Method for determining a mean radiation power ##EQU00001## of electromagnetic radiation of a radiation source, the radiation being intensity-modulated with modulation frequency .omega..sub.0, 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 .omega..sub.0-modulated power component P.sub.test,.omega..sub.0(t) of a reflected test radiation power P.sub.test(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.omega. ##EQU00002## of the measured .omega..sub.0-modulated power component P.sub.test,.omega..sub.0(t) of the reflected test radiation power P.sub.test(t) in the predetermined time interval, and determines the mean radiation power ##EQU00003## from the relationship.omega. ##EQU00004## where a is a predetermined constant.