Abstract: A system for positioning an end effector of a wafer handling robot with respect to a wafer to be extracted from a cassette, and for thereafter reading an indicial mark on the wafer. The system includes a pair of sensor units fixedly mounted on the end effector, which sensor units are capable of detecting the wafer edge upon approach of the end effector toward the wafer. Based on the positions of the end effector when the first and then the second sensor units detect the edge of the wafer, a computer may determine the orientation of the end effector with respect to the wafer, and adjust a position of the end effector to a center of the wafer. Thereafter, the indicial mark on the wafer may be read by a camera by withdrawing the wafer from the cassette on the end effector, positioning the center of the wafer over the central axis of rotation of the wafer handling robot, and rotating the robot until the indicial mark is located under the camera.