Depth-from-defocus optical apparatus with invariance to surface reflectance properties

• US 5,912,768 A
• Filed: 12/31/1996
• Issued: 06/15/1999
• Est. Priority Date: 12/31/1996
• Status: Expired due to Term

• Current Assignee: Cognex Corporation
• Original Assignee: Cognex Corporation
• Inventors: Sissom, Bradley, Sussman, Michael
• Primary Examiner(s): Pham, Hoa Q.
• Assistant Examiner(s): Ratliff, Reginald A.
• Application Number:
  US08/777,106
• Time in Patent Office: 896 Days
• Field of Search: 359/629, 359/625, 359/626, 359/640
• US Class Current: 359/629
• CPC Class Codes: G01B 11/026 by measuring distance between sensor and object G01B11/0608 takes precedence

Abstract

A depth-from-defocus optical apparatus is provided for use with a depth-from-defocus three-dimensional imaging system for obtaining a depth image of an object. The invention facilitates the formation of depth images of objects exhibiting specular reflection, either alone or in combination with diffuse reflection, thereby allowing the application of depth-from-defocus three-dimensional imaging to objects such as microelectronic packages. The optical apparatus of the invention generally includes an illumination source, a projection lens assembly for converging rays of incident light towards an object, and a viewing lens assembly for converging rays of reflected light towards an image plane. Importantly, the viewing lens assembly is of the same working f-number as the projection lens assembly. In preferred embodiments, both the projection lens assembly and the viewing lens assembly exhibit object-side telecentricity so as to substantially eliminate vignetting of off-axis specular object features, and consequently, substantially eliminate specular false depth. The invention can also include an uncrossed polarizer/analyzer pair to balance the dynamic range of specular reflections with the dynamic range of diffuse reflections so as to effectively utilize the limited dynamic range of a single image detector.

Claims

1. A depth-from-defocus optical apparatus for use with a depth-from-defocus three-dimensional imaging system for obtaining a depth image of an object, the optical apparatus comprising:

an illumination source for providing rays of incident light;

a projection lens assembly, characterized by a working f-number, for converging the rays of incident light towards an object plane;

a viewing lens assembly, also characterized by said working f-number, for converging rays of reflected light originating from a surface of the object towards an image plane, the object being disposed generally at said object plane; and

a beam splitting device disposed so as to permit a portion of said rays of incident light to reach said object, and disposed so as to permit a portion of said rays of reflected light to enter said viewing lens assembly.

2. The apparatus of claim 1, wherein said projection lens assembly includes an adjustable aperture stop.

3. The apparatus of claim 1, wherein said viewing lens assembly includes an adjustable aperture stop.

4. The apparatus of claim 1, wherein said viewing lens assembly and said projection lens assembly share at least one common optical element.

5. The apparatus of claim 1, wherein said illumination source comprises:

a light source;

a condenser lens; and

a periodic illumination mask.

6. The apparatus of claim 1, wherein said illumination source comprises:

a light source; and

a condenser lens.

7. The apparatus of claim 1, wherein said projection lens assembly includes a first polarizer in a first angular orientation, and said viewing lens assembly includes a second polarizer in a second angular orientation, said first angular orientation differing from said second angular orientation by a difference angle magnitude substantially different from 90 degrees, and substantially different from 270 degrees.

8. The apparatus of claim 1, further including a pair of polarizers, the relative orientation of the pair of polarizers being adjustable.

9. The apparatus of claim 7, wherein said first polarizer and said second polarizer are angularly oriented relative to each other such that a range of intensities of specular object feature reflections is substantially close to a range of intensities of diffuse object feature reflections.

10. The apparatus of claim 1, wherein said projection lens assembly exhibits object-side telecentricity.

11. The apparatus of claim 1, wherein said viewing lens assembly exhibits object-side telecentricity.

12. The apparatus of claim 1, further comprising:

only one image sensor.

13. The apparatus of claim 1, wherein said beam splitting device is disposed so as to make a focal plane of the viewing lens assembly coincident with a focal plane of the projection lens assembly.

14. The apparatus of claim 1, wherein said beam splitting device is disposed so as to fold a portion of a central axis of the viewing lens assembly into coaxial relationship with a portion of a central axis of the projection lens assembly.

15. A depth-from-defocus optical apparatus for use with a depth-from-defocus three-dimensional imaging system for obtaining a depth image of an object, the optical apparatus comprising:

an illumination source for providing rays of incident light;

a projection lens assembly, characterized by a working f-number, for converging the rays of incident light towards an object plane;

a viewing lens assembly exhibiting object-side telecentricity, also characterized by said working f-number, for converging rays of reflected light originating from a surface of the object towards an image plane, the object being disposed generally at said object plane; and

a beam splitting device disposed so as to permit a portion of said rays of incident light to reach said object, and disposed so as to permit a portion of said rays of reflected light to enter said viewing lens assembly.

16. The apparatus of claim 15, wherein said projection lens assembly includes an adjustable aperture stop.

17. The apparatus of claim 15, wherein said viewing lens assembly includes an adjustable aperture stop.

18. The apparatus of claim 15, wherein said viewing lens assembly and said projection lens assembly share at least one common optical element.

19. The apparatus of claim 15, wherein said illumination source comprises:

a light source;

a condenser lens; and

a periodic illumination mask.

20. The apparatus of claim 15, wherein said illumination source comprises:

a light source; and

a condenser lens.

21. The apparatus of claim 15, wherein said projection lens assembly includes a first polarizer in a first angular orientation, and said viewing lens assembly includes a second polarizer in a second angular orientation, said first angular orientation differing from said second angular orientation by a difference angle magnitude substantially different from 90 degrees, and substantially different from 270 degrees.

22. The apparatus of claim 15, further including a pair of polarizers, the relative orientation of the pair of polarizers being adjustable.

23. The apparatus of claim 21, wherein said first polarizer and said second polarizer are angularly oriented relative to each other such that a range of intensities of specular object feature reflections is substantially close to a range of intensities of diffuse object feature reflections.

24. The apparatus of claim 15, wherein said projection lens assembly exhibits object-side telecentricity.

25. The apparatus of claim 15, further comprising:

only one image sensor.

26. The apparatus of claim 15, wherein said beam splitting device is disposed so as to make a focal plane of the viewing lens assembly coincident with a focal plane of the projection lens assembly.

27. The apparatus of claim 15, wherein said beam splitting device is disposed so as to fold a portion of a central axis of the viewing lens assembly into coaxial relationship with a portion of a central axis of the projection lens assembly.

28. A depth-from-defocus optical apparatus for use with a depth-from-defocus three-dimensional imaging system for obtaining a depth image of an object, the optical apparatus comprising:

an illumination source for providing rays of incident light;

a projection lens assembly, characterized by a working f-number, for converging the rays of incident light towards an object plane, said projection lens assembly including a first polarizer in a first angular orientation;

a viewing lens assembly, also characterized by said working f-number, for converging rays of reflected light originating from the surface of the object towards an image plane, the object being disposed generally at said object plane, said viewing lens assembly including a second polarizer in a second angular orientation, said second angular orientation differing from said first angular orientation by a difference angle magnitude substantially different from 90 degrees and substantially different from 270 degrees; and

a beam splitting device disposed so as to permit a portion of said rays of incident light to reach said object, and disposed so as to permit a portion of said rays of reflected light to enter said viewing lens assembly.

29. The apparatus of claim 28, wherein said projection lens assembly includes an adjustable aperture stop.

30. The apparatus of claim 28, wherein said viewing lens assembly includes an adjustable aperture stop.

31. The apparatus of claim 28, wherein said viewing lens assembly and said projection lens assembly share at least one common optical element.

32. The apparatus of claim 28, wherein said illumination source comprises:

a light source;

a condenser lens; and

a periodic illumination mask.

33. The apparatus of claim 28, wherein said illumination source comprises:

a light source; and

a condenser lens.

34. The apparatus of claim 28, the orientation of the polarizer of the viewing assembly being adjustable with respect to the orientation of the polarizer of the projection assembly.

35. The apparatus of claim 28, wherein said first polarizer and said second polarizer are angularly oriented relative to each other such that a range of intensities of specular object feature reflections is substantially close to a range of intensities of diffuse object feature reflections.

36. The apparatus of claim 28, wherein said projection lens assembly exhibits object-side telecentricity.

37. The apparatus of claim 28, wherein said viewing lens assembly exhibits object-side telecentricity.

38. The apparatus of claim 28, further comprising:

only one image sensor.

39. The apparatus of claim 28, wherein said beam splitting device is disposed so as to make a focal plane of the viewing lens assembly coincident with a focal plane of the projection lens assembly.

40. The apparatus of claim 28, wherein said beam splitting device is disposed so as to fold a portion of a central axis of the viewing lens assembly into coaxial relationship with a portion of a central axis of the projection lens assembly.

41. A depth-from-defocus optical apparatus for use with a depth-from-defocus three-dimensional imaging system for obtaining a depth image of an object, the optical apparatus comprising:

an illumination source for providing rays of incident light;

a projection lens assembly, characterized by a working f-number, for converging the rays of incident light towards an object plane, said projection lens assembly including a first polarizer in a first angular orientation, said projection lens assembly being structured so as to exhibit object-side telecentricity;

a viewing lens assembly, also characterized by said working f-number, for converging rays of reflected light originating from the surface of the object towards an image plane, the object being disposed generally at said object plane, said viewing lens assembly being structured so as to exhibit object-side telecentricity, said viewing lens assembly including a second polarizer in a second angular orientation, said second angular orientation differing from said first angular orientation by a difference angle magnitude substantially different from 90 degrees and substantially different from 270 degrees; and

a beam splitting device disposed so as to permit a portion of said rays of incident light to reach said object, and disposed so as to permit a portion of said rays of reflected light to enter said viewing lens assembly.

42. The apparatus of claim 41, wherein said projection lens assembly includes an adjustable aperture stop.

43. The apparatus of claim 41, wherein said viewing lens assembly includes an adjustable aperture stop.

44. The apparatus of claim 41, wherein said viewing lens assembly and said projection lens assembly share at least one common optical element.

45. The apparatus of claim 41, wherein said illumination source comprises:

a light source;

a condenser lens; and

a periodic illumination mask.

46. The apparatus of claim 41, wherein said illumination source comprises:

a light source; and

a condenser lens.

47. The apparatus of claim 41, wherein said first polarizer and said second polarizer are angularly oriented relative to each other such that a range of intensities of specular object feature reflections is substantially close to a range of intensities of diffuse object feature reflections.

48. The apparatus of claim 41, further comprising:

only one image sensor.

49. The apparatus of claim 41, wherein said beam splitting device is disposed so as to make a focal plane of the viewing lens assembly coincident with a focal plane of the projection lens assembly.

50. The apparatus of claim 41, wherein said beam splitting device is disposed so as to fold a portion of a central axis of the viewing lens assembly into coaxial relationship with a portion of a central axis of the projection lens assembly.