Method for determining a mean radiation power P rad 0 _ of electromagnetic radiation of a radiation source, the radiation being intensity-modulated with modulation frequency ω0, in a predetermined time interval. The method provides a reflector designed to reflect electromagnetic radiation of the radiation source and electromagnetic radiation of a test radiation source, irradiates a predetermined area of the reflector with the source electromagnetic radiation, at least partially irradiates the predetermined area of the reflector with electromagnetic radiation of the test radiation source, measures a ω0-modulated power component Ptest,ω0(t) of a reflected test radiation power Ptest(t) of an electromagnetic radiation of the test radiation source, the radiation being reflected from the area, in the predetermined time interval, determines a mean value P test , ω 0 0 _ of the measured ω0-modulated power component Ptest,ω0(t) of the reflected test radiation power Ptest(t) in the predetermined time interval, and determines the mean radiation power P rad 0 _ from the relationship P rad 0 _ = a · P test , ω 0 0 _ , where a is a predetermined constant.