Semi-retroreflective scanners

Abstract

A semi-retroreflective optical scanning system has an optical scanning element which directs a scanning light beam via a stationary holographic optical element in a scan path across indicia to be read. Light from the indicia is collected by an optical collector, such as a lens or a mirror, which focuses it onto a small optical detector via the scanning element. The field of view of the optical detector scans across the indicia in synchronization with the outgoing scanning beam from the holographic optical element. Since the incoming reflected light is not limited by the aperture of the scanning element, the signal-to-noise ratio remains high even when a small scanning element is used. In a further embodiment the light source, scanning element collector and detector are provided in a common plane, the collector and scanning element sharing a common optical axis and a light source being provided substantially off axis to allow reduction in the scanning element size. In a further aspect of the invention a solid state reader is provided capable of establishing the distance to the target by sensing the exact position on the detector at which the target is imaged. According to yet a further aspect of the invention the problem of specular blinding is overcome by providing a plurality of sensors activated in a dynamic on-off sequence, for example a random sequence.