1. A method of distinguishing high quality elements from defective elements in an array of photo-sensitive elements, the method comprising: calculating a plurality of difference values for a first element of the array of photo-sensitive elements by comparing the output of the first element to the outputs of a corresponding plurality of elements neighboring the first element; determining whether the sign values of the plurality of calculated difference values fall within at least a first, second or third group wherein the first group being the sign values are all positive, the second group being the sign values are all negative, and the third group being the sign values are neither all positive nor all negative; in response to a determination that the sign values of the plurality of difference values fall within the first and second groups comparing each of the plurality of difference values with at least one defect determination threshold, and in response to a determination that the sign values of the plurality of difference values fall within the third group identifying the first element to be of high quality.