In my last post, I introduced you to the cutting edge photography Reflectance Transformation Imaging (RTI), a technique invented by Tom Malzbender at Hewlett Packard Labs. Here at the Museums, we have been using RTI to gain better understanding of objects in our permanent collection. We have just completed another round of RTI photography of this 5th-century Greek pelike.
When the conservators decided to remove the artwork from its display case at the Legion of Honor to study it in more detail, we decided that we should also shoot the pot using RTI while it was off view.
The pelike is carefully protected while it waits to be photographed.
Then we brought the piece into the photo studio. Note all the sandbags surrounding the base of the object. The sand bags are arranged to protect the pot in case an earthquake strikes and will prevent the pot from tipping over onto its side and breaking.
Next we had to decide which area of the pot might be interesting to shoot.
Here you an see the set up: Camera on monostand, artwork on table, two reflective spheres suspended near the pot, but in the same focal plane with the area we’re going to photograph.
Here’s a close up of the pot with the reflective spheres. By the way, in case you think those spheres are very expensive equipment, think again. They’re actually billiard balls!
The highly reflective billiard balls display a distinct highlight from the strobe light. This image shows the billiard ball reflecting the strobe light in one of our shots.
This distinct highlight is identifiable by the software, which allows it to deduce at what angle the light was coming from for a particular shot. This information is then synthesized by the software and the end result is that the person viewing the RTI image can interactively change the direction of the light. It is this changing light that emphasizes the topographical details that might not otherwise be visible to the naked eye.
As you can see by the number of strobes reflected in the composite shot, we photographed a lot of different views. Unfortunately, the first set we did was out of focus. It’s very important that the camera stay absolutely still. Even using a cable release to trip the shutter causes too much movement, so instead we use the computer to control the camera.
In this type of photography the camera and artwork remain absolutely still, while the strobe light moves around the artwork and a series of shots are taken with the light coming in from different angles. The software then combines all these images into one virtual composite; in addition, the software can mathematically accentuate the peaks and valleys of the topography, making the idiosyncrasies of the object's surface even more obvious.
I think we were successful, but we won't know the exact results of the RTI until the conservators weigh in. Adding to their vast knowledge of materials and techniques, the conservators will now be able to use the RTI photography we produced to help them gain insight into the makeup and material history of the artwork.
In the meantime, here are a few stills from the shoot in which you can see the emerging topography of the pelike: