A method and apparatus for the characterization and discrimination of bank notes and documents that are legal tender, the method for which is based on break down of the surfaces of documents into different characteristic zones, for the posterior detailed analysis of each zone under special conditions which are determined according to the characteristics of each. The method includes a process for self-calibration of the apparatus together with another for the automatic centering of documents. The apparatus includes an optical system composed of plates (3 and 4) between which the document moves, at least one mirror (11) which directs the image at a lineal photosensor (8), a diaphragm (10) which corrects luminosity and focus, and a meniscus lens (9), there also being a source of infrared illumination (13) and a broad spectrum light source (14).
