Patent Number: 061480622
Section: claims

1. A beam-shaping filter of variable area, which comprises a main frame in the form of a flat ring provided with a central circular opening for passage of a beam of radiation, at least one sliding rail fixed to one of the main surfaces of the main frame and at least one compensating plate made of a radiation-absorbent material, which can be moved along the rail between a retracted position in which the compensating plate lies outside a maximum field of view of the beam and active positions in which the compensating plate lies at least partly within the maximum field of view of the beam wherein the compensating plate comprises a first and second plate element which can be moved one relative to the other in such a way that, when the plate is in an active position, the thickness of absorbent material through which the beam passes is constant. 2. Filter according to claim 1 wherein the plate elements can be moved rotationally. 3. Filter according to claim 2 wherein the first plate element is joined by a first of its ends to the sliding rail and the second plate element is joined, so as to pivot by a first of its ends, to a second of the ends of the first plate element opposite the first end. 4. Filter according to claim 3 wherein the second plate element rotates with respect to the first plate element by the second plate element pivoting with respect to a pivot pin connected to the second end of the first plate element and to a first end of the second plate element. 5. Filter according to claim 4 wherein a second end of the second plate element has a curved elongate slot into which a stud engages, this stud being fixed to the first plate element and sliding in the slot in order to guide the second plate element during its rotation. 6. Filter according to claim 5 wherein the plate comprises a stressed spring joined respectively by one of its ends to the first and second plate elements so that the second plate element rotates with respect to the first plate element automatically under the action of the spring during the movement of the plate from its retracted position to an active position. 7. Filter according to claim 6 wherein a stop is fixed to the main frame so that, when the plate is in its retracted position, the second plate element is held in place against the force exerted by the spring. 8. Filter according to claim 4 wherein the plate comprises a stressed spring joined respectively by one of its ends to the first and second plate elements so that the second plate element rotates with respect to the first plate element automatically under the action of the spring during the movement of the plate from its retracted position to an active position. 9. Filter according to claim 8 wherein a stop is fixed to the main frame so that, when the plate is in its retracted position, the second plate element is held in place against the force exerted by the spring. 10. Filter according to claim 9 wherein the second plate element is a curved plate of constant width. 11. Filter according to claim 10 wherein the relative translation between the first plate element and the second plate element is provided by a pairs of studs fixed to the first plate element engaging in parallel straight slots provided in the second plate element. 12. Filter according to claim 1 wherein the plate elements can be moved in relative translation. 13. Filter according to claim 12 wherein the relative translation between the first plate element and the second plate element is provided by a pairs of studs fixed to the first plate element engaging in parallel straight slots provided in the second plate element. 14. Filter according to claim 1 wherein the plate elements have a minimum overlap area in the maximum field of view of the beam and in that they are bevelled in their parts corresponding to the minimum overlap area in such a way that the thickness of absorbent material through which the beam passes is constant. 15. Filter according to claim 1 comprising two diametrically opposed parallel straight rails and two compensating plates each of which can be moved respectively on one of the rails. 16. An imaging machine comprising a source of beam radiation, a shaping filter disposed in the beam and means for collecting the radiation and forming an image wherein the shaping filter comprises a main frame in the form of a flat ring provided with a central circular opening for passage of a beam of radiation, at least one sliding rail fixed to one of the main surfaces of the main frame and at least one compensating plate made of a radiation-absorbent material, which can be moved along the rail between a retracted position in which the compensating plate lies outside a maximum field of view of the beam and active positions in which the compensating plate lies at least partly within the maximum field of view of the beam wherein the compensating plate comprises a first and second plate element which can be moved one relative to the other in such a way that, when the plate is in an active position, the thickness of absorbent material through which the beam passes is constant.