A method of inspecting a plurality of wafers in an optical inspection tool. The method includes the steps of generating a reference wafer and polishing the reference wafer in a chemical mechanical polishing process following a metal deposition process such that the reference wafer is representative of a fully polished wafer. The optical inspection tool scans the reference wafer and a gray level map is generated. A number of further wafers are metalized, polished, scanned and gray level maps generated. The method includes the step of comparing a gray level map of the scanned reference wafer to a number of gray level maps of the scanned wafers. A determination (314) is then made as to whether the wafer exhibits an acceptable polishing quality based on the comparison.