Patent Number: 
Section: claims

1. A radiation source comprising:a source of radiation distributed along a given direction;a plate having an aperture formed therein, said aperture facing the source of radiation; andmeans for moving the source along the given direction when no radiation is desired on the side of the plate away from the source. 2. A radiation source according to claim 1 wherein the plate is a flat plate. 3. A radiation source according to claim 1 wherein the plate has a thickness that is greater than the smallest dimension of the aperture. 4. A radiation source according to claim 3 wherein the thickness is more than five times as large as the smallest dimension of the aperture, such that radiation exiting the aperture is collimated in the direction of the smallest dimension of the aperture. 5. A radiation source according to claim 4wherein the thickness is more than ten times as large as the smallest dimension of the aperture. 6. A radiation source according to claim 1wherein the aperture has a slit shape, such that the radiation exiting the slit forms a collimated fan beam. 7. A radiation source according to claim 1 wherein the radiation source is displaceable linearly with respect to the aperture, such that radiation from the radiation source does not pass through the aperture. 8. A radiation source comprising:a plurality of individual sources of radiation;a plate having a plurality of apertures formed therein, each said aperture facing a respective individual source of radiation; andmeans for moving the sources such that they do not face the apertures so that they are situated between the apertures when no radiation is desired on the side of the plate away from the source. 9. A radiation source according to claim 8 wherein the plate is a flat plate. 10. A radiation source according to claim 8 wherein the plate has a thickness that is greater than the smallest dimension of the apertures. 11. A radiation source according to claim 10 wherein the thickness is more than five times as large as the smallest dimension of the apertures, such that radiation exiting the aperture is collimated in the direction of the smallest dimension of the aperture. 12. A radiation source according to claim 11 wherein the thickness is more than ten times as large as the smallest dimension of the apertures. 13. A radiation source according to claim 8 wherein the apertures have a slit shape, such that the radiation exiting the slits forms collimated fan beams. 14. A radiation source according to claim 8 wherein the individual sources are situated along a line.