Disclosed is a method of determining a characteristic of a target on a substrate and corresponding metrology apparatus and computer program. The method comprises determining a plurality of intensity asymmetry measurements from pairs of complementary pixels comprising a first image pixel in a first image of the target and a second image pixel in a second image of the target. The first image is obtained from first radiation scattered by the target and the second image is obtained from second radiation scattered by the target, the first radiation and second radiation comprising complementary non-zero diffraction orders. The characteristic of the target is then determined from said plurality of intensity asymmetry measurements.