1. A method of distinguishing high quality elements from potentially defective elements in an array of photo-sensitive elements while illuminated with an object field of varying light intensity thereacross, comprising: calculating difference values between outputs of individual ones of the elements and neighboring elements, if the difference values for a given one of the individual elements have different signs, identifying the given element to be of high quality, and if the difference values for the given element have the same signs, identifying the given element to be potentially defective.