Systems and methods for measuring stray light in a lithographic apparatus are described using Radiometric Kirk Test (also known as Scanning SAMOS Test). The Radiometric Kirk Test of the present invention involves a test pattern having an isolated dark area within a much larger bright field. The radiometric Kirk test includes at least two continuous or stepped scans of an aperture of a detector in an image plane of a lithographic system. During a dark area measurement, the aperture of the detector is positioned such that at least at one point the aperture of the detector is centered within an image of the dark area. During a bright area measurement, the aperture of the detector is positioned within the image of the bright field. The integrated detector signal is correspondingly computed for the dark area measurement and the bright area measurement. The ratio of the integrated dark area measurement result and integrated bright area measurement result is a measure of stray light present in the lithographic apparatus.
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