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29 May 2007
A novel scanning technique brings ancient cuneiform tablets into sharp relief.
Researchers at Johns Hopkins University have developed an optical scanning technique for digitizing cuneiform tablets, the world’s oldest writing system. Digitizing the half-million known cuneiform tablets would make them more accessible to experts and the public, but most scanning technologies are unable to capture important 3D features on the patterned clay tablets.
Although laser-based scanning has previously been used for 3D digitization, this approach is relatively expensive and does not yield the resolution required by cuneiform scholars. In contrast, Daniel Hahn and colleagues have combined incoherent illumination from a standard commercial light source with novel data processing algorithms to achieve a lateral resolution of up to 5 µm (Applied Optics 46 2838).
The set-up uses a stationary camera and telecentric lens above the tablet, and a light source that rotates around it to vary the illumination angle. "The camera and lens sit about 6 inches above the test object, and the magnification factor of the lens and the pixel size of the detection camera determine the effective resolution of the technique," Hahn explained to optics.org.
The scanning process involves three distinct stages:
A surface color map is made by illuminating the object with solid primary colors. Twelve images are taken separately under red, green and blue light at 30° azimuthal increments.
A surface normal map and the bidirectional reflectance distribution function is constructed using photometric stereo data, collected from 12 images taken at 30° increments. This gives height results which are locally very precise, but accumulate large-scale inaccuracies.
A phase-stepped structured light technique projects a series of sinusoidal patterns onto the tablet and calculates relative heights by comparing the results to those from a flat white reference. Six sets of data at 60° increments are taken, giving results that are globally accurate but of low resolution, and so are complementary to the photometric results.
"These three scans are separate in that the data are gathered from different images, but the images are acquired in succession from the same camera only fractions of a second apart. The test object is not moved once a scan has begun, nor are any other physical changes made – the process is completely automated," explained Hahn. The three data sets are then processed through customized algorithms to give the final surface data, a process that currently takes about 10 minutes per scan.
Beyond the physical requirements for illuminating a fragile ancient artifact, the optics involved posed their own problems. "Photometric stereo requires a uniform and collimated source – these two features alone are hard to achieve," said Hahn. "Structured light adds the additional requirement of shape and intensity control. It’s not trivial to produce a light source that has all three characteristics. Our approach uses a projector as a source with a collimating lens, which does lead to some degradation in all three categories but the effects were manageable."
The team calculated that their scans had a lateral resolution as good as 5 µm and depth resolution of approximately 2 µm, allowing the depth-dependent meaning of the cuneiform indentations to be brought to fresh life. "These techniques have not been employed in artifact scanning before, nor have they been combined as we have done here," said Hahn. "The next step is to build a custom light source that’ll reduce the size and increase performance, and help take the system from a lab bench prototype to a turn-key prototype ready for commercial production."
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