Video contour measurement system employing moire interferometry having a beat frequency pattern

• US 5,436,462 A
• Filed: 12/21/1993
• Issued: 07/25/1995
• Est. Priority Date: 12/21/1993
• Status: Expired due to Fees

• Current Assignee: United Technologies Corporation (Raytheon Technologies Corporation d/b/a RTX)
• Original Assignee: United Technologies Optical Systems, Inc. (Raytheon Technologies Corporation d/b/a RTX)
• Inventors: Hull-Allen, C. Gregory
• Primary Examiner(s): Nelms, David C.
• Assistant Examiner(s): LE, QUE TAN
• Application Number:
  US08/171,559
• Time in Patent Office: 581 Days
• Field of Search: 250/550, 250/560, 250/561, 356/374, 356/376, 348/136
• US Class Current: 250/550
• CPC Class Codes: G01B 11/2536 using several gratings with variable grating pitch, projected on the object with the same angle of incidence
• CPC Class Codes: G01B 5/205 of turbine blades or propellers
• CPC Class Codes: G06T 7/521 from laser ranging, e.g. using interferometry; from the projection of structured light

Abstract

An electroptic metrology system uses Moire interferometry to generate contour information on the surface of an object. Interfering coherent beams generate a n image on the object'"'"'s surface that includes a stripe pattern. The surface is viewed with a high resolution camera and digitized using a frame grabber and computer. A reference pattern is generated in software and is mixed with the digitized object surface signals. The mixed signals are filtered to remove signals at the carrier frequency and signals containing the beat frequency indicative of the surface contours are output.

Claims

1. A method for measuring contours on the surface of an object, comprising the steps of:

illuminating the object surface with a beam of light whose intensity cross section includes periodically spaced regions of low intensity at a reference spatial frequency;

receiving an image reflected off the object surface having regions of low intensity at an object spatial frequency;

generating digital signal equivalents of said reflected object image;

mixing said reflected object image digital signals with reference signals corresponding to a pattern having periodically spaced regions of low intensity at a reference spatial frequency to generate combined image signals;

extracting, from said combined image signals, signals corresponding to an intensity pattern having regions of low intensity at a beat frequency; and

generating output signals indicative of said beat frequency pattern.

2. The method of claim 1 wherein said object illumination step further comprises the steps of interfering two coherent beams, thereby generating said periodically spaced regions of low intensity to have an infinite depth of field.

3. The method of claim 1 wherein said object illumination step further comprises the steps of expanding said object surface beam.

4. The method of claim 1 wherein said object illumination step further comprises the steps of collating said object surface beam.

5. The method of claim 1 further comprising the steps of filtering said combined image signals to remove therefrom signals corresponding to a carrier frequency.

6. The method of claim 1 further comprising the steps of computing, from said output signals, signals indicative of surface contours of the object.

7. The method of claim 1 further comprising the steps of generating a plurality of sets of combined image signals, with each of said combined image signal sets obtained at different positions of the object relative to said light beam.

8. The method of claim 7 further comprising the steps of stitching said sets of combined image signals so as to generate a single combined image signal set corresponding to multiple images of the object.

9. The method of claim 1 further comprising the steps of generating an image beam that provides an image of all of said outer surface of the object in a single combined image signal set.

10. The method of claim 5 wherein said reference and said object spatial frequencies are higher than said beat frequency and wherein said filtering step further comprises the steps of filtering said combined image signals through a non-linear morphological dilation filter and smoothing said filtered signals.

11. The method of claim 1 further comprising the step of fringe centroiding said output signals wherein intensity centroids are calculated along each of dark and light fringes to generate signals corresponding to a three dimensional image of the object.

12. The method of claim 1 further comprising the steps of:

generating a sequence of Moire interferogram signal sets comprises of an array of pixels, each interferogram signal set having a selected optical path length difference between the interfering beams;

computing a pixel by pixel phase difference signal set for said interferogram signal sets; and

computing from said pixel by pixel phase signal set a signal set indicative of relative height change of the object from said relative phase change.

13. The method of claim 12 wherein said phase difference is computed in accordance with the steps of:

generating N interferograms;

calculating phase using the equation;

space="preserve" listing-type="equation">tan φ

=Σ

I.sub.n sin (2π

n/N)/Σ

I.sub.n cos (2π

n/N)

where

φ

=phase angle;

N=number of interferograms

n=selected pixel

I=intensity at pixel, n.

14. The method of claim 5 wherein said filtering step includes the steps of comparing each of said combined signals against a threshold value and generating a signal indicative of a fringe should said compared signal exceed said threshold.

15. The method of claim 5 wherein said filtering step includes the steps of histogram equalization of said combined signals in said signal set.

16. The method of claim 5 wherein said fringe centroiding step further comprises the steps of linearly interpolating signal values in areas in between said light and dark fringes.

17. A system for measuring contours on the surface of an object, said apparatus comprising:

a means for illuminating the object surface with a beam of light whose intensity cross section includes periodically spaced regions of low intensity at a reference spatial frequency;

a means for receiving an image reflected off the object surface having regions of low intensity at an object spatial frequency;

a means for generating digital signal equivalents of said reflected object image;

a means for mixing said reflected object image digital signals with reference signals corresponding to a pattern having periodically spaced regions of low intensity at a reference spatial frequency to generate combined image signals;

a means for extracting, from said combined image signals, signals corresponding to an intensity pattern having regions of low intensity at a beat frequency; and

a means for generating output signals indicative of said beat frequency pattern.

18. The system of claim 17 wherein said object illumination means further comprises means for interfering two coherent beams, thereby generating said periodically spaced regions of low intensity to have an infinite depth of field.

19. The system of claim 17 wherein said object illumination means further comprises a means for expanding said object surface beam.

20. The system of claim 17 wherein said object illumination step further comprises a means for collating said object surface beam.

21. The system of claim 17 further comprising means for filtering said combined image signals to remove therefrom signals corresponding to a carrier frequency.

22. The system of claim 17 further comprising means for computing, from said output signals, signals indicative of surface contours of the object.

23. The system of claim 17 further comprising means for generating a plurality of sets of combined image signals, with each of said combined image signal sets obtained at different positions of the object relative to said light beam.

24. The system of claim 17 further comprising means for stitching said sets of combined image signals so as to generate a single combined image signal set corresponding to multiple images of the object.

25. The system of claim 17 further comprising means for generating an image beam that provides an image of all of said outer surface of the object in a single combined image signal set.

26. The system of claim 17 wherein said reference and said object spatial frequencies are higher than said beat frequency and wherein said filtering means further comprises a means for filtering said combined image signals through a non-linear morphological dilation filter and a means for smoothing said filtered signals.

27. The system of claim 17 further comprising a means for fringe centroiding said output signals wherein intensity centroids are calculated along each of dark and light fringes to generate signals corresponding to a three dimensional image of the object.

28. The system of claim 18 further comprising:

a means for generating a sequence of Moire interferogram signal sets comprises of an array of pixels, each interferogram signal set having a selected optical path length difference between the interfering beams;

a means for computing a pixel by pixel phase difference signal set for said interferogram signal sets; and

a means for computing from said pixel by pixel phase signal set a signal set indicative of relative height change of the object from said relative phase change.

29. The system of claim 28 wherein said phase difference computing means further comprises:

a means for generating N interferograms;

a means for calculating phase using the equation;

space="preserve" listing-type="equation">tan φ

=Σ

I.sub.n sin (2π

n/N)/Σ

I.sub.n cos (2π

n/N)

where

φ

=phase angle;

N=number of interferograms

n=selected pixel

I=intensity at pixel, n.

30. The system of claim 21 wherein said filtering means includes means for comparing each of said combined signals against a threshold value and generating a signal indicative of a fringe should said compared signal exceed said threshold.

31. The system of claim 21 wherein said filtering means includes means for providing histogram equalization of said combined signals in said signal set.

32. The system of claim 27 wherein said fringe centroiding means further comprises means for linearly interpolating signal values in areas in between said light and dark fringes.