1. A method, comprising: obtaining image information including a first vector and a second vector by respectively capturing an image of a first wafer and an image of a second, wherein the first wafer is known to be a good product, wherein the second wafer is known to be a defective product; calculating a projection vector based on a covariance matrix associated with the first vector and the second vector; obtaining image information including a third vector by capturing an image of a third wafer under a test; projecting each of the first vector, the second vector and the third vector onto the projection vector; and classifying the third wafer as either the good product or the defective product based on the projected first vector, the projected second vector and the projected third vector.