1. A method for detecting defects on a wafer, comprising: generating output for a wafer by scanning the wafer with an inspection system in first and second passes using a first optical state of the inspection system; generating first image data for the wafer using the output generated in the first pass and second image data for the wafer using the output generated in the second pass; combining the first image data and the second image data corresponding to substantially the same locations on the wafer thereby creating additional image data for the wafer; detecting defects on the wafer using the additional image data; and generating output for the wafer by scanning the wafer with a different inspection system, generating third image data for the wafer using the output generated using the different inspection system, combining the third image data with the first or second image data corresponding to substantially the same locations on the wafer thereby creating further additional image data for the wafer, and detecting defects on the wafer using the further additional image data.