A method of selecting a periodic modulation to be applied to a variable of a radiation source, wherein the source delivers radiation for projection onto a substrate and wherein there is relative motion between the substrate and the radiation at a scan speed, the method including: for a set of system parameters and for a position on the substrate, calculating a quantity, the quantity being a measure of the contribution to an energy dose delivered to the position that arises from the modulation being applied to the variable of the source, wherein the contribution to the energy dose is calculated as a convolution of: a profile of radiation, and a contribution to an irradiance of radiation delivered by the source; and selecting a modulation frequency at which the quantity for the set of system parameters and the position on the substrate satisfies a certain criteria.