Some of the sherd images on the Atlas pages are produced from images originally generated by a flat-bed scanner. The following notes describe the procedures and some of the pitfalls. This technique should be considered experimental.
I first encountered the technique of scanning shallow three-dimensional objects in the context of scanning botanical samples to produce images for an electronic herbarium (for example Herbier de la Drôme provençale).
Not all flat-bed scanners seem to have enough depth of field to capture such an image and some experimentation is necessary. The software/hardware combination employed for the Atlas images is listed at the bottom of this page.
A CCD (charge-coupled device) scanner gives far better depth of field than a CIS (contact image sensor) scanner, though the latter are simpler to manufacture have lower power consumption and are cheaper. The difference between the technologies is explained on this page (or here). There are several web sites describing experience with scanning 3D objects with sample scans of (for instance) flint arrowheads or antique pistols.
The initial technique investigated (which had proved successful with the botanical images) was to place the sherd directly on the glass plate of the scanner and place a shallow box lid with a white interior over this. The resulting image captures the essentials of the sherd, though it can be rather dark, particularly with larger sherds, and the background shows as a grey or brown colour. This can be adjusted using image editing tools, or the sherd can be cut out from the surrounding background.
To counter the problems of the background colour further experiments were conducted by supporting a small light panel with a daylight balanced tube over the scanner. The resulting images are brighter and have a paler background.
The light panel can propped over the scanner and sherds on blocks of wood or polystyrene. The best sherds are clearly those that are both relatively shallow and flat, though sherds can also be propped on their edge to enable the break to be scanned.
The glass surface of the scanner can (and should) be protected from the sherds by a thin transparent sheet of plastic film, such as that used on overhead projectors. When this becomes scratched it can be replaced. Any light leaking in from the sides into the scanning area can be blocked with polystyrene blocks (such as offcuts from insulation boards).
The highest resolution (600dpi) scans can produce a very detailed image similar to that seen when sherds are examined using a binocular microscope, but these can be too large for current web usage.
Further processing is not usually necessary or desirable, but the Unsharp Mask (see right) can be used emphasize the surface or texture of some sherds. This and other treatments can often be automated using batch tools such as those in the ImageMagick package without resorting to time-consuming GUI graphics programs.
The final image for the Atlas has a scale added as an overlay with a perl script using the ImageMagick module. A clear advantage of these images over those produced with a digital camera, for instance, is that they are at a known, fixed scale.