Method and apparatus for determining focus of a scanning system

1. A non-imaging tool for assisting in determining a focus in a scanning system that scans a planar object with a scanning beam in a fast scan direction, while the planar object is moved under the scanning beam in a slow scan direction perpendicular to the fast scan direction, the tool comprising:

a plate having a planar surface;

a focusing wedge mounted on the plate; and

a ramped grating which forms part of the focusing wedge and which has a tilted surface which extends from above the plate surface to below the plate surface; the ramped grating including alternating, parallel first stripes and second stripes, wherein the first and second stripes are one of, respectively, fluorescent and non-fluorescent stripes, storage phosphor and non-storage phosphor stripes, and reflective and non-reflective stripes.

2. The tool of claim 1 wherein the tool is removably contained in a cassette.

3. The tool of claim 1 wherein the focusing wedge has grating shields on either side of the ramped grating.

4. The tool of claim 1 wherein the tilted plane of the ramped grating crosses the plane of the plate surface at a ramp/plate crossing location and wherein the focusing wedge has a fiducial edge which is spaced from the crossing location a known distance and which locates the crossing location.

5. The tool of claim 4 including one of a fluorescent, storage phosphor and reflective patch bordering the fiducial edge.

6. The tool of claim 1 wherein the ramped grating stripes run perpendicularly to the fast-scan direction of a scanning beam.

7. The tool of claim 1 wherein the ramped grating stripes run perpendicularly to the slow-scan direction of a scanning beam.

8. The tool of claim 1 including at least one plate surface grating having parallel alternating first stripes and second stripes, wherein the first and second stripes are one of, respectively, fluorescent and non-fluorescent stripes, storage phosphor and non-storage phosphor stripes, and reflective and non-reflective stripes which are positioned parallel to the ramp grating and which are near to and parallel to the plate surface.

9. A method of determining a focus in a scanning system comprising:

providing a plate having a planar surface and having a focusing wedge with a ramped grating which has a tilted plane which extends from above the plate surface to below the plate surface, the ramped grating including alternating parallel first stripes and second stripes, wherein the first and second stripes are one of, respectively, fluorescent and non-fluorescent stripes, storage phosphor and non-storage phosphor stripes, and reflective and non-reflective stripes;

scanning the ramped grating in a raster pattern with a beam of radiation to cause the first stripes to emit or reflect radiation;

detecting the emitted or reflected radiation to produce a signal representative of the focus condition of the scanning beam of radiation; and

determining from the signal the best focus for the scanning beam of radiation.

10. The method of claim 9 wherein the plate is moved in a slow-scan direction as the scanning beam is scanned across the plate in a fast-scan direction; and wherein the ramped grating stripes run perpendicularly to the fast scan direction.

11. The method of claim 9 wherein the plate is moved in a slow-scan direction as the scanning beam is scanned across the plate in a fast-scan direction, and wherein the ramped grating stripes run perpendicularly to the slow-scan direction.

12. The method of claim 9 wherein the tilted plane of the ramped grating crosses the plane of the plate surface at a ramp/plate crossing location and wherein the focusing wedge has a fiducial edge which borders a patch which is one of a fluorescent, storage phosphor, and reflective material, and, which is spaced from the crossing location a known distance, and which locates the crossing location.

13. The method of claim 9 wherein the plate includes at least one plate surface grating having parallel alternating first stripes and second stripes, wherein the first and second stripes are one of, respectively, fluorescent and non-fluorescent stripes, storage phosphor and non-storage phosphor stripes, and reflective and non-reflective stripes, which are positioned parallel to the ramp grating, which are near to and parallel to the plate surface; and, which, when scanned by the scanning beam of radiation, produces a signal which is representative of plate focus shift along the length of the slow-scan motion of the raster scan.

14. The method of claim 9 wherein in the determining, the best focus position of the scanning beam of radiation is determined by the location(s) at which the signal is at a maximum.

15. The method of claim 14 wherein the signal maximum is determined by a peak detection circuit.