A new wafer inspection system basing on image to rule comparison was developed. For this system an exact characterization of the defects which have to be identified is very important. After that, inspection rules are established. In this connection the optical filters are useful to support the following digital image processing procedures. The influence of apodization-, edge enhancement filters and dark field methods on the images of wafers with crystal growth defects are discussed in detail. Especially dark field methods are suitable to enhance defects and to suppress structure information. Apodization filters are useful to smooth ringing effects. For edge enhancement methods (high pass filters) the phase information is characterized by dark edges.