At the beginning of April I was lucky enough to attend a RTI (Reflectance Transformation Imaging) workshop offered by Cultural Heritage Imaging (CHI) at Yale University. CHI is a non-profit organization that shares and teaches RTI and Photogrammetry technology with cultural heritage institutions around the world. The class I attended was a 4-day NEH grant sponsored course taught by three RTI experts from CHI, and it was amazing!
This is a composite image of all the highlight points from one RTI section. The software uses these highlight points to map the surface shape and color of your object.
So, what is RTI? CHI describes it on their website as “a computational photographic method that captures a subject’s surface shape and color and enables the interactive re-lighting of the subject from any direction”. For highlight RTI, which is the least expensive and most accessible method for most institutions and what I was taught in the class, you basically take a series of 36 to 48 images of an object where everything is constant (settings and position of objects, camera and spheres) except for the light position. With a reflective black sphere (or 2) set up next to your object, you move you light source around the object at varying angles. Then, you take that set of images and plug them into the free RTI software provided by CHI and the algorithm detects the sphere(s) and the highlight points (from your light) captured on the sphere(s) and voila!…you have an fun and interactive way to look at your object’s surface texture.
Before I attended this fantastic training opportunity, our conservator and I knew right away what the subject of our first capture would be when I returned…a 16th century German Reformation text by Martin Luther with a highly decorated cover that is practically invisible under normal illumination.
Here’s a time lapse video of our first (and second) capture in the Lab…
That day (Tuesday) were were able to capture the upper and lower covers of the Reformation text (from ARB), the original silk cover from a 17th century Chinese manuscript (from Hebrew Union College) and an illuminated page from a German vellum prayer book (from PLCH). And here our some snapshots of our results from two of those captures (click on the thumbnails for a larger view of the image)…
This possibly 13th century German Prayer Book has a full stiff vellum binding and an illuminated first page. The varying modes highlight condition issues like worn/abraded parchment and flaking gold illumination, as well as the overall surface texture of the illumination.
I hope you’ve enjoyed getting a little sneak peek into RTI. I will be demoing and discussing in further depth this afternoon from 1:30 to 3pm at the Lab’s annual Preservation Week Open House. I also hope to do more RTI captures/processes in the future and share them here.
Jessica Ebert (UCL) – Conservation Technician