Contamination, either in the form of particulate matter (46), e.g., dust and non-wetting liquid, or surface discontinuities (48), such as a smooth film or cratering, is collected on an exposed glass plate (12). Illumination at an angle incident with respect to a surface (14) of the glass plate causes the particulate matter to scatter light. A further light source illuminates the inside volume of the glass, causing light to scatter from the surface discontinuities. Either source of light scattering is detected by an optically sensitive detector (18) positioned beneath the glas plate. A bandpass filter (24) between the glas plate and the detector rejects spurious radiation.
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