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From Mackinaw City; taken while chewing a stick a’ Blackjack licorice gum.

Photograph by Marlin Lum

Reflectance Transformation Imaging, or RTI, is a computational photographic technique that produces detailed computer models of surfaces from photographs. Typically, when capturing a sequence of RTI photographs, one places the camera on a stable platform and takes many pictures while moving a light source around the object one wants to model. From these photographs, computer software is able to analyze the subject and generate a topographical model. This model can then be interactively re-lit or manipulated to produce highly detailed images. Oftentimes the RTI model conveys far more information than any single photograph that went into its production. You can think of it as a powerful method of analyzing surfaces; more powerful than any photograph could possibly be.

For many fields of scientific or cultural inquiry, such robust analysis is exciting and meaningful. Think of a painting conservator able to see new detail in brushstrokes on the surface of a canvas; or an archeologist divining new insights into the methods used to produce a petroglyph far out in the field. Such new methods of seeing hold the potential to unlock new knowledge, and greatly facilitate important scholarship.

In the spirit of advancing these methods, the National Museum of the American Indian participated in a grant from the National Center for Preservation Technology and Training to bring the non-profit group Cultural Heritage Imaging to Washington DC for a week of hands-on instruction in RTI.

On June 8-11, 2009 I joined Mark Christal (Multimedia Co-ordinator for the NMAI), a group of conservators from our museum and the Museum Conservation Institute, and the staff of C-H-I to learn the techniques involved in capturing a good RTI sequence.

Cultural Heritage Imaging is a 501(c)3 non-profit based in California whose mission is to advance the use of advanced digital imaging techniques in the field of cultural preservation. C-H-I travels the world training archeologists, museum professionals, and material cultural documentation experts in RTI and other imaging techniques.

By the end of our week together, we had generated quite a few excellent RTI sequences which I hope to post to my website soon. Part of the NMAI’s role in the grant was to help generate educational materials for the C-H-I website, in order to facilitate future training. Mark and I spent the last day of training with C-H-I staff shooting video clips of the RTI process for the site. It is our hope that future students of RTI will be able to use the clips to better learn the technique.