Patent Number: 
Section: claims

1. A filter for use in adjusting a spectrum of passing radiation, said filter comprising:a support plate having an aperture for passage therethrough of radiation, the support plate being disc-shaped and having a circumference;a plurality of filter plates supported by the support plate within the circumference and having mutually different filter characteristics; anda moving device configured to selectively move the plurality of filter plates to a first position to close the aperture and a second position to open the aperture, the moving device configured to bias the plurality of filter plates to the second position and to extend the plurality of filter plates radially inwardly toward the first position. 2. A filter according to claim 1, wherein the moving device comprises:a spring device configured to operate the plurality of filter plates in a direction away from the aperture;a plurality of ratchet plates supported by the support plate in proximity to the plurality of filter plates, each of the plurality of ratchet plates comprising a notched portion;a first support point positioned to pivotably support the plurality of filter plates on the support plate;a second support point positioned to pivotably support the plurality of ratchet plates on the support plate;a leaf spring connected to the plurality of filter plates and the plurality of ratchet plates, the leaf spring configured to cause the plurality of filter plates and the plurality of ratchet plates to pivot interlockedly with each other; anda pawl engageable with the notched portion of each of the plurality of ratchet plates, the pawl configured to move in order to pivot the plurality of ratchet plates, thereby making the plurality of filter plates movable to the first position to close the aperture. 3. A filter according to claim 2, wherein the support plate is formed with the aperture at a center thereof. 4. A filter according to claim 2, further comprising a ring configured to rotate along the circumference of the support plate, the ring comprising the pawl. 5. A filter according to claim 1, wherein the support plate comprises a top layer and a bottom layer parallel to each other, and the plurality of filter plates are supported by the top layer and the bottom layer. 6. A filter according to claim 5, wherein the support plate is formed with the aperture at a center thereof. 7. A filter according to claim 5, further comprising a ring configured to rotate along the circumference of the support plate, the ring comprising a pawl. 8. A filter according to claim 1, wherein the support plate is formed with the aperture at a center thereof. 9. A filter according to claim 8, further comprising a ring configured to rotate along the circumference of the support plate, the ring comprising a pawl. 10. A filter according to claim 1, further comprising a ring configured to rotate along the circumference of the support plate, the ring comprising a pawl. 11. An X-ray imaging system for use in radiographing a subject with X-ray passing through a filter, the X-ray imaging system comprising:an X-ray irradiator; anda filter positioned with respect to the X-ray irradiator, the filter comprising:a support plate having an aperture for passage therethrough of radiation, the support plate being disc-shaped and having a circumference;a plurality of filter plates supported by the support plate within the circumference and having mutually different filter characteristics; anda moving device configured to selectively move the plurality of filter plates to a first position to close the aperture and a second position to open the aperture, the moving device configured to bias the plurality of filter plates to the second position and to extend the plurality of filter plates radially inwardly toward the first position. 12. An X-ray imaging system according to claim 11, wherein the moving device comprises:a spring device configured to operate the plurality of filter plates in a direction away from the aperture;a plurality of ratchet plates supported by the support plate in proximity to the plurality of filter plates, each of the plurality of ratchet plates comprising a notched portion;a first support point positioned to pivotably support the plurality of filter plates on the support plate;a second support point positioned to pivotably support the plurality of ratchet plates on the support plate;a leaf spring connected to the plurality of filter plates and the plurality of ratchet plates, the leaf spring configured to cause the plurality of filter plates and the plurality of ratchet plates to pivot interlockedly with each other; anda pawl engageable with the notched portion of each of the plurality of ratchet plates, the pawl configured to move in order to pivot the plurality of ratchet plates, thereby making the plurality of filter plates movable to the first position to close the aperture. 13. An X-ray imaging system according to claim 12, wherein the support plate is formed with the aperture at a center thereof. 14. An X-ray imaging system according to claim 12, wherein the filter further comprises a ring configured to rotate along the circumference of the support plate, the ring comprising the pawl. 15. An X-ray imaging system according to claim 11, wherein the support plate comprises a top layer and a bottom layer parallel to each other, and the plurality of filter plates are supported by the top layer and the bottom layer. 16. An X-ray imaging system according to claim 15, wherein the support plate is formed with the aperture at a center thereof. 17. An X-ray imaging system according to claim 15, wherein the filter further comprises a ring configured to rotate along the circumference of the support plate, the ring comprising a pawl. 18. An X-ray imaging system according to claim 11, wherein the support plate is formed with the aperture at a center thereof. 19. An X-ray imaging system according to claim 18, wherein the filter further comprises a ring configured to rotate along the circumference of the support plate, the ring comprising a pawl. 20. An X-ray imaging system according to claim 11, wherein the filter further comprises a ring configured to rotate along the circumference of the support plate, the ring comprising a pawl.