A measurement system (10) for determining a polarization parameter of an optical system (50) has an illumination system (12) providing an optical radiation (14), a measurement mask (22) arranged between the illumination system and the optical system and including measurement structures (24) arranged at a plurality of field points (26) of the measurement mask, a polarization variation device (28) arranged in a beam path of the optical radiation and configured to vary a polarization state of the optical radiation in a field-point-dependent manner, such that at the same point in time one of the field points is irradiated with the optical radiation (14-1) in a first polarization state and another of the field points is irradiated with the optical radiation (14-2) in a second polarization state, and a detection module (32), which is configured to detect the optical radiation after it has interacted with the optical system.