1. A method of calibrating a position measurement device which device utilizes signals from a plurality of detectors, more than two of which view at least one edge between a dark region and a light region at any one position, comprising: measuring the signals of the plurality of detectors at a plurality of positions with respect to the edge; and calculating at least one average of some of the measured signals from individual ones of the detectors that do not view the edge at the position; and calibrating the individual detectors based on averages.