Patent US 7,202,491

Wafers in a cassette are mapped without having to open the cassette. The cassette is at least partially transparent to a particular type of radiation. A source of the radiation is directed into the cassette, through a transparent or translucent part of the cassette, and an imaging sensor sensitive to the radiation detects radiation that is reflected off the wafers inside the cassette. A second source of radiation and a second camera preferably provide additional images of the wafers from a different angle. By processing these images, the spatial orientation of the wafers and loading status of the cassette can be determined.