Image-based comparison of pre-modern coins and medals

In Numismatics it is of central importance to find out which dies were used to mint a specific coin. For this purpose, coins have to be compared directly with each other. However, different states of preservation turn a comparison into a challenge. Partly degenerated regions which are caused by abrasion and corrosion present considerable difficulties. Furthermore, within a comparison based on images, different directions of illumination can lead to completely different impressions. In this contribution, it is described how the acquisition of coin images can be enhanced by a ThreeColor Selective Stereo Gradient Method. The resulting reflection images can be aligned with each other by a Modified Particle Swarm Optimization Algorithm. This technique helps the numismatist in deciding, and thus decreases the effort of the comparison. So far, the proposed illumination and registration technique has been tested on 14 die-identical coin images of 6 different coin types.