Abstract: A method and apparatus for receiving two sets of digitized scan data from two optical detectors which simultaneously scan two supposedly identical portions of a photomask for comparing the two sets of scan data to detect defects, and for evaluating the defect data to determine whether or not it represents real defect information or false defect information. Scan lines containing defect data are scanned twice to produce two sets of defect data and the two sets are then compared to produce a real defect data set including only defects detected in both scans. False defects are thus eliminated from the final data set.