Apparatus and methods for surface contour measurement

• US 6,690,474 B1
• Filed: 01/10/2000
• Issued: 02/10/2004
• Est. Priority Date: 02/12/1996
• Status: Expired due to Term

• Current Assignee: Massachusetts Institute of Technology
• Original Assignee: Massachusetts Institute of Technology
• Inventors: Shirley, Lyle
• Primary Examiner(s): SMITH, ZANDRA V
• Application Number:
  US09/480,043
• Time in Patent Office: 1,492 Days
• Field of Search: 356/511, 356/512, 356/603, 356/604, 356/605, 382/154
• US Class Current: 356/603
• CPC Class Codes: G01B 11/2527 with phase change by in-plane movement of the patern
• CPC Class Codes: G01B 11/2531 using several gratings, projected with variable angle of incidence on the object, and one detection device

Abstract

Apparatus and methods of measuring three-dimensional position information of a point on the surface of an object. In one embodiment, the method includes the steps of providing two sources of radiation having a spectral distribution, illuminating the surface with each of the sources to produce a first fringe pattern, moving the first fringe pattern to a second position, generating a first wrapped cycle map, estimating fringe numbers in the first fringe pattern, changing the first fringe pattern, moving the second fringe pattern to a second position, generating a second wrapped cycle map, estimating fringe numbers in the second fringe pattern, and determining position information in response to the estimated fringe numbers in the second fringe pattern and the second wrapped cycle map. In another embodiment the apparatus includes two sources of radiation having a spectral distribution and being coherent with respect to one another, a control system for moving each of the sources relative to each other, a detector positioned at the point on the surface of the object to receive radiation illuminating the point on the surface of the object, and a processor for receiving signals from the detector. The processor calculates position information of the point on the surface of the object in response to the movement of the sources and the radiation received at the point on the surface of the object.

Claims

1. A method for determining, on an object having a surface, three-dimensional position information of a point on said surface of said object, said method comprising the steps of:

a) providing two sources of radiation having a spatial distribution of spectral regions;

b) illuminating said surface with said radiation from each of said sources to produce a first fringe pattern at a first position on said surface;

c) moving said first fringe pattern to a second position;

d) generating a first wrapped cycle map in response to said first and second positions of said first fringe pattern;

e) estimating fringe numbers in said first fringe pattern;

f) changing said first fringe pattern to generate a second fringe pattern at a first position;

g) moving said second fringe pattern to a second position;

h) generating a second wrapped cycle map in response to said first and second positions of said second fringe pattern;

i) estimating fringe numbers in said second fringe pattern in response to said estimated fringe numbers in said first fringe pattern; and

j) determining said three-dimensional position information of said point on said surface in response to said estimated fringe numbers in said second fringe pattern and said second wrapped cycle map.

2. The method of claim 1 wherein said two sources of radiation are coherent with respect to one another.

3. The method of claim 1 wherein said two sources of radiation have a spectral distribution that is narrowband.

4. The method of claim 1 wherein the step of providing two sources of radiation comprises splitting radiation from a single source of radiation.

5. The method of claim 1 wherein the step of providing two sources of radiation comprises providing two sources of laser radiation.

6. The method of claim 1 wherein the steps of estimating fringe numbers in said first fringe pattern and said second fringe pattern comprise applying a mathematical model.

7. The method of claim 1 wherein the steps of estimating fringe numbers in said first fringe pattern and said second fringe pattern comprise applying a linear estimation model.

8. The method of claim 1 wherein the steps of estimating fringe numbers in said first fringe pattern and said second fringe pattern comprise applying three-dimensional position information determined for a different surface.

9. The method of claim 1 wherein each of said spectral regions of one of said sources is separated from a respective spectral region of the other of said sources by a respective distance proportional to the respective wavelength of said spectral regions.

10. The method of claim 9 wherein said distance is linearly proportional to said wavelength of said spectral regions.

11. The method of claim 1 wherein said step of changing said first fringe pattern comprises separating each of said spectral regions relative to the other of said spectral regions by a distance.

12. The method of claim 11 wherein said distance comprises a midpoint equidistant from each of two respective spectral regions and wherein the step of separating each of said spectral regions comprises changing said distance and keeping said midpoint fixed.

13. The method of claim 1 wherein said step of moving said second fringe pattern comprises the step of changing the phase of a spectral region in said spatial distribution from one of said sources relative to the phase of a respective spectral region in said spatial distribution from the other of said sources.

14. A method for synthesizing a wrapped cycle map corresponding to a fringe pattern of a predetermined spacing on a surface, said method comprising the steps of:

a) providing two sources of radiation having a spatial distribution of spectral regions;

b) illuminating said surface with said radiation from each of said sources to produce a first fringe pattern at a first position on said surface;

c) moving said first fringe pattern to a second position on said surface;

d) generating a first wrapped cycle map in response to said first and second positions of said first fringe pattern;

e) changing said first fringe pattern to generate a second fringe pattern at a first position;

f) moving said second fringe pattern to a second position;

g) generating a second wrapped cycle map in response to said first and second positions of said second fringe pattern;

h) subtracting said second wrapped cycle map from said first wrapped cycle map; and

i) wrapping said difference between said second wrapped cycle map and said first wrapped cycle map to generate a wrapped cycle map corresponding to said fringe pattern of said predetermined spacing.

15. The method of claim 14 wherein said two sources of radiation are coherent with respect to one another.

16. The method of claim 14 wherein said two sources of radiation have a spectral distribution that is narrowband.

17. The method of claim 14 wherein the step of providing two sources of radiation comprises splitting radiation from a single source of radiation.

18. The method of claim 14 wherein the step of providing two sources of radiation comprises providing two sources of laser radiation.

19. The method of claim 14 wherein each of said spectral regions of one of said sources is separated from a respective spectral region of the other of said sources by a distance proportional to the respective wavelength of said spectral regions.

20. The method of claim 19 wherein said distance is linearly proportional to said wavelength of said spectral regions.

21. The method of claim 14 wherein said step of changing said first fringe pattern comprises separating each of said spectral regions relative to the other of said spectral regions by a distance.

22. The method of claim 21 wherein said distance comprises a midpoint equidistant from each of two respective spectral regions and wherein the step of separating each of said spectral regions comprises changing said distance and keeping said midpoint fixed.

23. The method of claim 14 wherein said step of moving said second fringe pattern comprises the step of changing the phase of a spectral region in said spatial distribution from one of said sources relative to the phase of a respective spectral region in said spatial distribution from the other of said sources.

24. The method of claim 14 wherein said two sources of radiation have a first frequency and wherein said step of changing said first fringe pattern to generate a second fringe pattern comprises changing said first frequency to a second frequency.

25. A method for determining, on an object having a surface, three-dimensional position information of a point on said surface of said object, said method comprising the steps of:

a) generating a first and a second beam of radiation, said first and second beams of radiation having a first frequency and being coherent with respect to one another;

b) illuminating said surface with said second beam of radiation;

c) generating a first interference pattern at a first position in response to said first beam of radiation and radiation from said second beam of radiation scattered by said surface;

d) moving said first interference pattern to a second position;

e) generating a first wrapped cycle map in response to said first and second positions of said first interference pattern;

f) estimating intensity cycles in said first interference pattern;

g) changing said first interference pattern to generate a second interference pattern at a first position;

h) moving said second interference pattern to a second position;

i) generating a second wrapped cycle map in response to said first and second positions of said second interference pattern;

j) estimating intensity cycles in said second interference pattern in response to estimated intensity cycles in said first interference pattern; and

k) calculating said three-dimensional position information in response to said estimated intensity cycles in said second interference pattern and said second wrapped cycle map.

26. The method of claim 25 wherein said step of changing said first interference pattern to generate a second interference pattern comprises changing said first frequency to a second frequency.

27. The method of claim 25 further comprising the step of imaging said surface onto an image plane.

28. The method of claim 27 wherein said image plane comprises a detector.

29. A method for determining, on an object having a surface, three-dimensional position information of a point on said surface of said object, said method comprising the steps of:

a) generating a first and a second beam of radiation, said first and second beams of radiation being coherent with respect to one another;

b) illuminating said surface with said second beam of radiation;

c) generating a first interference pattern in response to said first beam of radiation and radiation from said second beam of radiation scattered by said surface;

d) phase shifting one of said first and second beams of radiation with respect to the other of said first and second beams of radiation to generate a first phase shifted interference pattern;

e) generating a first wrapped cycle map in response to said first interference pattern and said first phase shifted interference pattern;

f) changing said first interference pattern to generate a second interference pattern;

g) phase shifting one of said first and second beams of radiation with respect to the other of said first and second beams of radiation to generate a second phase shifted interference pattern;

h) generating a second wrapped cycle map in response to said second interference pattern and said second phase shifted interference pattern;

i) subtracting said second wrapped cycle map from said first wrapped cycle map; and

j) wrapping said difference between said second wrapped cycle map and said first wrapped cycle map to generate a wrapped cycle map.

30. The method of claim 29 wherein said first and second beams of radiation have a first frequency and wherein said step of changing said first interference pattern to generate a second interference pattern comprises changing said first frequency to a second frequency.

31. The method of claim 29 further comprising the step of determining said three-dimensional position information by using said wrapped cycle map.

32. The method of claim 29 further comprising the step of imaging said surface onto an image plane.

33. The method of claim 32 wherein said image plane comprises a detector.

34. A method for determining, on an object having a surface, three-dimensional position information of a point on said surface of said object, said method comprising the steps of:

a) projecting a first cyclical pattern to produce a first projected pattern at a first position on said surface;

b) moving said first projected pattern to a second position;

c) generating a first wrapped cycle map in response to said first and second positions of said first projected pattern;

d) estimating cycle numbers in said first projected pattern;

e) changing said first projected pattern to generate a second projected pattern at a first position;

f) moving said second projected pattern to a second position;

g) generating a second wrapped cycle map in response to said first and second positions of said second projected pattern;

h) estimating cycle numbers in said second projected pattern in response to said estimated cycle numbers in said first projected pattern; and

i) determining said surface in response to said estimated cycle numbers in said second projected pattern and said second wrapped cycle map.

35. The method of claim 34 wherein the step of changing said first projected pattern comprises projecting a second cyclical pattern to generate a second projected pattern at a first position.

36. A method for synthesizing a wrapped cycle map corresponding to a projected cyclical pattern of a predetermined spacing on a surface, said method comprising the steps of:

a) projecting a first cyclical pattern to produce a first projected pattern at a first position on said surface;

b) moving said first projected pattern to a second position on said surface;

c) generating a first wrapped cycle map in response to said first and second positions of said first projected pattern;

d) changing said first projected pattern to generate a second projected pattern at a first position;

e) moving said second projected pattern to a second position;

f) generating a second wrapped cycle map in response to said first and second positions of said second projected pattern;

g) subtracting said second wrapped cycle map from said first wrapped cycle map; and

h) wrapping said difference between said second wrapped cycle map and said first wrapped cycle map to generate a wrapped cycle map corresponding to said projected cyclical pattern of said predetermined spacing.

37. The method of claim 36 wherein the step of changing said first projected pattern comprises projecting a second cyclical pattern to generate a second projected pattern at a first position.

38. A method for mitigating the effects of speckle on a measurement of a point on a surface of an object, said method comprising the steps of:

a) generating a coherent fringe pattern from two sources of radiation separated by a distance;

b) projecting the coherent fringe pattern onto the surface of the object;

c) detecting the fringe pattern and the speckle in an image of the surface of the object;

d) translating the two sources such that the fringe pattern remains substantially stationary and the speckle changes;

e) detecting the new fringe pattern and the changed speckle in an image of the surface of the object; and

f) determining the fringe pattern substantially without speckle in response to the detected fringe patterns and the change in speckle.

39. The method of claim 38 further comprising the step of iteratively repeating steps d) and e).

40. The method of claim 38 wherein the two sources of radiation are laser radiation.

41. A method for mitigating the effects of speckle on a measurement of a point on a surface of an object, said method comprising the steps of:

a) generating a coherent fringe pattern from two sources of radiation separated by a distance;

b) projecting the coherent fringe pattern onto the surface of the object;

c) detecting the fringe pattern and the speckle in an image of the surface of the object;

d) translating the object laterally with respect to the two sources of radiation along a path parallel to the equiphase plane of the fringe pattern such that the fringe pattern remains substantially stationary with respect to the surface of the object and the speckle changes;

e) detecting the new fringe pattern and the changed speckle in an image of the surface of the object; and

f) determining the fringe pattern substantially without speckle in response to the detected fringe patterns and the change in speckle.

42. The method of claim 41 further comprising the step of iteratively repeating steps d) and e).

43. The method of claim 41 wherein the two sources of radiation are laser radiation.

44. A method for mitigating the effects of speckle on a measurement of a point on a surface of an object, said method comprising the steps of:

a) projecting a coherent fringe pattern onto the surface of the object;

b) providing a lens having a transmission function having a gradual transmission drop off at the lens edge; and

c) detecting the fringe pattern in an image of the surface of the object, wherein the transmission function substantially reduces the speckle influence on the measurement.

45. A method for mitigating the effects of speckle on a measurement of a point on a surface of an object, said method comprising the steps of:

a) generating a first fringe pattern from two sources of radiation having a first frequency and separated by a distance;

b) changing said first fringe pattern to generate a second fringe pattern by changing said first frequency to a second frequency; and

c) changing said distance in response to the difference between said first frequency and said second frequency, wherein the ratio of said distance to the difference between said first frequency and said second frequency is substantially constant.

46. A method for projecting fringes onto a surface of object, said method comprising the steps of:

a) providing two sources of radiation separated by a distance, each of said sources having a spectral distribution and being coherent with respect to the other of said sources;

b) illuminating said point on said surface of said object with said radiation from each of said sources;

c) moving one of said sources relative to the other of said sources;

d) detecting radiation scattered by said point on said surface of said object; and

e) calculating position information in response to said movement of said sources and said detected radiation scattered by said point on said surface of said object.

47. The method of claim 46 wherein the step of moving one of said sources relative to the other of said sources comprises changing said distance between a spectral component of one of said sources relative to a respective spectral component of the other of said sources to change said distance between each of said spectral components of said sources.

48. The method of claim 47 wherein said distance comprises a midpoint equidistant from each of said spectral components of said sources and wherein the step of moving one of said sources relative to the other of said sources comprises changing said distance between a spectral component of one of said sources relative to a respective spectral component of the other of said sources to change said distance between each of said spectral components of said sources and keeping said midpoint fixed.

49. The method of claim 46 wherein the step of providing two sources of radiation comprises:

a) providing an initial beam of radiation having a spectral width;

b) generating a first radiation beam at a first beam angle and a second radiation beam at a second beam angle from said initial beam of radiation; and

c) imaging said first radiation beam and said second radiation beam to form said two sources of radiation.

50. The method of claim 49 further comprising amplitude modulating said initial beam of radiation.

51. The method of claim 49 wherein said generating a first radiation beam and a second radiation beam comprises acousto-optically modulating with an acoustic field said initial beam.

52. The method of claim 51 wherein said step of acousto-optically modulating comprises the step of phase modulating said acoustic field.

53. The method of claim 51 wherein said step of acousto-optically modulating comprises the step of amplitude modulating said acoustic field.

54. The method of claim 46 wherein said step of moving one of said sources relative to the other of said sources comprises acousto-optically modulating said initial beam by frequency modulating an acoustic field.

55. An apparatus for projecting fringes onto a surface of an object, said apparatus comprising:

a) two sources of radiation separated by a distance, each of said sources having a spectral distribution and being coherent with respect to the other of said sources;

b) control system moving each of said sources relative to the other of said sources;

c) a detector positioned to receive radiation scattered from said point on said surface of said object; and

d) a processor receiving signals from said detector, said processor calculating position information in response to said movement of said sources and said received radiation scattered from said point on said surface of said object.

56. The apparatus of claim 55 wherein said control system moves each of a spectral component of one of said sources of radiation relative to a respective spectral component of the other of said sources of radiation to change said distance between each of said spectral components of said two sources of radiations.

57. The apparatus of claim 56 wherein said distance comprises a midpoint equidistant from each of said spectral components of said sources and said control system moves each of said spectral components of said sources relative to the other of said spectral components of said sources to change said distance and keep said midpoint fixed.

58. The apparatus of claim 55 wherein said detector comprises a plurality of photodetectors and wherein said processor comprises a multiprocessor system having a plurality of processing units and each of said plurality of photodetectors is in electrical communication with a respective one of said plurality of processing units.

59. The apparatus of claim 55 wherein said two sources of radiation comprise:

a) an initial source of a beam of radiation having a spectral width;

b) a beam separator in optical communication with said initial source of a beam of radiation, said beam separator generating a first optical beam and a second optical beam; and

c) an imaging system in optical communication with said beam separator, said imaging system generating said two sources of radiation each respective source of radiation corresponding to said first optical beam and said second optical beam, respectively.

60. The apparatus of claim 59 wherein said beam separator is a diffraction grating.

61. The apparatus of claim 55 wherein a point at a wavelength in said spectral distribution of one source is separated from a respective point at said wavelength in said spectral distribution of the other source by said distance, said distance being substantially proportional to said wavelength of said points.

62. The apparatus of claim 55 wherein said control system further comprises a phase shifter.

63. The apparatus of claim 62 wherein said phase shifter is a translator for changing the position of said beam separator relative to said beam of radiation.

64. The apparatus of claim 55 wherein said control system comprises a zoom lens having a variable focal length, said zoom lens varying said distance of separation in response to a change in said variable focal length.

65. The apparatus of claim 55 wherein said control system comprises a plurality of lenses of different focal length, said distance of separation corresponding to a respective focal length of one of said plurality of lenses.

66. The apparatus of claim 60 wherein said diffraction grating is a spatial light modulator.

67. The apparatus of claim 66 wherein said spatial light modulator has an adjustable grating period, said grating period being responsive to an electrical signal applied to said spatial light modulator.

68. The apparatus of claim 59 wherein said beam separator is an acousto-optic modulator.

69. The apparatus of claim 59 further comprising an amplitude modulator in optical communication with said initial source of a beam of radiation.

70. The apparatus of claim 59 further comprising an order-blocking mask.

71. The apparatus of claim 70 wherein the order-blocking mask is a double-slit mask.

72. The apparatus of claim 59 further comprising an amplitude modulator in optical communication with said beam separator, said amplitude modulator modulating the amplitude of said initial source.

73. A method for determining, on an object having a surface, three-dimensional position information of a point on said surface of said object, said method comprising the steps of:

a) providing two sources of radiation, each of said two sources of radiation having a spectral distribution and being coherent with respect to the other of said two sources;

b) providing a detector at said point on said surface;

c) illuminating said point on said surface of said object with said radiation from each of said sources;

d) moving each of said sources relative to each other;

e) detecting said radiation at said point on said surface of said object; and

f) calculating position information in response to said movement of said sources and said radiation detected at said point on said surface of said object.

74. The method of claim 73 wherein said two sources of radiation have a spectral distribution that is narrowband.

75. The method of claim 73 further comprising the step of changing the phase of a spectral component in said spectral distribution of said radiation from one of said sources relative to the phase of a respective spectral component in said spectral distribution of said radiation from the other of said sources as measured at said point on said surface of said object.

76. The method of claim 73 wherein said sources of radiation are virtual sources of radiation.

77. The method of claim 73 wherein the step of moving each of said sources comprises changing the distance between a spectral component of one of said sources relative to a respective spectral component of the other of said sources to change the distance between each of said spectral components of said sources.

78. The method of claim 77 wherein said distance comprises a midpoint equidistant from each of said spectral components of said sources and wherein the step of moving each of said sources comprises changing said distance between a spectral component of one of said sources relative to a respective spectral component of the other of said sources to change said distance between each of said spectral components of said sources and keeping said midpoint fixed.

79. An apparatus for determining, on an object having a surface, three-dimensional position information of a point on said surface of said object, said apparatus comprising:

a) two sources of radiation, each of said sources having a spectral distribution and being coherent with respect to one another;

b) a control system moving each of said sources relative to each other;

c) a detector positioned at said point on said surface of said object to receive radiation illuminating said point on said surface of said object; and

d) a processor receiving signals from said detector, said processor calculating position information of said point on said surface of said object in response to said movement of said sources and said radiation received at said point on said surface of said object.

80. The apparatus of claim 79 wherein said two sources of radiation have a spectral distribution that is narrowband.

81. The apparatus of claim 79 wherein said control system changes the phase of a spectral component in said spectral distribution of said radiation from one of said sources relative to the phase of a respective spectral component in said spectral distribution of said radiation from the other of said sources as measured at said point on said surface of said object.

82. The apparatus of claim 79 wherein said sources of radiation are virtual sources of radiation.

83. The apparatus of claim 79 wherein said control system moves each of a spectral component of one of said sources of radiation relative to a respective spectral component of the other of said sources of radiation to change said distance between each of said spectral components of said two sources of radiation.

84. The apparatus of claim 83 wherein said distance comprises a midpoint equidistant from each of said spectral components of said sources and said control system moves each of said spectral components of said sources relative to the other of said spectral components of said sources to change said distance and keep said midpoint fixed.