A casing for an electromagnetic radiation source includes a window made of a material that is transparent to the radiation emitted by the source and which includes at least one chamber in which a material that is opaque to electromagnetic radiation can move. The chamber is designed such that the material that is opaque to radiation can be introduced from the exterior of the chamber. Inside the chamber, the material that is opaque to radiation surrounds a region for passage of a radiation beam such that the area of the beam passage region varies as a function of the volume of opaque material in the chamber, by virtue of which parasitic extrafocal radiation is eliminated from the beam.