Optics: measuring and testing – Shape or surface configuration – Triangulation
Reexamination Certificate
2000-02-25
2003-10-28
Smith, Zandra V. (Department: 2877)
Optics: measuring and testing
Shape or surface configuration
Triangulation
C356S604000, C382S286000
Reexamination Certificate
active
06639685
ABSTRACT:
TECHNICAL FIELD
The present invention relates generally to image processing techniques for fringe pattern analysis and, in particular, to optical measurement techniques that utilize phase-shift analysis of a fringe pattern to extract information contained within the fringe pattern.
BACKGROUND OF THE INVENTION
There are three basic types of fringe patterns used in optical metrology: carrier patterns, Young's patterns, and phase-shifted patterns. With the continuing advances in image capturing capabilities and increased processing power available from computers and other digital processing devices, automated processes for interpretation of these different types of fringe patterns are continually being developed and refined.
In carrier pattern processing techniques, carrier fringes are utilized to obtain modulated information fringes. The carrier is demodulated afterward in order to retrieve the desired information. See, for example, P. Plotkowski, Y. Hung, J. Hovansian, G. Gerhart, 1985 (“Improvement Fringe Carrier Technique for Unambiguous Determination of Holographically Recorded Displacements,”
Opt. Eng
., Vol. 24, No. 5, 754-756); T. Yatagai, S. Inaba, H. Nakano, M. Suzuki, 1984 (“Automatic Flatness Tester for Very Large Scale Integrated Circuit Wafers,”
Opt. Eng
., Vol. 23, No. 4, 401-405); and C. Sciammarella, J. Gilbert, 1976 (“A Holographic-Moiré Technique to Obtain Separate Patterns for Components of Displacement,”
Exp. Mech
., Vol. 16, No. 6., 215-219). In these techniques both the pure carrier pattern and modulated pattern are utilized so that the difference can be calculated. See, also, U.S. Pat. No. 5,671,042, issued Sep. 23, 1997 to C. A. Sciammarella, which discloses a holographic moiré interferometry technique using images taken before and after object deformation for purposes of strain measurement.
Another carrier pattern processing approach is the Fourier Transform Method (FTM). See, for example, M. Takeda, H. Ina, S. Kobayashi, 1982 (“Fourier-Transform Method of Fringe-Pattern Analysis for Computer-Based Topography and Interferometry,”
J. Opt. Soc. Am
., Vol. 72, No. 1, 156-160), and M. Takeda, Q. Ru, 1985 (“Computer-based Sensitive Electron-wave Interferometry,”
Appl. Opt
., Vol. 24, No. 18, 3068-3071). This FTM approach can be used with single patterns and involves removing the carrier by shifting the spectrum in the frequency domain. However, this spectrum shift introduces error and requires that the carrier fringes be equally spaced. M. Takeda, K. Mutoh, 1983 (“Fourier Transform Profilometry for the Automatic Measurement of 3-D Object Shapes,”
Appl. Opt
., Vol. 22, No. 24, 3977-3982) discloses another FTM approach in which carrier information is pre-saved in a computer and the carrier is then removed in the spatial domain. D. Bone, H. Bachor, R. Sandeman, 1986 (“Fringe-Pattern Analysis Using a 2-D Fourier Transform,”
Appl. Opt
., Vol. 25, No. 10, 1653-1660A) discloses a refinement of FTM that involves introducing a nonlinear response function and suggests using pure carrier fringe area for carrier demodulation. Yet another refinement is disclosed in J. Gu, F. Chen, 1995 (“Fast Fourier Transformation, Iteration, and Least-Square-Fit Demodulation Image Processing or Analysis of Single-Carrier Fringe Pattern,”
JOSA. Am. A
, Vol. 12, No. 10, 2159-2164). This modified FTM technique uses iteration and a global least squares fit to construct the phase of the carrier.
Processing of Young's patterns is commonly carried out in speckle metrology to obtain optical measurements of deformed objects, including measurements of slope, displacement, strain, and vibration. In speckleometry, two exposures are recorded on a specklegram—one before and one after the object being measured is deformed. The specklegram is then probed pointwise by a narrow laser beam and the resulting Young's fringes are displayed in a diffraction field behind the specklegram. Various automated systems and algorithms have been proposed, including one-dimensional integration, one-dimensional autocorrelation, one-dimensional and two-dimensional Fourier transformation, two-dimensional Walsh transformation, and maximum-likelihood techniques. See, for example, D. W. Robinson, 1983 (“Automatic Fringe Analysis with a Computer Image Processing System,”
Appl. Opt.
22, 2169-2176); J. M. Huntley, 1986 (“An Image Processing System for the Analysis of Speckle Photographs,”
J.Phys.E.
19, 43-48); D. J. Chen and F. P. Chiang, 1990 (“Digital Processing of Young's Fringes in Speckle Photography,”
Opt. Eng.
29, 1413-1420); J. M. Huntley, 1989 (“Speckle Photography Fringe Analysis by the Walsh Transform,”
Appl. Opt.
25, 382-386); and J. M. Huntley, 1992 (“Maximum-Likelihood Analysis of Speckle Photography Fringe Patterns,”
Appl. Opt.
31, 4834-4838). A whole-field processing approach to analyzing Young's patterns has been proposed by J. Gu, F. Chen, 1996 (“Fourier-Transformation, Phase-Iteration, and Least-Square-Fit Image Processing for Young's Fringe Pattern,”
Appl. Opt.
35, 232-239). In the proposed process, two-dimensional fast-Fourier transform (FFT) filtering of a Young's pattern is used to produce data for an initial phase calculation, following which phase iteration can be used to improve the phase, if desired or necessary. Finally, a global least-squares regression is carried out to fit the phase to a reference plane.
Synchronous detection has been used in processing of carrier and Young's patterns. See, for example, K. H. Womack, 1984 (“Interferometric Phase Measurement Using Spatial Synchronous Detection,”
Optical Engineering, Vol.
23, No. 4, 391-395); S. Toyooka, M. Tominaga, 1984 (“A Spatial Fringe Scanning for Optical Phase Measurement”); S. Toyooka, Y. Iwaasa, M. Kawahashi, K. Hosoi, M. Suzuki, 1985 (“Automatic Processing of Young's Fringes in Speckle Photography,”
Opt. Lasers Eng
., Vol. 6, 203-212); S. Tang, Y. Hung, 1990 (“Fast Profilometer for the Automatic Measurement of 3-D Object Shapes,”
Appl. Opt
., Vol. 29, No. 20, 3012-3018); and M. Kujawinska, 1993 (“Spatial Phase Methods in Interferogram Analysis,”
IOPP, D
. Robinson and G. Reid eds., Chapter 5, 180-185).
Synchronous methods utilize a reference spacing for correlation calculation. Generally, the accuracy of the method is determined by how close the reference spacing and the real fringe spacing are to each other. Synchronous methods typically start with either estimating a reference spacing or assuming that the reference spacing is equal to the carrier fringe spacing. Because of the mismatch between the reference spacing and the actual fringe spacing, the method has limited application. As discussed in J. Gu, Y. Shen, 1997 (“Iteration of Phase Window Correlation and Least Square Fit for Young's Fringe Pattern Processing,”
Appl. Opt
., Vol. 36, No. 4, 793-799), synchronous detection can be combined with a least squares fit. Iterations of these two steps and the phase calculation provide good accuracy for Young's pattern analysis. However, this iteration method is applicable only to equal-spaced fringe patterns and is not useful in analyzing unequally spaced fringe patterns, such as exist when projection fringe patterns are used in three-dimensional (3-D) surface contour measurement.
For purposes of 3-D surface contour measurement, a number of optical projection fringe pattern techniques have been proposed. For example, moiré techniques can be used to obtain surface contour maps, as disclosed in H. Takasaki, 1970 (“Moiré Topography,”
Appl. Opt
., Vol. 9, 1467-1472); D. M. Meadows, W. O. Johnson, J. B. Allen, 1970 (“Generation of Surface Contours by Moiré Patterns,”
Appl. Opt
., Vol. 9, 942-947); and J. Hovanesian, Y. Hung, 1971 (“Moiré Contour-Sum Contour-Difference, and Vibration Analysis of Arbitrary Objects,”
Appl. Opt
., Vol. 10, 2734). As is known, a moiré fringe pattern is obtained when a reference grating is superimposed over another grating. Projection moiré techniques often utilize a projection of light through a first grating, with the light then being
Barber Gary C.
Gu Jie
Tung Simon Chin-Yu
General Motors Corporation
Marra Kathryn A.
Smith Zandra V.
LandOfFree
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