Calibration apparatus, system and method

• US 20030038933A1
• Filed: 04/19/2002
• Published: 02/27/2003
• Est. Priority Date: 04/19/2001
• Status: Abandoned Application

• Current Assignee: Dimensional Photonics, Inc.
• Original Assignee: Dimensional Photonics, Inc.
• Inventors: Derr, Nathan D., Shirley, Lyle G., Swanson, Gary J.
• Application Number:
  US10/126,187
• Publication Number:
  US 20030038933A1
• Time in Patent Office: Days
• US Class Current: 356/243.1
• CPC Class Codes: G01B 11/25 by projecting a pattern, e.g. one or more lines, moiré fringes on the object G01B11/255 takes precedence ; image analysis for depth or shape recovery G06T7/50
• CPC Class Codes: G01B 11/2504 Calibration devices
• CPC Class Codes: G01B 21/042 Calibration or calibration artifacts G01B3/30, G01B9/02072 take precedence
• CPC Class Codes: G01N 21/278 Constitution of standards

Abstract

An aspect of the invention relates to a calibration standard for a three-dimensional measurement system and various calibration methods and techniques. The calibration standard typically includes a calibration standard surface and a plurality of optical targets. The optical targets being are affixed to the calibration standard surface and define a three-dimensional distribution of optical reference points. The optical targets can be serve as active, passive calibration targets, or combinations of both. In one embodiment, the optical targets include an optical source and a diffusing target, and each of the optical sources are configured to illuminate the respective diffusing target. The optical targets can be removably affixed to the calibration standard surface.

Claims

1. A calibration standard for a three-dimensional measurement system comprising:

a calibration standard structure; and

a plurality of optical targets, each of the optical targets being affixed to the calibration standard structure and defining a three-dimensional distribution of optical reference points.

2. The calibration standard of claim 1 wherein at least one of the optical targets comprises a passive calibration target.

3. The calibration standard of claim 1 wherein at least one of the optical targets comprises an active calibration target.

4. The calibration standard of claim 1 wherein at least one of the plurality of optical targets comprises an optical source and a diffusing target, and the optical source is configured to illuminate the respective diffusing target.

5. The calibration standard of claim 1 wherein the optical targets are removably affixed to the calibration standard surface.

6. The calibration standard of claim 1 wherein at least one of the optical targets further comprises an optical target surface, wherein the optical target surface comprises a retroreflective material.

7. The calibration standard of claim 1 further comprising a plurality of detectors adapted for measuring the local fringe intensity of a projected fringe pattern.

8. The calibration standard of claim 7 wherein at least one of the detectors is colocated with a respective one of the optical targets.

9. The calibration standard of claim 3 further comprising an active calibration target control system to independently activate and deactivate each of the plurality of active calibration targets.

10. The calibration standard of claim 1 wherein the calibration standard structure further comprises a contoured structure chosen to resemble a surface of an object of interest.

11. The calibration standard of claim 4 wherein the optical source is a light emitting diode.

12. The calibration standard of claim 1 further comprising a plurality of supports having a first end and a second end, the first end of each of the supports being affixed to the calibration standard structure, the second end of each of the supports being affixed to a calibration target surface.

13. The calibration standard of claim 1 wherein the plurality of optical targets comprise a plurality of pyramid targets, each of the pyramid targets having at least three diffuse sides and a vertex, the plurality of vertices being distributed in three dimensions.

14. The calibration standard of claim 1 further comprising a wireless module connected to at least one active calibration target.

15. An optical calibration target for use in a three-dimensional measurement system comprising:

a calibration target surface; and

an optical calibration target support attached to the calibration target surface.

16. The optical calibration target of claim 15 wherein the calibration target support further comprises an optical calibration target housing, wherein the optical calibration target housing comprises at least one of an optical source, and an optical detector, and a diffusing target.

17. The optical calibration target of claim 15 wherein the calibration target surface comprises a retroreflective coating.

18. The optical calibration target of claim 15 wherein the calibration target surface comprises an interference fringe intensity detector.

19. The optical calibration target of claim 15 wherein the target can be removably affixed to a geometric locus of interest on an object being measured by the three dimensional measurement system.

20. A calibration system for use in a three-dimensional measurement system comprising:

an optical receiver, an optical source, a calibration standard, and at least one optical calibration target wherein the optical source is disposed to illuminate the calibration standard, wherein the optical receiver is positioned to view at least one of the calibration standard and optical calibration target.

21. The system of claim 20 wherein the optical source has an annular structure adapted for mounting to an imaging system.

22. The system of claim 20 wherein the calibration standard comprises at least one fringe intensity detector.

23. The system of claim 20 wherein the calibration standard further comprises a calibration standard surface chosen to resemble a surface of an object of interest.

24. The system of claim 20 wherein the three-dimensional measurement system comprises an interference fringe projector.

25. A method for positioning an object at a focal point of an optical imaging device adapted for use in three-dimensional measurement system comprising the steps of:

providing a first movable orienting device fixed relative to the optical imaging device, wherein the first movable orienting device has a first projection element;

providing a second movable orienting device fixed relative to the optical imaging device wherein the second movable orienting device has a second projection element;

configuring the first and second movable orienting devices such that the first and second projection elements intersect at a focal point of the imaging device when the first and second movable orienting devices are moved in a prescribed manner; and

positioning the object at the focal point.

26. A device for positioning an object at a focal point of an optical imaging device adapted for use in three-dimensional measurement system comprising:

a first movable orienting device fixed relative to an optical imaging device wherein the first movable orienting device has a first projection element, and

a second movable orienting device fixed relative to the optical imaging device wherein the second movable orienting device has a second projection element;

wherein

the first and second projection elements intersect at a focal point of the imaging device when the first and second movable orienting devices are moved in a prescribed manner.

27. The device of claim 26 wherein the first movable orienting device is a laser beam projector with a first laser beam projection element.

28. A method for calibrating a measurement system for determining three-dimensional information of an object, the method comprising the steps of:

acquiring two-dimensional fringe data representative of a calibration object, having three-dimensional truth data, using the measurement system;

determining three-dimensional coordinate data for the calibration object in response to the two-dimensional fringe data;

comparing the three-dimensional coordinate data and the three-dimensional truth data to generate a deviation measure; and

adjusting a calibration parameter if the deviation measure is greater than a predetermined value.

29. The method of claim 28 further comprising repeating the steps of acquiring, determining and comparing if the deviation measure is greater than the predetermined value.

30. The method of claim 28 wherein the calibration parameter comprises one of a source head relative position, a source head relative orientation, and a camera magnification.

31. The method of claim 28 wherein the calibration parameter comprises one of a projected fringe pattern lens distortion parameter and a camera lens distortion parameter.

32. The method of claim 28 comprising the additional step of changing at least one of an orientation or a position of the object by a specified amount.

33. The method of claim 28 wherein the deviation measure comprises a plurality of difference data.

34. The method of claim 28 wherein the deviation measure comprises a statistical measure.

35. The method of claim 28 wherein the three-dimensional coordinate data for the calibration object is determined at a plurality of locations on the object surface.

36. A depth of field independent method for calibrating a measurement system for determining three-dimensional surface information of an object, the method comprising the steps of:

providing a plurality of fringe detectors fixed in known spatial relationships;

providing at least one fringe source, which projects fringes

detecting the fringes at the plurality of fringe detectors to acquire a fringe data set; and

determining three-dimensional coordinate data for the spatial locations of the at least one fringe source.

37. A method of improving the fringe projection imaging of an object having a geometric locus comprising the steps of:

positioning at least one active calibration target at the geometric locus on the object; and

projecting fringes on the object.

38. The method of claim 37 further comprising the steps of:

detecting fringe projection data at the fringe intensity detector; and

using the fringe projection data to extrapolate imaging data for the geometric locus.

39. The method of claim 37 wherein the geometric locus is a hole in the object.

40. The method of claim 37 wherein the geometric locus is an edge of the object.

41. A method for compensating for projection lens imperfections in a fringe projection system, the method comprising the steps of:

determining an ideal spherical wavefront output for a projection lens;

determining an actual wavefront output for the projection lens;

comparing the ideal spherical wavefront output with the actual wavefront output;

determining a first wavefront error for a first point source;

determining a second wavefront error for a second point source;

determining a fringe phase error from the first and second wavefront errors;

converting the fringe phase error into a correction factor; and

using the correction factor to compensate for projection lens imperfections.

42. A method for compensating for lens imperfections in a fringe projection system, the method comprising the steps of:

(a) projecting a fringe on a fringe detector;

(b) measuring a fringe intensity;

(c) measuring a first pixel coordinate (i) and a second pixel coordinate (j);

(d) determining a three dimensional coordinate from the given fringe intensity, first pixel coordinate, and the second pixel coordinate;

(e) determining a correction factor to determine a correction fringe intensity; and

(f) determining a corrected three dimensional coordinate based on the correction fringe intensity.

43. A method for compensating for lens imperfections in a fringe projection system, the method comprising the steps of

(a) projecting a fringe on a fringe detector;

(b) measuring a fringe number, wherein N is the fringe number;

(c) measuring a first pixel coordinate (i) and a second pixel coordinate (j);

(d) determining a relative coordinate in a pupil plane from corresponding fringe number;

(e) constructing an approximate phase correction map from the relative coordinates;

(f) determining a correction fringe number; and

(g) determining a corrected three dimensional coordinate based on the correction fringe number.

44. A method for compensating for distortion in an optical imaging system, the method comprising the steps of:

providing a calibration target comprising optical grating lines;

providing an optical imaging system comprising a focal plane array, and a plurality of system parameters, wherein the focal plane array further comprises pixels;

aligning the optical grating lines of the calibration target with the pixels;

imaging a calibration target on a focal plane array of an optical imaging system;

adjusting system parameters based on an iterative process to generate a data set;

simulating a Moiré

pattern from the data set and an image of the calibration target; and

generating distortion coefficients to compensate for distortion in the optical imaging system from the simulated Moiré

pattern.

45. A method for compensating for distortion in an imaging optical system, the method comprising the steps of:

(a) designating a first distortion free pixel coordinate (i), a second distortion free pixel coordinate (j), and a distortion free radius in a sensing array;

(b) designating a distortion center comprising a first distortion coordinate, a second distortion coordinate, and a distortion radius in a sensing array; and

(c) designating a distortion parameter relating the distortion free radius and the distortion radius.

46. The method of claim 45 further comprising the steps of

imaging a calibration target to establish the distortion parameter;

and

minimizing the distortion parameter.

47. The method of claim 45 further comprising the steps of

imaging a calibration target to establish the distortion parameter;

and

using the distortion parameter to minimize a distortion error in an imaging measurement.

48. A method for appending a plurality of related three-dimensional images of an object, each of the three-dimensional images having a unique orientation with respect to a three-dimensional measurement system, the method comprising the steps of:

projecting an orientation pattern at a fixed position on the object;

acquiring a first three-dimensional measurement of the object, the three-dimensional measurement system being at a first position relative to the object;

moving the three-dimensional measurement system to a second position relative to the object;

acquiring a second three-dimensional measurement of the object, the orientation pattern being at the fixed position on the object and the three-dimensional measurement system being at a second position relative to the object.

49. The method of claim 48 wherein the orientation pattern comprises a plurality of laser spots.

50. The method of claim 48 wherein the orientation pattern comprises a projected optical pattern.