Patent Number: 
Section: claims

1. A beam diaphragm comprising:a pair of control rings having coaxial apertures for the passage of X-rays therethrough, the pair of control rings being opposed to each other axially through a spacing and coaxially rotatable independently of each other;a blade positioned between the pair of control rings;anda position adjusting device including a first pin moveably engaging a slot extending through a first end of the blade, wherein, in accordance with a relative rotation of the pair of control rings, the position adjusting device causes the blade to move toward or away from a common axis of the apertures so as to describe a sectorial plane having a radius that increases or decreases continuously, wherein the radius is defined by a distance between a second end of the blade and the first pin. 2. A beam diaphragm according to claim 1, wherein the position adjusting device further comprises:a first groove radially formed outside the aperture in one of the pair of control rings;a second groove arcuately formed outside the aperture in the other control ring anda second pin for bringing the second end of the blade into engagement with the first and second grooves simultaneously. 3. A beam diaphragm according to claim 1, wherein a planar shape of the blade is symmetrical with respect to a center line. 4. A beam diaphragm according to claim 3, wherein the thickness of the blade decreases gradually toward both right and left sides. 5. A beam diaphragm according to claim 4, wherein the decrease in thickness of the blade is asymmetrical. 6. An X-ray imaging apparatus for photographing a radioscopic image by radiating X-rays emitted from an X-ray source to a subject through apertures of a beam diaphragm, the beam diaphragm comprising:a pair of control rings having coaxial apertures for the passage of X-rays therethrough, the pair of control rings being opposed to each other axially through a spacing and coaxially rotatable independently of each other;a blade positioned between the pair of control rings; anda position adjusting device including a first pin moveably engaging a slot extending through a first end of the blade, wherein, in accordance with a relative rotation of the pair of control rings, the position adjusting device causes the blade to move toward or away from a common axis of the apertures so as to describe a sectorial plane whose radius increases or decreases continuously, wherein the radius is defined by a distance between a second end of the blade and the first pin. 7. An X-ray imaging apparatus according to claim 6, wherein the position adjusting device further comprises:a first groove radially formed outside the aperture in one of the pair of control rings;a second groove arcuately formed outside the aperture in the other control ring anda second pin for bringing the second end of the blade into engagement with the first and second grooves simultaneously. 8. An X-ray imaging apparatus according to claim 6, wherein a planar shape of the blade is symmetrical with respect to a center line. 9. An X-ray imaging apparatus according to claim 8, wherein the thickness of the blade decreases gradually toward both right and left sides. 10. An X-ray imaging apparatus according to claim 9, wherein the decrease in thickness of the blade is asymmetrical.