Patent Number: 
Section: claims

1. A filter comprising:more than two plural filter plates that form a layer crossing X-rays, wherein each of said plural filter plates has a constant thickness and the thickness of each filter plate is successively doubled with the thinnest filter plate defined as a reference; andan adjusting device that adjusts a combination of filter plates forming the layer by individually moving the plural filter plates so as to come in and out of an X-ray passing space. 2. A filter according to claim 1, wherein the adjusting device has a pair of moving in/out mechanisms for moving the plural filter plates in and out from both sides of the X-ray passing space, wherein a first of said mechanisms moves a first set of said plural filter plates comprising every other of said plural filter plates, and a second of said mechanisms moves the remaining of said plural filter plates. 3. A filter according to claim 2, wherein the moving in/out mechanisms move the filter plates in and out by a reciprocating movement of a link based upon a rotation of a plate cam. 4. A filter according to claim 3, wherein plural plate cams present at the same side with respect to the X-ray passing space have a common rotation axis. 5. A filter according to claim 2, wherein the moving in/out mechanisms move the filter plates in and out by a swing movement of an arm driven by a motor. 6. A filter according to claim 2, wherein the moving in/out mechanisms move the filter plates in and out by using a reciprocating movement of a movable section of an electromagnetic solenoid. 7. A filter according to claim 2, wherein the moving in/out mechanisms move the filter plates in and out by using a reciprocating movement of a movable section of an air cylinder. 8. A filter according to claim 2, wherein the moving in/out mechanisms move the filter plates in and out by using a reciprocating movement of a movable section of a hydraulic cylinder. 9. An X-ray imaging apparatus for imaging a subject by X-rays via a filter, wherein the filter comprises:more than two plural filter plates that form a layer crossing X-rays, wherein each of said plural filter plates has a constant thickness and the thickness of each filter plate is successively doubled with the thinnest filter plate defined as a reference; andan adjusting device that adjusts a combination of filter plates forming the layer by individually moving the plural filter plates so as to come in and out of an X-ray passing space. 10. An X-ray imaging apparatus according to claim 9, wherein the adjusting device has a pair of moving in/out mechanisms for moving the plural filter plates in and out from both sides of the X-ray passing space, wherein a first of said mechanisms moves a first set of said plural filter plates comprising every other of said plural filter plates, and a second of said mechanisms moves the remaining of said plural filter plates. 11. An X-ray imaging apparatus according to claim 10, wherein the moving in/out mechanisms move the filter plates in and out by a reciprocating movement of a link based upon a rotation of a plate cam. 12. An X-ray imaging apparatus according to claim 11, wherein plural plate cams present at the same side with respect to the X-ray passing space have a common rotation axis. 13. An X-ray imaging apparatus according to claim 10, wherein the moving in/out mechanisms move the filter plates in and out by a swing movement of an arm driven by a motor. 14. An X-ray imaging apparatus according to claim 10, wherein the moving in/out mechanisms move the filter plates in and out by using a reciprocating movement of a movable section of an electromagnetic solenoid. 15. An X-ray imaging apparatus according to claim 10, wherein the moving in/out mechanisms move the filter plates in and out by using a reciprocating movement of a movable section of an air cylinder. 16. An X-ray imaging apparatus according to claim 10, wherein the moving in/out mechanisms move the filter plates in and out by using a reciprocating movement of a movable section of a hydraulic cylinder. 17. A filter comprising:plural filter plates that form a layer crossing X-rays, wherein each of said plural filter plates has a constant thickness; andan adjusting device that adjusts a combination of filter plates forming the layer by individually moving the plural filter plates so as to come in and out of an X-ray passing space, the adjusting device having a pair of moving in/out mechanisms for moving the plural filter plates in and out from both sides of the X-ray passing space, wherein a first of said mechanisms moves a first set of said plural filter plates comprising every other of said plural filter plates, and a second of said mechanisms moves the remaining of said plural filter plates.