Cultural Heritage Imaging

Capturing Reflectance Transformation Images by cdschroer
September 30, 2009, 1:52 am
Filed under: Technology, Training | Tags: , ,

By Debra Dooley, Executive Director, and Marlin Lum, Imaging Director, CHI

We have quite a bit of information on Reflectance Transformation Imaging (RTI) on our website. But what’s the process used to create a RTI? There will be several blog entries on the process. For more detailed information we recommend that you sign up for the Digital Imaging Techniques for Conservation & Education 3-hour workshop. Check out a Flickr photostream of one of the workshops.

There are four different parts to the process: Preparation, Capture, Processing, and Viewing. This blog entry is a high-level summary of the Capture process.

There are two different methods used to capture RTI. One is called Highlight RTI and the other uses a hardware system. CHI calls the custom hardware systems we build “RTI Capture Hardware Systems” and they are usually built for specific types of objects.

Highlight RTI

The highlight method is easy to use in the field as well as in a controlled setting. You use a tripod, digital camera, a light source (strobes or continuous lighting) and other generally available equipment. That’s one of the big pluses about highlight RTI —  “over the counter,” pro-sumer photographic equipment will yield professional, museum-quality,  high-grade digital RTI surrogates.

This picture was taken after one of the workshops. This is a classic “camera down” or “floor setup,” and is pretty basic. Basics include: a tripod with the camera mounted in the down position, a wireless trigger set (pocket wizards), a 580EXII flash strobe (with a string attached), and a priceless stone tool (insert your artwork here) in the field of view.

Carefully placed next to the object are two round black shiny reflective spheres. When the strobe hits the shiny surface, it produces a “highlight.” Bling! Bling! Software used in processing the images finds the highlights in each image to derive a light position (LP).

Also in this image is a Canon EOS Digital Rebel, tethered to a laptop and being controlled by the Canon EOS utility software. The blue tape minimizes movements and vibration during the capture process. The sandbag hanging from the ball head also dampens the wiggles. Again, though basic, the results are high tech.

Highlight RTI setup

Highlight RTI setup

RTI Capture Hardware System

The RTI Capture Hardware System is better suited to controlled settings such as in a museum conservation photo/lab. One benefit of using a rig is efficiency.  Think mass production, or in this case, mass-documentation. A RTI rig/dome is extremely favorable if one needs to document a large quantity of similar sized objects: coins, signatures, stone tools, paper samples, paintings, objects, etc. etc.

The most noticeable feature of an RTI Capture Hardware system is the light array — many lights (up to 40) mounted at different angles, all directed toward the center, at the object. And of course the camera is also mounted at the apex of the unit. A PC laptop controls the light sequence and asks the camera to open the shutter when a lamp is on, capturing the image with different light positions.

This capture sequence is  managed by open source software that we teach you how to use in our workshops, so you should look into it if this is interesting to you.

4-Day Training: Reflectance Transformation Imaging:
“Generating Digital Representations of Cultural Heritage Objects”

The following image is of a custom RTI Capture Hardware System built as part of the Developing Advanced Technologies for the Imaging of Cultural Heritage Objects project. CHI built 2 identical systems, one for the University of Southern California’s West Semitic Research Project and one for CHI.

Custom RTI Capture Hardware System

Processing the Images. After capture comes the processing of your images. Stay tuned for an update on that soon.