Abstract: A method and apparatus for generating a beam of radiation in which a beam of coherent radiation (2) is fed to a pair of A/O modulators (3,4). The beam is modulated by the modulator (4) in response to a chopped control function which chops between two levels, one of the levels comprising a reference level (V.sub.low). The radiation beam is sampled at times corresponding to the application of the reference level control function to the modulator (4) and a feedback signal corresponding to the sampled power is fed to a comparator (12) where it is compared with a reference voltage (V.sub.ref). The resultant error signal is applied to the modulator (3) so as to maintain at a substantially constant level the power of the sampled radiation when the chopped control function is at the reference level.