research and preservation of the collections both within the Smithsonian and outside. We have found the technique to be a great indicator of the condition of objects as seen in the imaging of a 2×2.5” daguerreotype. A daguerreotype is one of the earliest photographic processes that formed an image on a silver coated copper plate. The highly reflective and relatively flat surface proved to be an ideal candidate for the RTI process. Using a 21-megapixel full-frame sensor, our final RTI file revealed the detailed information of polish marks and scratches on the metal surface and allowed very close examination due to the high resolution.

The image acquisition for the RTI process has definitely been an adventure of trial and error. When initially imaging the daguerreotype, we ran into issues with flare on the object. We crafted a homemade snoot from cardstock, black velvet and rubber bands, which quickly solved the problem and generated a beautiful final RTI file. This was not our first run-in with stray light. Working with larger objects on the wall and even smaller objects on the copy stand, we have experienced stray light from surroundings including the floor and ceiling tiles and the copy stand post, issues that we remedied with available non-reflective diffuse material like dark moving blankets or scrap velvet. A final example of our adventures of image acquisition includes a human skull and a ring flash. We were losing information to shadows in the roof of the mouth of a skull while imaging the gold filings in the teeth. To resolve this loss of information we used a (stationary) ring flash during the image acquisition in addition to the standard repositioned light source. Before processing the files in RTI Builder, we batch-edited the files removing the ring flash highlight on the reference spheres, which produced a successful final RTI file.

In our image acquisition explorations, we have adopted a creative process of trial and error to our RTI workflow that can incorporate crafting homemade devices and using available materials to remedy unexpected hurdles.


1 Comment so far
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Wow! Love the use of Reflectance Transformation Imaging (RTI). You should probably explain to users it is software driven photographic method that captures a subject’s surface shape and color and enables re-lighting of the subject from any direction, thereby enhancing the subject’s surface shape and color attributes. I have used it a lot actually in scientific television.

Comment by Eran

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