The invention is related to a method and apparatus for modifying the irradiation distribution of a radiation source. In accordance with the method the radiation source (1) is used to direct radiation to an essentially planar target surface (6). In accordance with the invention between the radiation source (1) and the target surface (6), several plates (4), which are essentially transparent to the radiation and have spaces between them, are placed close to the radiation source (1), in order to use the reflection and absorption of the transparent plates (4) to attenuate the radiation to desired areas.