This blog is a brief intro to RTI (Reflectance Transformation Imaging) and my personal tips and tricks. You find everything on the website of the group developing the tool.
I must say I was not that excited the first time I heard about it while I was fellow in conservation science at the Metropolitan Museum in NY in 2010. There was an RTI training section and yes, I thought it was a kind of enhanced-raking light-Photoshop method. Though, after testing it myself, I have been captured by the fact that it produces an actually useful documentation with basic equipment and pretty fast and simple procedure. I find RTI especially useful for my work in support for appraisal and authentication professionals. When you want to share the “feeling” of the painting’s surface with the team you are working with — but they are scattered around the world and you are the only one who has access to see the painting. So, I was pretty happy to introduce this imaging method in my multispectral imaging documentation service.
What you need. I have to do Reflectance Transformation Imaging - RTI — all alone with as a light equipment as I can. So this is what I figured out.
- Camera. I recommend an high-resolution camera. Since stitching photos to get bigger images is not an option here (you should do the same stitching at least 24 times!!) you should use a camera with pixel count enough to get the details you need. I use my beloved Nikon D800, 36 MP.
- Black Spheres. You can be creative and try to experiment using everything you find suitable. I have used black marbles (this were pretty awesome), ping-pong ball painted black, mini billiard number 8 ball (it is already black and you find it on eBay sold as a key chain) and ball bearings.
- Lighting. I prefer halogen spotlights, IKEA mainly. I always avoid using Flash light to document art. Their CRI — color rendering index — is pretty low.
- Software. Manuals and software on the RTI group’s website.
- Metering tool. You need to put the lamp at the same distance while you move it around the object and at different angles. I use Bosh DLE 40 laser rangefinder. It’s precise (1.5 mm), small and lightweight. It fits perfectly in one hand.
- Remote shutter. As i said I work alone. don’t get me wrong. I’d love company but that would dramatically increase traveling cost and so on.
Let’s start! This video briefly describe the hardware and software process. It’s just a brief description all the details you need to know are on the CHI group website.
Infrared RTI. Yes, you read right. I did some Infrared RTI. It has some advantages. When it comes to infrared reflected, the better contrast to read the underdrawing can be achieved at different lighting angle for different areas. So, for example bright areas have a different behavior than dark areas. See the shots below from an Infrared RTI I did on a mock-up painting with a underdrawing squaring grid. More details about this painting can be found here. It result that on bright areas the underdrawing grids are better visible in raking light while in the darker areas they are better seen in top lighting. Compared with the normal digital infrared image, it seems, Infrared RTI can give a better imaging of the underdrawing.