Abstract
Motivations for studying inverse optical problems in the Central Laboratory of the Landis & Gyr Corporation are the encoding of information in diffracting or scattering structures tailored to purpose, the reading of such information using optical sources and detectors, and the retrieval of structural information in the presence of random scatterers (such as a rough surface), with emphasis on the automatic optical checking of identity or authenticity. One example of a product based on such procedures is the PHONOCARD currently being manufactured by Sodeco-Saia, a company of the Landis & Gyr group. The PHONOCARD is a pay telephone operated by pre-paid cards (see Fig. 1) with optically stored value units, whose authenticity is checked by an optical reading system.