Abstract: A wafer cassette mapper detects the presence or absence of a wafer in a particular corresponding pair of slots in a wafer cassette, as well as a cross slotted condition in which a wafer is not aligned in a corresponding pair of slots. The wafer cassette mapper includes a base member for receiving a standard slotted wafer cassette, the base member supporting a transmitter/receiver module adjacent the rear of the wafer cassette. The transmitter/receiver module includes two vertical columns of light emitting apertures, coupled to an infrared transmitter array, and a single vertical column, between the two columns of light emitting apertures, of light receiving apertures coupled to an infrared receiver array. Corresponding light emitting apertures in each of the two columns are positioned to illuminate the rear edge of each of the wafers in the wafer cassette at an angle that will cause the diffused reflected light to enter an associated one of the light receiving apertures.