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: calculate a plurality of difference values between outputs of individual ones of the elements and a plurality of neighboring elements, determine the signs of the difference values for a given one of the individual elements, examine the signs of the difference values, if the difference values for a given one of the individual elements have different signs, identify the given element to be of high quality, and if the difference values for the given element have the same signs, identify the given element to be potentially defective and only thereafter proceed to compare the difference values with at least one threshold.