Look at an ancient coin under ordinary light and the chances are that its features, worn down by its passage from hand to hand, will be hard to make out. Point a spotlight at it, though, so that the face of the coin is illuminated from an acute angle, and the resulting shadows will emphasise any minor details.
This is the basic principle behind a novel technique that is helping archaeologists reveal previously invisible clues hidden in the worn or damaged surfaces of any objects they uncover. From wall paintings in Herculaneum to Scandinavian stone tools to rock art in Libya, polynomial texture mapping, as the process is known, is proving an invaluable way to illuminate the past.
The lighting method was originally developed by Tom Malzbender, a computer scientist at HP’s laboratory in Palo Alto, California, to generate better 3-D imagery for computer games. In its most basic form, the process involves capturing between 30 and 50 digital photos of an object of interest. The pictures are taken from directly above the object in a darkened room. Though the camera is fixed, the object is lit from a different angle in every shot. The photos are then combined on a computer to create an image that can have a “virtual” light shone from various angles to reveal any hidden surface detail. The wavelength of this virtual light can also be changed using the computer, allowing colour-sensitive details of the artefact’s surface to be brought out more clearly.