Optics: measuring and testing – Range or remote distance finding – With photodetection
Reexamination Certificate
1999-11-24
2001-09-11
Buczinski, Stephen C. (Department: 3662)
Optics: measuring and testing
Range or remote distance finding
With photodetection
C356S496000, C356S456000
Reexamination Certificate
active
06288776
ABSTRACT:
FIELD OF THE INVENTION
The invention relates generally to the field of processing range images captured by a range imaging system, and in particular to the estimation and unwrapping of phase images captured by a scanner-less laser radar range imaging system.
BACKGROUND OF THE INVENTION
A scanner-less laser radar range imaging system is described in U.S. Pat. No. 4,935,616. The system described therein illuminates a scene object with an amplitude modulated laser source, wherein the amplitude modulation is in the range of 10MHz. The image capture portion of the system includes a micro-channel plate that is capable of modulating the optical signal reflected from the scene object. The phase shift of the intensity modulation reflected from the scene object can be calculated by capturing two images, one without modulating the optical signal, and another with the optical signal modulated by the micro-channel plate in phase with the same amplitude modulated frequency as used to modulate the laser source. Both images are registered spatially, and the difference between them is caused by the interference of the two modulating wave patterns, which produces a dc signal proportional to the phase shift. Once the phase shift has been established, range to the object can be computed.
Since the phase shift can only be determined modulo 27&pgr; the resulting range can only be found to within one wavelength of the modulation of the laser. To calculate the range at each point in the image, the correct integer number of phase cycles must be added to each phase measurement; that is, the phase must be “unwrapped”. It is therefore desirable to resolve the ambiguous (or wrapped) phase measurements to determine unambiguous (or unwrapped) phase.
The unambiguous phase, in turn, can be used to calculate unambiguous range. The aforementioned '616 patent suggests modulating the laser and receiver with different frequencies in order to produce two range images with different modulating frequencies. This would yield range unambiguous to within one wavelength of the wave whose frequency is the greatest common factor of the frequencies of the laser and receiver, which is a lower frequency than either of the two modulating frequencies. Even though this may reduce ambiguity problems in many situations, they still exist albeit on a smaller scale.
There are two main types of methods for solving the phase ambiguity problem: branch-cut methods and weighted least-squares methods.
Branch-cut methods (such as those described in Goldstein, Zebker, and Werner, “Satellite radar interferometry: two-dimensional phase unwrapping”,
Radio Science
, Vol. 23, pp. 713-720, 1998; and Prati, Giani, and Leurati, “SAR interferometry: A 2-D phase unwrapping technique based on phase and absolute values informations”,
Proc. Int. Geoscience
&
Remote Sensing Symposium IGARSS
1990, Washington, D.C., pp. 2043-2046, 1990) use lines to connect phase inconsistencies (or residues). Branch-cut methods fail to perform adequately when the number of residues is high. They often resort to local averaging, which is undesirable because it can dampen high frequency information. Least-squares methods (such as those described in Ghiglia and Romero, “Robust two-dimensional weighted and unweighted phase unwrapping that uses fast transforms and iterative methods”,
Journal of the Optical Society of America,
Vol. 11, pp.107-117, 1994; and Pritt and Shipman, “Least-squares two dimensional phase unwrapping using FFT's”,
IEEE Transactions on Geoscience and Remote Sensing,
Vol. 32, pp. 706-708, 1994) determine a phase finction that minimizes the error in the gradient estimates. If there are areas in the ambiguous phase image with a high noise content, nearby areas with a low noise content can be distorted by a least-squares phase unwrapping algorithm.
SUMMARY OF THE INVENTION
An object of the invention is to provide a method of unambiguous range estimation without the drawbacks in the known branch-cut and weighted least squares methods.
It is a further object of this invention to unambiguate (or unwrap) the measured phase shifts using a morphological technique.
It is a further object of this invention to provide a method of unwrapping phase shifts obtained by a scanner-less laser radar range imaging system that produces an image bundle (at least three images corresponding to distinct phase offsets of the capture device and/or illumination source).
The present invention is directed to overcoming one or more of the problems set forth above. Briefly summarized, according to one aspect of the present invention, a method of unambiguous range estimation is provided for use with a range imaging system that derives phase images from image pixels of a digital image. The method involves generating (a) a first phase image having one or more ambiguous phase intervals and (b) at least one additional phase image that is generated by shifting the phase intervals of the first phase image. Then at least one region of intersection between phase intervals in the two phase images is identified. Next, the phase of at least one of the ambiguous phase intervals in the first phase image is adjusted based on values of the phase of the image pixels that belong to the region of intersection. As a result, the phase adjustment unwraps the phase ambiguity in the phase intervals of the first phase image.
Instead of using a path-following technique or a technique based on a least-squares solution, this invention will resolve phase ambiguities by the use of a “region-moving” algorithm. By utilizing a model where ambiguous phase is determined by capturing images from at least three different phase shifts of either the illumination source (e.g., a laser) or the capture device (e.g., an intensifier) to obtain a wrapped phase image, a parameter in the model is then perturbed that spatially shifts regions of ambiguity in the wrapped phase image.
Using morphological image processing techniques, the spatial movements of the regions of ambiguity are followed. Appropriate phase offsets are determined by this region-moving technique, generating an unambiguous phase image. The advantage of this region-moving technique in relation to other phase-unwrapping techniques is that it determines unambiguous phase without distortions (i.e. given only the output of the region-moving technique, the input can be found exactly).
These and other aspects, objects, features and advantages of the present invention will be more clearly understood and appreciated from a review of the following detailed description of the preferred embodiments and appended claims, and by reference to the accompanying drawings.
REFERENCES:
patent: 4935616 (1990-06-01), Scott
patent: 5581345 (1996-12-01), Oki et al.
patent: 5621807 (1997-04-01), Eibert et al.
patent: 5995223 (1999-11-01), Power
“Least-Squares Two-Dimensional Phase Unwrapping Using FFT's” by Mark D. Pritt and Jerome S. Shipman,IEEE Transactions on Geoscience and Remote Sensing, vol. 32, No. 3, May 1994, pp. 706-708.
“Robust two-dimensional weighted and unweighted phase unwrapping that uses fast transforms and iterative methods” by Dennis C. Ghiglia and Louis A. Romero.J. Opt. Soc. Am. A/vol. 11, No. 1, Jan. 1994, pp. 107-117.
“Satellite radar interferometry: Two-dimensional phase unwrapping” by Richard M. Goldstein, Howard A. Zebker, and Charles L. Werner.Radio Science, vol. 23, No. 4, pp. 713-720. Aug. 1988.
“SAR Interferometry: A 2-D Phase Unwrapping Technique Based on Phase and Absolute Values Informations” by C. Prati, M. Giani and N. Leuratti.Proc. Int. Geoscience & Remote Sensing Symposium IGARSS 1990, Washington. D.C. pp. 2043-2046, 1990.
Cahill Nathan D.
Ray Lawrence A.
Buczinski Stephen C.
Eastman Kodak Company
Woods David M.
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