Method and apparatus for measuring surface distances from a reference plane

Abstract

A method of measuring the distance of an examined surface from a reference plane, by: (a) directing a parallel beam of radiation along a first optical path in which the parallel beam is focussed as a spot on the examined surface and reflected therefrom; (b) converting the reflected beam to a parallel beam; (c) directing a part of the reflected parallel beam through a second optical path in which the spot is focussed on a surface of a first detector located such that the position of the spot on the first detector includes both drawback errors caused by variations in reflectivity, scattering, and/or interference in the examined surface; and a plane displacement error, representing the distance between the plane of the examined surface and the reference plane; (d) directing another part of the reflected parallel beam through a third optical path in which the spot is focussed on a surface of a second detector located such that the position of the spot on the second detector includes only the drawback errors; (e) and utilizing the outputs of the first and second detectors for providing a measurement of the distance of the examined surface from the reference plane.

What is claimed is: 
     
       1. A method of measuring the distance of an examined surface from a reference plane, comprising the steps: (a) directing a parallel beam of radiation along a first optical path which includes first focussing means for focussing the parallel beam as a spot on the examined surface and reflecting it therefrom as a reflected beam;   (b) converting the reflected beam to a parallel beam;   (c) directing a part of the reflected parallel beam through a second optical path which includes second focussing means for focussing the spot on a surface of a first detector located such that the position of the spot on the first detector includes both: (i) drawback errors caused by variations in reflectivity, scattering, and/or interference in the examined surface; and   (ii) a plane displacement error, representing the distance between the plane of the examined surface and the reference plane;     (d) directing another part of the reflected parallel beam through a third optical path which includes third focussing means for focussing the spot on a surface of a second detector located such that the position of the spot on the second detector includes only said drawback errors (i); .Iadd.and .Iaddend.   (e) .[.and.]. utilizing the outputs of said first and second detectors for providing a measurement of the distance of the examined surface from the reference plane.   
     
     
       2. The method according to claim 1, wherein .[.the magnification in.]. said second optical path .Iadd.and said third optical path include respectively different magnification means .Iaddend..[.is different from that in said third optical path.].; said step (e) .[.including.]. .Iadd.includes .Iaddend.adjusting the drawback errors (i) detected by the second detector in-step (e) by the ratio between the two magnifications in utilizing the outputs of the first and second detectors for providing a measurement of said plane displacement error (ii) alone. 
     
     
       3. The method according to claim 1, wherein said surface of the first detector is coplanar with a virtual axis of said first focussing means. 
     
     
       4. The method according to claim 1, wherein said surface of the second detector is perpendicular to a virtual axis of said first focussing means. 
     
     
       5. The method according to claim 4, wherein said third optical path includes a first focussing lens focussing the spot on the optical axis of a second focussing lens which in turn focusses the spot on the optical axis of a third focussing lens whose optical axis is perpendicular to said surface of the second detector. 
     
     
       6. The method according to claim 1, wherein said third optical path is a straight path separate and distinct from said second optical path. 
     
     
       7. The method according to claim 1, wherein the part of the parallel beam in said third optical path is reflected back towards said first optical path and is focussed on said spot of the examined surface and then on said second detector. 
     
     
       8. The method according to claim 7, wherein one of said detectors is a CCD camera and includes image processing means which processes the output of the camera to determine from the image focussed thereon the drawback errors corresponding to those detected by the first detector. 
     
     
       9. The method according to claim 8, wherein said second detector is the CCD camera and includes said image processing means. 
     
     
       10. The method according to claim 1, wherein said first optical path includes a variable intensity filter which is controlled by the output of at least one of said detectors to maintain a constant beam intensity. 
     
     
       11. Apparatus for measuring the distance of an examined surface from a reference plane, comprising: (a) a first detector;   (b) a second detector;   (c) means for directing a parallel beam of radiation along a first optical path which includes first focussing means for focussing the parallel beam as a spot on the examined surface and reflecting it therefrom as a reflected beam;   (d) means for converting the reflected beam to a parallel beam;   (e) means for directing a part of the reflected parallel beam through a second optical path which includes second focussing means for focussing the spot on a surface of said first detector located such that the position of the spot on the first detector includes both: (i) drawback errors caused by variations in reflectivity, scattering, and/or interference in the examined surface; and   (ii) a plane displacement error, representing the distance between the plane of the examined surface and the reference plane;     (f) means for directing another part of the reflected parallel beam through a third optical path including third focussing means for focussing the spot on a surface of said second detector located such that the position of the spot on the second detector includes only said drawback errors (i); .Iadd.and .Iaddend.   (g) .[.and.]. means for utilizing the outputs of said first and second detectors for providing a measurement of the distance of the examined surface from the reference plane.   
     
     
       12. The apparatus according to claim 11, wherein .[.the magnification in.]. said second optical path .[.is different from that in said third optical path.]. .Iadd.and said third optical path include respectively different magnification means.Iaddend.; said means (g) .[.including.]. .Iadd.includes .Iaddend.means for adjusting the drawback error (i) detected by the second detector by the ratio between the two magnifications in utilizing the outputs of the first and second detectors for providing a measurement of said plane displacement error (ii) alone. 
     
     
       13. The apparatus according to claim 11, wherein said surface of the first detector is coplanar with a virtual axis of said first focussing means. 
     
     
       14. The apparatus according to claim 11, wherein said surface of the second detector is perpendicular to a virtual axis of said first focussing means. 
     
     
       15. The apparatus according to claim 14, wherein said third optical path includes a first focussing lens focussing the spot on the optical axis of a second focussing lens which in turn focusses the spot on the optical axis of a third focussing lens whose optical axis is perpendicular to said surface of the second detector. 
     
     
       16. The apparatus according to claim 11, wherein said third optical path is a straight path separate and distinct from said second optical path. 
     
     
       17. The apparatus according to claim 11, wherein said third optical path includes a retro-reflector which reflects said parallel beam in said path back towards said first optical path, and focussing means which focusses said reflected back beam on said spot of the examined surface and then on said second detector. 
     
     
       18. The apparatus according to claim 17, wherein one of said detectors is a CCD camera and includes image processing means which processes the output of the camera to determine from the image focussed thereon the drawback errors corresponding to those detected by the first detector. 
     
     
       19. The apparatus according to claim 18, wherein said second detector is the CCD camera and includes said image processing means. 
     
     
       20. The apparatus according to claim 11, wherein said first optical path includes a variable intensity filter, said apparatus further including control means for controlling said filter in accordance with the output of at least one of said detectors to maintain a constant beam intensity. .Iadd.21. A method of measuring the distance of an examined surface from a reference plane, comprising the following steps: (a) directing a radiation beam along a first optical path which includes first focussing means for focussing said radiation beam as a spot on said examined surface and reflecting it therefrom as a reflected radiation, said first focussing means arranged to collect and convert said reflected radiation to a reflected beam;   (b) directing a part of said reflected beam through a second optical path which includes second focussing means for focussing said part of said reflected beam onto a first spot on a surface of a first detector located such that the position of said first spot on said first detector includes both: (i) drawback errors caused by variations in reflectivity, scattering, and/or interference in said examined surface; and   (ii) a plane displacement, representing the distance between the plane of said examined surface and said reference plane;     (c) directing another part of said reflected beam through a third optical path which includes third focussing means for focussing said other part of said reflected beam onto a second spot on a surface of a second detector, such that the position of said second spot on said second detector includes only said drawback errors; and   (d) utilizing the outputs of said first and second detectors for providing a measurement of the distance of said examined surface from said reference plane. .Iaddend..Iadd.22. The method of claim 21 wherein said detectors are position-sensitive detectors. .Iaddend..Iadd.23. The method of claim 21 wherein said detectors are charge-coupled device cameras and image processing means which process the output of said cameras to determine from images focussed thereon any drawback errors corresponding to those detected by said second detector and both, plane displacement errors and drawback errors corresponding to those detected by said first detector. .Iaddend..Iadd.24. The method of claim 21 wherein one of said detectors is a position-sensitive detector. .Iaddend..Iadd.25. The method of claim 21 wherein one of said detectors is a charge-coupled device camera and includes image processing means. .Iaddend..Iadd.26. The method of claim 21 wherein said radiation beam is a parallel beam. .Iaddend..Iadd.27. The method of claim 21 wherein said reflected beam is a parallel beam. .Iaddend..Iadd.28. The method of claim 21 wherein said radiation beam and said reflected beam are parallel beams. .Iaddend..Iadd.29. The method of claim 21 wherein said first focussing means comprises at least one lens. .Iaddend..Iadd.30. The method of claim 21 wherein said first focussing means comprises at least one lens for focussing said radiation beam and for collecting and converting said reflected radiation to a reflected   
     
     
        beam. .Iaddend..Iadd.31.  The method of claim 21 wherein said first focussing means comprises one lens for focussing said radiation beam and another lens for collecting and converting said reflected radiation to a reflected beam. .Iaddend..Iadd.32. The method of claim 21 wherein said second and third optical paths include magnification means, the magnification means in said second optical path being different from that in said third optical path; said step (d) includes adjusting the drawback errors detected by said second detector in step (d) by the ratio between said magnifications in utilizing the outputs of said first and second detectors for providing a measurement of said plane displacement error alone. .Iaddend..Iadd.33. The method of claim 21 wherein said surface of said first detector is coplanar with a virtual axis Of said first focussing means. .Iaddend..Iadd.34. The method of claim 21 wherein said surface of said second detector is perpendicular to a virtual axis of said first focussing means. .Iaddend..Iadd.35. The method of claim 21 wherein said third optical path includes a first focussing lens for focussing a first spot on an optical axis of a second focussing lens which in turn focusses a second spot onto said onto an optical axis of a third focussing lens whose optical axis is perpendicular to said surface of said second detector. .Iaddend..Iadd.36. The method of claim 21 wherein said third optical path is a straight path separate and distinct from said second optical path. .Iaddend..Iadd.37. The method of claim 21 wherein said part of said reflected beam in said third optical path is a parallel beam and said third optical path includes means for reflecting said beam back towards said first optical path and focussing said beam onto said spot on said examined surface and then onto said second detector. 
     
     
        .Iaddend..Iadd.  .  The method of claim 37 wherein said first detector is a charge-coupled device camera and includes image processing means. .Iaddend..Iadd.39. The method of claim 37 wherein said detectors are charge-coupled device cameras and include image processing means. .Iaddend..Iadd.40. The method of claim 37 wherein said radiation beam and said reflected beam are parallel beams. .Iaddend..Iadd.41. The method of claim 37 wherein said first focussing means comprises at least one lens for focussing said radiation beam and for collecting and converting said reflected radiation to a reflected beam. .Iaddend..Iadd.42. The method of claim 37 wherein said first focussing means comprises one lens for focussing said radiation beam and another lens for collecting and converting said reflected radiation to a reflected beam. .Iaddend..Iadd.43. The method of claim 37 wherein said first focussing means for focussing said radiation beam comprises at least one lens. .Iaddend..Iadd.44. Apparatus for measuring the distance of an examined surface from a reference plane, comprising: (a) a first detector;   (b) a second detector;   (c) means for directing a radiation beam along a first optical path which includes first focussing means for focussing said radiation beam as a spot on said examined surface and reflecting it therefrom as reflected radiation; said first focussing means arranged to collect and convert said reflected radiation to a reflected beam;   (d) means for directing a part of said reflected beam through a second optical path which includes second focussing means for focussing said part of said reflected beam onto a first spot on a surface of said first detector located such that the position of said first spot on said first detector includes both: (i) drawback errors caused by variations in reflectivity, scattering, and/or interference in said examined surface; and   (ii) a plane displacement, representing the distance between the plane of said examined surface and said reference plane; and     (e) means for directing another part of said reflected beam through a third optical path which includes third focussing means for focussing said other part of said reflected beam into a second spot on a surface of said second detector, said second detector being located such that the position of said second spot on said second detector includes only said drawback errors;   (f) whereby the outputs of said first and second detectors can be utilized to provide a measurement of the distance of said examined surface from   
     
     
        said reference plane. .Iaddend..Iadd.45.  The apparatus of claim 44 wherein said detectors are position-sensitive detectors. .Iaddend..Iadd.46. The apparatus of claim 44 wherein said detectors are charge-couple device cameras and image processing means which process said output of said cameras to determine from images focussed thereon any drawback errors corresponding to those detected by said second detector and both, plane displacement errors and drawback errors corresponding to those detected by said first detector. .Iaddend..Iadd.47. The apparatus of claim 44 wherein one of said detectors is a position-sensitive detector. .Iaddend..Iadd.48. The apparatus of claim 44 wherein one of said detectors is a charge-coupled device camera and includes image processing means. .Iaddend..Iadd.49. The apparatus of claim 44 wherein said radiation beam is a parallel beam. .Iaddend..Iadd.50. The apparatus of claim 44 wherein 
     
     
        said reflected beam is a parallel beam. .Iaddend..Iadd.51.  The apparatus of claim 44 wherein said radiation beam and said reflected beam are parallel beams. .Iaddend..Iadd.52. The apparatus of claim 44 wherein said first focussing means comprises at least one lens. .Iaddend..Iadd.53. The apparatus of claim 44 wherein said first focussing means comprises at least one lens for focussing said radiation beam and for collecting and converting said reflected radiation to a reflected beam. .Iaddend..Iadd.54. The apparatus of claim 44 wherein said first focussing means comprises one lens for focussing said radiation beam and another lens for collecting and converting said reflected radiation to a reflected beam. .Iaddend..Iadd.55. The apparatus of claim 44 wherein said second and third optical paths include magnification means, the magnification means in said second optical path being different from that in said third optical path; said step (f) includes adjusting the drawback errors detected by said second detector in step (f) by the ratio between said magnifications in utilizing the outputs of said first and second detectors for providing a measurement of said plane displacement error alone. .Iadd.56. The apparatus of claim 44 wherein said surface of the first detector is coplanar with a virtual axis of said first focussing means. .Iaddend..Iadd.57. The apparatus of claim 44 wherein said surface of the second detector is perpendicular to a virtual axis of said first focussing means. .Iaddend..Iadd.58. The apparatus of claim 44 wherein said third optical path includes a first focussing lens for focussing a first spot onto the optical axis of a second focussing lens which in turn focusses a second spot onto an optical axis of a third focussing lens whose optical axis is perpendicular to said surface of said second detector. 
     
     
        .Iaddend..Iadd.59.  The apparatus of claim 44 wherein said third optical path is a straight path separate and distinct from said second optical path. .Iaddend..Iadd.60. The apparatus of claim 44 wherein said part of said reflected beam in said third optical path includes a retro-reflector which reflects said reflected beam which is a parallel beam in said third optical path back towards said first optical path, and focussing means which focusses said reflected back beam as said spot on said examined surface and then onto said second detector. .Iaddend..Iadd.61. The apparatus of claim 60 wherein said first detector is a charge-coupled device camera which includes image processing means. .Iaddend..Iadd.62. The apparatus of claim 60 wherein said detectors are charge-coupled device cameras and includes image processing means. .Iaddend..Iadd.63. The apparatus of claim 60 wherein said radiation beam and said reflected beam are parallel beams. .Iaddend..Iadd.64. The apparatus of claim 60 wherein said first focussing means comprises at least one lens for focussing said radiation beam and for collecting and converting said reflected radiation to a reflected beam. .Iaddend..Iadd.65. The apparatus of claim 60 wherein said first focussing means comprises one lens for focussing said radiation beam and another lens for collecting and converting said reflected radiation to a reflected beam. .Iaddend..Iadd.66. The apparatus of claim 60 wherein said first focussing means for focussing said radiation beam comprises at least one lens. .Iaddend.