Image Based Measurement of Ancient Coins

Museums catalogue their ancient coins using top view photographs in which a ruler is placed next to the coin. We present an automatic method that allow numismatists to faster and more accurately process such images. The method consists of two parts. The first part uses a novel algorithm to calculate the images’ scale by a Fourier analysis of the characteristic pattern produced by the ruler. Therefore, distances can be measured in terms of the units on the ruler. The second part uses a shape-adaptive thresholding method to isolate the coin from the background. This makes it possible to determine the real-world size of the coin. Experimental results on 30 images show the high accuracy and robustness of the two algorithms with respect to different coins and rulers as well as the ruler's orientation, position and size.