Abstract: A system, method, a lithographic apparatus and a software product configured to determine a drift in an attribute of an illumination and a corresponding drift correction. The system includes a lithographic apparatus that includes at least two sensors, each configured to measure a property related to an illumination region provided for imaging a substrate. Furthermore, a processor is configured to: determine, based on a ratio of the measured property, a drift of the illumination region with respect to a reference position; determine, based on the drift of the illumination region, a drift in an attribute related to the illumination upstream of the illumination region measured by the at least two sensors, and determine, based on the drift in the attribute, the drift correction to be applied to the attribute to compensate for the drift in the attribute.