Abstract: A method for fabricating a geometric beamsplitter involves applying a reflective coating having at least one metallic layer to a transparent substrate. A pattern of holes containing numerous holes that are preferably randomly distributed over its reflective surface is created in the reflective coating using laser processing. The method allows inexpensively fabricating beamsplitters that have accurately defined transmittances. Beamsplitters in accordance with the invention are suitable for use as dosimetry mirrors on, for example, the illumination systems of microlithographic projection exposure systems.