US Patent 7274032 Document processor with optical sensor arrangement

An apparatus for document processing comprises an optical sensor including a light source, a light detector and an optical element. The optical sensor is adapted so that, during operation of the apparatus, at least a first portion of light from the source that enters the optical element travels along paths in the optical element so as to be re-directed by total internal reflection toward the detector and wherein the total internal reflection is maintained when the optical element is wet. Signals form the optical sensor may be used to determine, for example, the state of a document storage cassette or the location of a document with respect to the cassette.