An apparatus for detecting a degree of particulate contamination on a flat panel is disclosed, including: an illuminator for producing a radiation beam which results in scattered radiation from its scattering by contaminants on a surface of the flat panel under test and reflected radiation from its reflection by the surface of the flat panel under test; a detector for collecting the scattered radiation, the detector having a radiation collection surface perpendicular to a normal of the surface of the flat panel under test; and a beam trimmer for separating the reflected radiation from the scattered radiation, the beam trimmer including a first optical member, disposed in correspondence with the scattered radiation and configured for directing the scattered radiation to be collected by the detector, and a second optical member disposed in correspondence with the reflected radiation and configured for directing the reflected radiation not to be collected by the detector.