Note: Descriptions are shown in the official language in which they were submitted.
CA 02275411 1999-06-16
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Apparatus and Method for Rapid 3D Image Parametrization
Field of the Invention
This invention relates to the measurement and modeling of a three dimensional
surface. More particularly, this invention relates to measuring and modeling a
three
dimensional surface by projecting a speckle pattern upon the three dimensional
surface,
imaging the speckle pattern with a plurality of cameras to obtain a plurality
of two
dimensional digital images, and obtaining from the plurality of two
dimensional digital
images a three dimensional digital image and a model parameter set
representing the three
dimensional illuminated surface.
Background of the Invention
Speckle techniques, both photographic and interferometric, are known to
produce
images of surfaces containing a rich set of spatial frequencies thereby
providing spatial
information on a wide range of spatial scales.A survey of speckle techniques
is provided
in Jones, Robert, Holographic and speckle interferometry: a discaession of the
theory,
practice, and application of the techniques, 2nd ed., Cambridge University
Press (1989),
which is herein incorporated by reference.
Summary of the Invention
A preferred embodiment of the present invention provides a method for
measuring
and modeling a three dimensional surface. This method has the steps of: (i)
illuminating
the three dimensional surface with a speckle pattern; (ii) imaging the speckle
pattern to
obtain a plurality of two dimensional digital images; and (iii) processing the
plurality of
two dimensional digital images to obtain a three dimensional digital
characterization of
the illuminated three dimensional surface. A further embodiment includes the
step of
modeling the illuminated surface, based upon the plurality of two dimensional
digital
images, to obtain a parameter set characterizing the illuminated surface. In
addition, the
embodiment includes performing steps (i)-(iii) as outlined above more than
once to
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provide an ensemble of speckle patterns and an ensemble of two dimensional
digital
images, wherein the ensemble of speckle patterns contains at least two
distinct speckle
patterns. In accordance with an alternate embodiment of the invention, the
method also
has the step of modeling the illuminated surface to obtain a parameter set
characterizing
the illuminated surface, wherein the modeling is based upon the ensemble of
two
dimensional digital images.
In accordance with a further aspect of the present invention in one of its
embodiments, there is provided an apparatus for rapid three dimensional image
parametrization of a three dimensional surface. The apparatus has a speckle
pattern
generator for providing a speckle pattern upon the three dimensional surface;
a plurality of
cameras for imaging the speckle pattern to provide a plurality of two
dimensional digital
images; and a processor in communication with the plurality of cameras for
processing the
plurality of two dimensional digital images to obtain a three dimensional
digital
characterization of the three dimensional surface. The processor may further
provides a
parameter set characterizing the three dimensional surface. The speckle
pattern generator
may have a source of optical radiation coupled through an optical fiber, as
well as a
speckle pattern shifter for varying the speckle pattern projected upon the
three
dimensional surface as a function of time. The speckle pattern shifter may be
a
mechanical strain inducer for applying strain to the optical C ber, and the
mechanical strain
inducer may be a piezoelectric element.
Yet another embodiment of the present invention is an apparatus for rapid
three
dimensional image parametrization of a three dimensional surface, where the
apparatus
comprises a speckle pattern generator for providing a speckle pattern upon the
three
dimensional surface; a plurality of cameras for imaging the speckle pattern to
provide a
plurality of two dimensional digital images; a memory in communication with
the
plurality of cameras for storing the plurality of two dimensional digital
images; and a
processor in communication with the memory for processing the plurality of two
dimensional digital images to obtain a three dimensional digital
characterization of the
three dimensional surface.
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Brief Description of the Drawing
Fig. 1 illustrates a flow diagram for a preferred embodiment of the invention.
Detailed Description of Specific Embodiments
An embodiment of a method for measuring and modeling a three dimensional
surface of an object is illustrated in Fig. 1. An object 10 is illuminated by
a laser speckle
generator 20. The techniques of the present invention are broadly applicable
to various
optical inspection modalities known in the art, and the application of these
techniques to
any of such modalities is considered within the scope of the invention and of
the
appended claims.
In a preferred embodiment, laser speckle generator 20 is a laser coupled to
one end
of an optical fiber, using optical coupling techniques known to persons of
ordinary skill in
the art. The end of the optical fiber distal to the laser is used to
illuminate the object. The
laser speckle generator projects a speckle pattern upon object 1 (1. A speckle
pattern is a
I S pattern of illumination in which the intensity profile of the illumination
appears as a
realization of a random illumination pattern. A laser coupled to an
inexpensive, low
quality optical fiber may provide a speckle pattern, and it is this
combination which serves
as a speckle generator in a preferred embodiment. The speckle may be
diffraction limited
thereby providing a random pattern including the highest spatial frequencies
attainable.
Furthermore, in a preferred embodiment a mechanical strain may be applied to
the optical
fiber so that an entire ensemble of uncorrelated speckle patterns can be
generated by
changing the mechanical strain. This can be accomplished by wrapping the
optical fiber
around a piezoelectric material, so that a voltage applied to the
piezoelectric material
causes it to apply a mechanical stress to the optical fiber. In a preferred
embodiment,
object 10 is illuminated with an ensemble of laser speckle patterns, as
indicated in Fig. 1
by the index n = 1,2, 3, . . ..
A plurality of cameras 30 is used to image the speckle patterns illuminated on
object 10. In a preferred embodiment, cameras 30 provide digital images. Due
to
parallax, the images obtained from cameras 30 will be different from each
other. From
the differences in the digital images obtained from cameras 30, and from
knowledge of
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the relative positions of the cameras to each other, three dimensional
coordinates
describing the three dimensional surface of object 10 may be obtained for each
speckle
illumination. Use is made of the random nature of the speckle illumination
pattern in
determining the differences in the digital images due to parallax. If the
speckle pattern is
sufficiently random, then small portions of the speckle pattern will be
sufficiently
different from other small portions of the speckle pattern, and determining
the relative
shifts of the digital images will be facilitated by making comparisons among
these
uniquely identifiable small portions.
Parameters of a model may be chosen to fit the digital images obtained from
cameras 30 according to a model parameter fitting algorithm, as indicated in
step 40 of
Fig. 1. Finite element modeling of a surface is the subject of P. Charette et
al., "Large
deformation mechanical testing of biological membranes using speckle
interferometry in
transmission. II. Finite element modeling," Applied Optics, vol. 36( 10), pp.
2246-51
(1997), which is incorporated herein by reference. The parameters may be
obtained by a
least squares fit using the finite element method in which the basis functions
are cubic-
Hermite functions. The parameters obtained from model parameter fitting step
40 are used
in 50 to obtain a model of the three dimensional surface of object 10. These
parameters
may be used to display the three dimensional surface of object 10, or they may
be used to
fabricate or synthesize new three dimensional surfaces which characterize the
three
dimensional surface of object 10.
As indicated by flow control lines 60 and 70, the illumination and imaging
process
may be repeated a number of times with or without changing the speckle
pattern. For
example, the surface of object 10 may be changing as a function of time, in
which case the
process must be repeated to obtain parameter sets indexed by time. This is
indicated by
temporal bandwidth control line 60. There is also the spatial frequency
aspects of the three
dimensional surface which must be properly captured. This is indicated by the
spatial
bandwidth control line 70, in which object 10 is repeatedly illuminated with
different
speckle patterns generated by laser speckle generator 20 for each illumination
step. By
using different speckle patterns which are statistically uncorrelated from
each other, it is
possible to capture features of the surface of object 10 that might otherwise
be missed if
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the same speckle pattern was used throughout the measurement and modeling
process.
Statistical averaging may be employed over the ensemble of digital images
obtained by
illuminating object 10 with different speckle patterns, so that a single
parameter set is
obtained in which the spatial features of the surface of object 10 are
properly captured.
Numerous modifications may be made to the embodiments described above
without departing from the spirit and scope of the invention.
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