Abstract: This invention is a method for calibrating a two beam rotationally scanned light beam interruption coordinate data input device. In particular, four reference objects which can be sensed by the effectively rotating directional sensor are disposed at measured equal interval distances along an essentially straight line. The four reference objects are positioned relative to the effectively rotating directional sensor so that each of the objects can be essentially individually sensed in turn in the course of effective rotation of the sensor. The effective angular velocity and effective center of rotation of the sensor is then determined from the known distance between adjacent reference objects and the measured time intervals between detection of adjacent pairs of reference objects by the effectively rotating directional sensor.