Abstract: An irradiation apparatus for obtaining an irradiation beam having precise and easily adjustable dimensions. This apparatus comprises more particularly a secondary collimation device having an axis Y--Y, removably fixed to the mobile arm of the irradiation apparatus and comprising a conical pot, two pairs of frames which support collimating plates and are movable about shafts fixed to said conical pot, each of said frames being connected to said conical pot by means of a compass system formed from legs hinged together about shafts. With said frames are associated mobile antipenumbra plates whose position is checked by means of a feeler system.