Alain Daubigney

Statistical Matching of the Directional Content of an Illumination Source Application to the Restoration of the Legibility of Roman Inscriptions

The visibility of erased inscriptions on Roman stones is improved by illuminating them with a lateral source of light. The optimal angular location and incidence of the source depend on the directional content of the graven letters as well as their degree of erosion. A general multidirectional illumination scheme has been worked out by statistically matching the respective intensities of a set of n primary sources surrounding the stone. The amount of directional information accounted for by each primary source with respect to the corresponding primary image of the stone, disclosed by the source. Multispectral information compression techniques can therefore be applied to the n-dimensional space spanned by the general source. Principal images are computed from the analysis of variance of the directional information in this space. Global or local criteria have to be implemented, depending on degree of erosion of the inscriptions. Full resolution principal images are synthetized by linear combination of photographs of the primary images. Application to the deciphering and reading of Raman inscriptions are presented. The principle of a real time scanner based on the multiplexing properties of white light is introduced as a conclusion. It takes advantage of the achromaticity of the studied objects for coding a spatial frequency scanner upon the chromatic variable. The princi-ple of a generalized source, that illuminates a scene in a pattern recognition context, results from mixing two features : the directional content is statiscally matched to that of the scene, and the chromatic content allows a spatial frequencies sampling