System for obtaining a uniform illumination reflectance image during periodic structured illumination

• US 6,025,905 A
• Filed: 12/31/1996
• Issued: 02/15/2000
• Est. Priority Date: 12/31/1996
• Status: Expired due to Term

• Current Assignee: Cognex Corporation
• Original Assignee: Cognex Corporation
• Inventors: Sussman, Michael
• Primary Examiner(s): Hellner, Mark
• Application Number:
  US08/777,107
• Time in Patent Office: 1,141 Days
• Field of Search: 348/86, 348/131, 348/135, 382/106, 382/153, 382/154, 250/559.22, 901/47, 356/376, 356/394, 356/3.01
• US Class Current: 356/3.01
• CPC Class Codes: G01N 21/8806 Specially adapted optical and illumination features

Abstract

The invention provides an apparatus and method for obtaining a uniform illumination reflectance image of an object, even though the object is illuminated only using periodic structured illumination. The uniform illumination reflectance image so-produced has precise geometric and photometric correspondence with images produced using the periodic structured illumination. To obtain the uniform illumination reflectance image, a sum of a spanning set of periodic structured illumination images is computed. The resulting summation image bears substantially no trace of periodic structured illumination. Various embodiments of the apparatus of the invention are disclosed employing illuminator motion, object motion, and ray deflection to obtain a plurality of periodic structured illumination images of different phase. The invention is useful with triangulation ranging systems using a striped periodic illumination mask, with depth-from-focus ranging systems, and with depth-from-defocus ranging systems.

Claims

1. A method for forming a uniform-illumination reflectance image of an object in a system employing periodic structured illumination, the uniform-illumination reflectance image being characterized by both substantial geometric correspondence and substantial photometric correspondence with at least one captured periodic structured illumination image, the method comprising the steps of:

capturing a first periodic structured illumination image of the object to provide a first captured periodic structured illumination image characterized by a period and a first phase;

capturing a second periodic structured illumination image of the object to provide a second captured periodic structured illumination image characterized by a second phase and by a period that is substantially identical to the period of the first captured periodic structured illumination image; and

combining at least the first captured periodic structured illumination image with the second captured periodic structured illumination image so as to provide a substantially uniform-illumination reflectance image characterized by both substantial geometric correspondence with at least one of the captured periodic structured illumination images, and substantial photometric correspondence with said at least one of the captured periodic structured illumination images.

2. The method of claim 1, wherein the second phase is in 180 degree phase relationship with the first phase.

3. The method of claim 1, wherein the second phase is different from the first phase.

4. The method of claim 1, wherein only two periodic structured illumination images of the object are combined to provide the uniform-illumination reflectance image of the object.

5. The method of claim 1, wherein the step of capturing a second periodic structured illumination image of the object includes the step of:

shifting a structured illumination mask.

6. The method of claim 5, wherein the structured illumination mask is physically shifted.

7. The method of claim 5, wherein the structured illumination mask is electronically shifted.

8. The method of claim 1, wherein the step of capturing a second periodic structured illumination image of the object includes the step of:

shifting the object.

9. The method of claim 1, wherein the step of capturing a second periodic structured illumination image of the object includes the step of:

optically steering rays that together project the periodic structured illumination.

10. The method of claim 9, wherein the step of optically steering is accomplished by the steps of:

transmitting the rays through a transparent plate; and

tilting the transparent plate.

11. The method of claim 1, wherein said first and second periodic structured illumination images are striped images.

12. The method of claim 1, wherein said first and second periodic structured illumination images are two-dimensional periodic array images.

13. An apparatus for forming a uniform-illumination reflectance image of an object in a system employing periodic structured illumination, the uniform-illumination reflectance image being characterized by both substantial geometric correspondence and substantial photometric correspondence with at least one captured periodic structured illumination image, the apparatus comprising:

means for capturing a first periodic structured illumination image of the object to provide a first captured periodic structured illumination image characterized by a period and a first phase;

means for capturing a second periodic structured illumination image of the object to provide a second captured periodic structured illumination image characterized by a second phase and by a period that is substantially identical to the period of the first captured periodic structured illumination image; and

image combination means, cooperative with said means for capturing said first periodic structured illumination image and said second structured illumination image, for combining at least the first captured periodic structured illumination image with the second captured periodic structured illumination image so as to provide a substantially uniform-illumination reflectance image characterized by both substantial geometric correspondence with at least one of the captured periodic structured illumination images, and substantial photometric correspondence with said at least one of the captured periodic structured illumination images.

14. The apparatus of claim 13, wherein the second phase is in 180 degree phase relationship with the first phase.

15. The apparatus of claim 13, wherein the second phase is different from the first phase.

16. The apparatus of claim 13, wherein only two periodic structured illumination images of the object are combined to provide the uniform-illumination reflectance image of the object.

17. The apparatus of claim 13, wherein the means for capturing a second periodic structured illumination image of the object includes:

means for physically shifting a structured illumination mask.

18. The apparatus of claim 13, wherein the means for capturing a second periodic structured illumination image of the object includes:

means for electronically shifting a structured illumination mask.

19. The apparatus of claim 13, wherein the means for capturing a second periodic structured illumination image of the object includes:

means for shifting the object.

20. The apparatus of claim 13, wherein the means for capturing a second periodic structured illumination image of the object includes:

means for optically steering rays that together project the periodic structured illumination.

21. The apparatus of claim 20, wherein the means for optically steering rays includes:

a transparent plate; and

means for tilting the transparent plate.

22. The apparatus of claim 13, wherein the first and second periodic structured illumination images are stripped images.

23. The apparatus of claim 13, wherein the first and second periodic structured illumination images are two-dimensional periodic array images.

24. An apparatus for forming a uniform-illumination reflectance image of an object in a system employing periodic structured illumination, the uniform-illumination reflectance image being characterized by both substantial geometric correspondence and substantial photometric correspondence with at least one captured periodic structured illumination image, the apparatus comprising:

an illumination assembly, including a periodic structured illumination mask;

a viewing assembly, optically coaxial with respect to the illumination assembly, for forming a plurality of periodic structured illumination images;

an image capture module, cooperative with the viewing assembly;

a processor, cooperative with the image capture module, for combining the plurality of periodic structured illumination images to provide a uniform illumination reflectance image.

25. The apparatus of claim 24, further comprising:

an actuator, controlled by the processor, for facilitating the forming of the plurality of periodic structured illumination images.

26. The apparatus of claim 24, wherein said illumination assembly includes:

a tiltable transparent plate, the plate being moveable by the actuator.

27. The apparatus of claim 25, wherein the illumination assembly further comprises:

a moveable mask carriage for moveably supporting the periodic structured illumination mask, the moveable mask carriage being moved by the actuator.

28. The apparatus of claim 25, further comprising:

a motion stage for moveably supporting the object, the motion stage being moved by the actuator.