A method for characterizing a surface of a sample object, the method including dividing the surface into pixels which are characterized by a parameter variation, and defining blocks of the surface as respective groups of the pixels. The method further includes irradiating the pixels in multiple scans over the surface with radiation having different, respective types of polarization, and detecting returning radiation from the pixels in response to each of the scans. For each scan, respective block signatures of the blocks are constructed, in response to the returning radiation from the group of pixels in each block. Also for each scan, a block signature variation using the respective block signatures of the blocks is determined. In response to the block signature variation, one or more of the types of polarization for use in subsequent examination of a test object are selected.