Abstract: Machine readable identifying indices for a semiconductor wafer having a top surface, a bottom surface, and at least one flat edge is placed on the bottom surface of the semiconductor wafer, positioned in a predetermined relationship to the flat edge, desirably parallel to the flat edge. Means is provided for directing a first beam of radiant energy at the machine readable code and a second beam of radiant energy at the semiconductor wafer proximate to the machine readable code. Sensing means detects the first and second beams after impinging on the semiconductor wafer. A differential amplifier produces a series of output pulses representing the machine readable code. The output from the sensing means constitutes the inputs to the differential amplifier. In a preferred form, the machine readable code is a bar code including error check symbols. This approach gives a simplified means for identifying semiconductor wafers that is highly reliable.