Image analysis – Image transformation or preprocessing – Measuring image properties
Reexamination Certificate
1998-12-31
2002-08-20
Mancuso, Joseph (Department: 2621)
Image analysis
Image transformation or preprocessing
Measuring image properties
C382S149000, C382S141000, C356S004090
Reexamination Certificate
active
06438272
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates generally to a method and apparatus for three dimensional surface contouring. In particular, the present invention uses a digital video projection system for digitally generating fringe patterns in three dimensional surface contouring.
Three dimensional surface contouring techniques have numerous applications in design and manufacturing. For example, surface contouring can be used for inspection of industrial parts whose dimensions and geometry need to be checked against their design specifications during or after manufacturing. These techniques can also be used in reverse engineering where construction of a Computer Aided Design (CAD) model from a physical part is required. In recent years, rapid prototyping technology based on a layered manufacturing concept has been established which allows for rapid fabrication of physical concept models, functional parts, and toolings directly from CAD models. Surface contouring techniques can help extend the capabilities of current rapid prototyping systems to include building physical parts and toolings from hand-crafted models or parts for which a CAD model is not available. Surface contouring techniques can also help improve the accuracy of constructed models by introducing in-process or post-process inspection into the rapid prototyping process.
Many optical three dimensional contouring methods have been developed and are well known in the art. The methods can generally be categorized into two groups: scanning and non-scanning imaging techniques. (Bieman, Leonard H., “Survey of design considerations for 3-D imaging systems,” Proc. SPIE, Vol. 1005, 138-144 (1998)). The scanning techniques are represented by point triangulation (Blais, F. and Rioux, M., “BIRIS: a simple 3-D sensor,” Proc. SPIE, Vol. 723, 235 (1986)), laser radar (Svetkoff, D. J., Leonard, P. F., and Sampson, R. E., “Techniques for real-time, 3-D feature extraction using range information,” Proc. SPIE, Vol. 521, 302 (1984)), and structured line methods. Point triangulation and structured line methods are based on the triangulation principle and the laser radar methods are based on the measurement of the travel time or phase of either a pulsed or modulated laser. All these techniques require either one-dimensional or two-dimensional scanning of the laser to cover the entire surface of the object. This generally makes the systems more sophisticated and the measurement more time consuming.
Typical non-scanning techniques include stereo vision and moiré interferometry. Stereo vision obtains three-dimensional information of an object by viewing a scene from two different perspectives and then locating common features in both images. (Hobrough, G. and Hobrough, T., “Stereopsis for robots by iterative stereo image matching,” Proc. SPIE, Vol. 449, 62 (1983)). The processing of the images is computationally intensive, which makes the technique unsuitable for high-speed 3-D contouring.
Moiré interferometry is one of the most commonly used techniques for 3-D surface contouring. Compared to other techniques, it has the primary advantage of fast measurement speed due to the fact that it does not require scanning to cover the whole object surface and the image processing for extracting 3-D contour information is relatively simple. Moiré contouring techniques can be classified as either shadow moiré (Chiang, F. P., “Moiré Methods for Contouring, Displacement, Deflection, Slope, and Curvature,” Proc. SPIE, Vol. 153, 113-119 (1978)) or projection moiré (Khetan, R. P. and F. P. Chiang, “On the theory of two projection moiré methods,” Univ. Of Ill. at Chicago press, 8, 16-18 (1977); Halioua, M., Krishnamurthy, R. S., Liu, H., and Chiang, F. P., “Projection moiré with moving gratings for automated 3-D topography,” Appl. Opt. 22, 850-855 (1983)). Shadow moiré uses the same grating for both illumination and observation, while projection moiré uses separated gratings. Another surface contouring technique is fringe projection which uses only one grating and measures surface height by triangulation.
An advantage of shadow moiré is that it is easy to obtain quantitative contour information from the moiré pattern because the grating is flat and its period known. However, the contouring of large objects is difficult because a grating with approximately the same size as the object must be used. Large gratings are difficult to make and have limited mobility.
Projection moiré and fringe projection offer advantages in their ability to contour large objects and the ease with which phase measuring techniques can be implemented to increase the measurement resolution. Their primary limitation is the tedium associated with obtaining quantitative height information. This limitation arises because it is necessary to calibrate both the projection geometry and the magnification factor.
In order to increase contouring resolution, phase shifting techniques developed in interferometry have been widely adopted and used in moiré and fringe projection methods for 3-D surface contouring. The resolution of the moiré and fringe projection contouring methods depends on the density of the fringe projected on the object. Generally, higher fringe density means higher resolution. However, there is a limit to the fringe density that can be applied because overly dense fringes may not be resolvable by the camera. To solve this dilemma, phase shifting techniques have been developed and widely used in optical contouring applications (Halioua, M. and Liu, H. -C., “Optical Three-Dimensional Sensing by Phase Measuring Profilometry,” Opt. Lasers Eng., 11(3), 185-215 (1989); Moore, D. T. and Truax, B. E., “Phase-Locked Moiré Fringe Analysis for Automated Contouring of Diffuse Surfaces,” Appl. Opt., 18(1), 91-96 (1979); Srinivasan, V. H., Liu, H. -C., and Halioua, M., “Automated Phase-Measuring Profilometry of 3-D Diffuse Objects,” Appl. Opt., 23(18), 3015-3018 (1984); Srinivasan, V. H., Liu, H. -C., and Halioua, M., “Automated Phase-Measuring Profilometry of 3-D Diffuse Objects,” Appl. Opt., 24(2), 185-188 (1985); Boehnlein, A. J. and Harding, K. G., “Adaptation of a Parallel Architecture Computer to Phase Shifted Moiré Interferometry,” Proc. SPIE, Vol. 728, 183-193 (1986); Kujawinska, M., “Use of Phase-Stepping Automatic Fringe Analysis in Moiré Interferometry,” Appl. Opt. 26(22), 4712-4714 (1987); Toyooka, S. and Iwaasa, Y., “Automatic Profilometry of 3-D Diffuse Objects by Spatial Phase Detection,” Appl. Opt.,25(10), 1630-1633 (1986)). Phase shifting dramatically increases measurement resolution without the need of using high density fringes. Traditional phase shifting is accomplished by mechanically shifting a grating to create a series of phase shifted fringe patterns. The phase shifted fringe patterns then are processed to extract the phase of each pixel of the image using algorithms well known in the art.
Phase shifted images are generally obtained by mechanically translating a grating. The shortcomings are that the system becomes more complicated because of the introduction of moving parts into the system and the phase shifting may not be accurate due to mechanical errors. The Phase Shifting And Logical Moiré (PSALM) was proposed to eliminate some of the problems with traditional phase shifting techniques (Asundi, A., “Projection moiré using PSALM,” Proc. SPIE, Vol. 1554B, 257-265 (1991)). PSALM uses only one grating with the other grating generated by software in a computer. The phase-shifted moiré fringes are obtained through logic calculations on the image of the object and the software created grating. Since no moving parts are necessary, this technique greatly simplifies the contouring system. The problem with this technique is that the contouring result is subject to possible errors due to surface reflectivity changes and existing surface marks. Other attempts to simplify the contouring system used a Liquid Crystal Display (LCD) panel as the projection system (Asundi, A., “Fringe Analysis in Moiré Interferometry,” Proc. SPIE, Vol. 1554B, 472-480 (1991); Arai, Y
Chiang Fu-Pen
Huang Peisen S.
Hoffman & Baron LLP
Kassa Yosef
Mancuso Joseph
The Research Foundation of State University of NY
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