Disclosed are apparatus and methods for detecting defects on a semiconductor sample. The system includes an illumination module for directing a nonzero-order Gaussian illumination beam towards a plurality of locations on a sample and a collection module for detecting light scattered from the sample in response to the nonzero-order Gaussian illumination beams and generating a plurality of output images or signals for each location on the sample. The system further comprises a processor system for detecting defects by (i) processing the output images or signals so as to retain filtered image or signal portions that substantially match a point spread function of the one or more nonzero-order Gaussian illumination beams, and (ii) analyzing the filtered image or signal portions to detect defects on the sample.