U.S. patent application number 12/553526 was filed with the patent office on 2009-12-31 for multiple channel interferometric surface contour measurement system.
This patent application is currently assigned to DIMENSIONAL PHOTONICS INTERNATIONAL, INC.. Invention is credited to D. Scott Ackerson, Robert F. Dillon, Neil Judell, Yi Qian, Gurpreet Singh, Yuqing Zhu.
Application Number | 20090324212 12/553526 |
Document ID | / |
Family ID | 36739915 |
Filed Date | 2009-12-31 |
United States Patent
Application |
20090324212 |
Kind Code |
A1 |
Dillon; Robert F. ; et
al. |
December 31, 2009 |
MULTIPLE CHANNEL INTERFEROMETRIC SURFACE CONTOUR MEASUREMENT
SYSTEM
Abstract
Described is a multiple channel interferometric surface contour
measurement system. The measurement system includes a multiple
channel interferometer projector, a digital camera and a processor.
The projector includes two or more interferometer channels. Each
channel has an optical axis spatially separate from the optical
axes of the other channels. Each channel projects a fringe pattern
onto the surface of an object to be measured. Image data for the
fringe patterns projected on the object surface are acquired by the
digital camera. The processor controls the projection of the fringe
patterns of different spatial frequencies, adjusts the phase of
each fringe pattern and generates surface contour data in response
to the camera image data. The multiple channel interferometric
surface contour measurement system provides numerous advantages
over conventional single channel interferometric systems, including
reduced sensitivity to optical noise, improved stability and
increased measurement accuracy.
Inventors: |
Dillon; Robert F.;
(Chelmsford, MA) ; Judell; Neil; (Newtonville,
MA) ; Qian; Yi; (Acton, MA) ; Zhu; Yuqing;
(Andover, MA) ; Ackerson; D. Scott; (Windham,
NH) ; Singh; Gurpreet; (Providence, RI) |
Correspondence
Address: |
GUERIN & RODRIGUEZ, LLP
5 MOUNT ROYAL AVENUE, MOUNT ROYAL OFFICE PARK
MARLBOROUGH
MA
01752
US
|
Assignee: |
DIMENSIONAL PHOTONICS
INTERNATIONAL, INC.
Wilmington
MA
|
Family ID: |
36739915 |
Appl. No.: |
12/553526 |
Filed: |
September 3, 2009 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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11910638 |
Oct 4, 2007 |
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PCT/US2006/012438 |
Apr 4, 2006 |
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12553526 |
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60669039 |
Apr 6, 2005 |
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Current U.S.
Class: |
396/428 |
Current CPC
Class: |
G01B 11/2527 20130101;
G01B 11/2441 20130101 |
Class at
Publication: |
396/428 |
International
Class: |
G03B 17/00 20060101
G03B017/00 |
Claims
1. A crossbar for improved orientation stability between two system
components in an interferometric measurement system, comprising: an
outer cylindrical tube having a tube axis, an inner surface and two
ends; an inner cylindrical tube having a tube axis collinear with
the tube axis of the outer tube and having an outer surface and two
ends, the inner cylindrical tube disposed inside the outer tube and
extending outside the two ends of the outer cylindrical tube, each
end of the inner cylindrical tubes adapted for attaching one of the
system components; and a pair of O-rings disposed between the inner
surface of the outer cylindrical tube and the outer surface of the
inner cylindrical tube, each O-ring being disposed along a length
of the inner cylindrical tube at a predetermined position to reduce
changes in the orientation between the two system components as the
crossbar is reoriented.
2. The crossbar of claim 1 wherein the outer cylindrical tube and
the inner cylindrical tube are manufactured from materials having
similar coefficients of thermal expansion.
3. The crossbar of claim 1 wherein the outer cylindrical tube and
the inner cylindrical tube are manufactured from aluminum.
4. The crossbar of claim 1 further comprising a mounting plate
secured to the outer tube.
Description
RELATED APPLICATIONS
[0001] This application is a divisional of U.S. patent application
Ser. No. 11/910,638 filed on Oct. 4, 2007, which claimed the
benefit of and priority to U.S. Provisional Patent Application Ser.
No. 60/669,039, filed Apr. 6, 2005, titled "Multiple Channel
Interferometric Surface Contour Measurement Methods and Apparatus,"
the entirety of which is incorporated herein by reference.
FIELD OF THE INVENTION
[0002] The invention relates generally to the measurement of
surface contours and more particularly to a non-contact
interferometric system and method for the measurement of surface
contours.
BACKGROUND OF THE INVENTION
[0003] Surface measurement systems are used in a variety of
applications to generate three dimensional surface data of objects.
Such systems are employed at various stages in the fabrication and
assembly of complex objects across a variety of industries to
ensure that the shape and size of the objects meet strict
manufacturing tolerances.
[0004] Interferometric surface measurement systems have been
developed which permit measurements of the surface of an object
without physical contact. Coherent optical sources are used to
generate a fringe pattern on the surface of the object and a camera
acquires images of the fringes on the surface for analysis. In some
systems, a diffraction grating is positioned in the path of a laser
beam to generate multiple coherent laser beams at various angles to
the original beam path. A focusing objective and spatial filter are
used to isolate the desired diffracted beam pair. One or more
additional diffraction gratings are utilized to project at least
one additional set of fringes onto the object surface. This
multiplexing of different gratings into the beam path poses many
challenges. Moving different gratings into the beam path and
shifting each grating to implement phase shifts generally requires
multiple mechanical components that add weight, size, complexity
and cost to the system. The frequent movement of components affects
the stability and therefore the accuracy of the measurement data.
Moreover, measuring the displacement of a diffraction grating
during the phase shift process with sufficient precision and
accuracy can require expensive measurement components such as
capacitance gauges.
[0005] Other system components can limit the applications for the
system. For example, the focusing objective and spatial filter are
used for multiple gratings and, therefore, their optical parameters
are not optimal for the individual gratings. Moreover, the depth of
field of the camera can limit the maximum spatial frequency of the
projected fringe pattern, thereby limiting the measurement
resolution.
[0006] Noise sources also typically limit the measurement data.
When laser light is scattered from a surface, a high-contrast,
granular speckle pattern is typically observed. Speckle results in
part from the roughness of the object surface. In particular, the
microscopic roughness of the surface contributes randomly phased
contributions of the scattered laser light. These contributions
interfere with one another to produce complex intensity variations
across the surface of the object as viewed from a distance. Speckle
introduces fine-scale intensity fluctuations (i.e., intensity
noise) in the observed fringe pattern on the object surface. Shot
noise contributions from the individual detectors in the camera can
further limit the accuracy of measurement data.
SUMMARY OF THE INVENTION
[0007] In one aspect, the invention features a crossbar for
improved orientation stability between two system components in an
interferometric measurement system. The crossbar includes an outer
cylindrical tube, an inner cylindrical tube and a pair of O-rings.
The outer cylindrical tube has a tube axis, an inner surface and
two ends. The inner cylindrical tube has a tube axis collinear with
the tube axis of the outer tube and also has an outer surface and
two ends. The inner cylindrical tube is disposed inside the outer
tube and extends outside the two ends of the outer cylindrical
tube. Each end of the inner cylindrical tube is adapted for
attachment to one of the system components. The O-rings are
disposed between the inner surface of the outer cylindrical tube
and the outer surface of the inner cylindrical tube. Each O-ring is
disposed along a length of the inner cylindrical tube at a
predetermined position to reduce changes in the orientation between
the two system components as the crossbar is reoriented.
BRIEF DESCRIPTION OF THE DRAWINGS
[0008] The above and further advantages of this invention may be
better understood by referring to the following description in
conjunction with the accompanying drawings, in which like numerals
indicate like structural elements and features in the various
figures. The drawings are not necessarily to scale, emphasis
instead being placed upon illustrating the principles of the
invention.
[0009] FIG. 1 is a diagram of a surface contour measurement system
in which the surface of an object is irradiated with structured
light patterns generated according to interferometric
principles.
[0010] FIG. 2 is another view of the system of FIG. 1 and includes
a magnified view of a surface contour of the object.
[0011] FIG. 3 depicts a portion of an image of the surface of the
object of FIG. 2 as irradiated by a first fringe pattern.
[0012] FIG. 4 shows a view of a portion of an image of the surface
of the object of FIG. 2 as irradiated by a second fringe
pattern.
[0013] FIG. 5A and FIG. 5B illustrate front and rear perspective
views, respectively, of a multiple channel interferometer projector
according to an embodiment of the invention.
[0014] FIG. 6A and FIG. 6B illustrate exploded perspective views of
the projector shown in FIG. 5A and FIG. 5B, respectively.
[0015] FIG. 7 shows a perspective view of a diffraction grating
module of the projector of FIG. 6A and FIG. 6B according to an
embodiment of the invention.
[0016] FIG. 8A and FIG. 8B illustrate rear and front perspective
views, respectively, of the diffraction grating module of FIG. 7
with the grating substrate removed.
[0017] FIG. 9 illustrates an exploded perspective view of a
multiple channel interferometer projector having an intensity
shaping module according to another embodiment of the
invention.
[0018] FIG. 10 is a cross-sectional diagram of a crossbar for
rigidly coupling a camera to a projector according to an embodiment
of the invention.
[0019] FIG. 11 is a perspective view of a crossbar for rigidly
coupling a camera to a projector according to an embodiment of the
invention.
DETAILED DESCRIPTION
[0020] In brief overview, the present invention relates to a
multiple channel interferometric surface contour measurement
system. The measurement system includes a multiple channel
interferometer projector, a digital camera and a processor. The
projector includes two or more interferometer channels and each
interferometer channel has an optical axis spatially separate from
the optical axes of the other channels. Each channel projects a
fringe pattern onto the surface of an object to be measured. As
used herein, a digital camera means an electronic imaging device or
system that provides digital image data indicative of intensity as
a function of image position. The digital camera acquires image
data of the fringe patterns projected on the object surface. The
processor communicates with the multiple channel interferometer
projector and the digital camera. Signal sent from the processor to
the projector control the projection of the fringe patterns of
different spatial frequencies and the phase of each fringe pattern.
The processor generates surface contour data in response to the
camera image data. The multiple channel interferometric surface
contour measurement system provides numerous advantages over
conventional single channel interferometric systems, including
reduced sensitivity to optical noise, improved stability and
increased measurement accuracy.
[0021] FIG. 1 is an illustration of a surface measurement system 10
in which a projector 14 irradiates a surface of an object of
interest 18 with structured light patterns that are generated based
on interferometric principles. Images of the irradiated object
surface 34 are acquired by a digital camera 22 that is rigidly
coupled to the projector 14 via a crossbar 26. The camera 22 is
electronically coupled to a processor 30 which is configured to
process the acquired images. The processor 30 can be coupled to the
projector 14 to control various aspects of the structured light
patterns.
[0022] The surface measurement system 10 is configured to determine
a three-dimensional profile of the surface contour 34. In
particular, for a given portion of the surface contour 34 that is
within a field of view of the digital camera 22, the processor 30
calculates relative coordinates in three dimensional space for a
significant number of points on the object surface 34.
[0023] A coordinate system 38 is shown in FIG. 1 to provide a
reference for the surface contour measurements. The y-axis of the
coordinate system 38 is perpendicular to the plane of the figure.
An image plane 42 (e.g., a detector array) of the digital camera 22
lies in an x-y plane in space, and the optical axis 44 of the
camera 22 (which is perpendicular to the image plane 42) is
parallel to the z-axis of the coordinate system 38. The x-y
coordinates of a point P in the camera field of view generally are
directly determinable from the known geometry and optical
parameters of the camera 22; however, a z-coordinate z.sub.P of the
point P is not directly determinable.
[0024] Structured light patterns are used to calculate the
z-coordinates of points on the object surface 34. Images of
structured light patterns irradiating the object surface 34 provide
information from which the processor 30 can calculate distances
z.sub.P between points P on the object surface and the camera image
plane 42. Such calculations are based on triangulation, using a
known distance L between the digital camera 22 and the projector
14, and a known angle .theta. between a central axis 46 of the
projector 14 and an axis of the crossbar 26.
[0025] In various implementations of the projector 14, light from a
coherent light source (e.g., a laser) is separated into two beams.
The two beams are directed toward the object surface 34 and caused
to interfere with each other so as to generate a pattern of
interference fringes comprising periodic bright lines separated by
dark lines. As used herein, the process of separating light from a
coherent light source into two beams and causing the two beams to
interfere to generate a fringe pattern is referred to as "single
channel interferometry."
[0026] FIG. 2 is an illustration of the surface measurement system
10 of FIG. 1 and includes a magnified view of the surface contour
34 of the object of interest 18 around a point of interest P for
which the distance z.sub.P is to be determined. The projector 14 is
configured to irradiate the object surface 34 with a fringe pattern
50 depicted as a periodic wave. The peaks of the periodic wave
represent the highest intensities and the troughs of the periodic
wave represent the lowest intensities. Thus the areas around the
peaks represent bright lines or "fringes" on the object surface 34
that can be observed by the digital camera 22 and the areas around
the troughs represent dark lines between neighboring fringes.
[0027] The spatial frequency (i.e., periodicity) of the fringe
pattern 50 generally is a function of the configuration of the
projector 14 and the arrangement of two coherent light sources 14A
and 14B or, equivalently, the apparent sources of two spatially
distinct beams from a single coherent light source, that interfere
with each other. The sources 14A and 14B are separated by a
distance a (hereafter the "source separation distance") along an
axis 48 perpendicular to the central axis 46 of the projector 14.
The spatial frequency of the fringe pattern 50 is determined in
part by the source separation distance a. The spatial frequency
decreases (i.e., the fringes become "coarser") as the source
separation distance a decreases.
[0028] In one example, a single channel interferometer projector
transmits a laser beam through a diffraction grating having a pitch
d (i.e., the spacing between adjacent light perturbing components
of the diffraction grating). The laser beam is divided into pairs
of coherent beams propagating at various deflection angles from the
grating according to their order and the pitch d of the grating. A
portion of the laser beam passes through the diffraction grating
undeflected as a zero order beam. As the pitch d increases (i.e.,
as the grating becomes coarser), the deflection angles of the
non-zero order beams decrease.
[0029] The single channel interferometer utilizes a focusing
objective and a spatial filter to isolate the pair of first order
beams propagating from the diffraction grating and thereby provide
two coherent beams required for interference. Specifically, the
focusing objective directs the two first order beams towards two
apertures spaced apart at the source separation distance a in the
spatial filter. This source separation distance a is determined
from the known angles at which the first order beams exit the
diffraction grating and the optical properties of the focusing
objective. Accordingly, the spatial frequency of the fringe pattern
50 generated by the single channel interferometer is based on the
diffraction grating pitch d.
[0030] The fringe pattern 50 includes a central fringe having a
fringe number N.sub.o and an intensity peak 54, and is associated
with a point P.sub.o on the object surface 34 which is intersected
by the central axis 46 of the projector 14. As illustrated, the
coherent light sources 14A and 14B are disposed symmetrically about
the central axis 46. The peak 54 of the central fringe N.sub.o
serves as a reference point for the fringe pattern 50, on either
side of which are other fringes consecutively labeled N.sub.+1,
N.sub.+2, . . . moving to the right in the figure and N.sub.-1,
N.sub.-2, . . . moving to the left in the figure. Accordingly, the
peak 54 of the central fringe N.sub.o is associated with a
reference phase of 0.degree. for the fringe pattern 50 and the
phase between adjacent peaks, or adjacent troughs, is 360.degree.
or 2.pi. radians.
[0031] The determination of the distance z.sub.P between the camera
22 and a point of interest P can be determined from the phase .phi.
between point P.sub.o at the intensity peak 54 and point P at an
intensity 58 on the fringe N.sub.-1. By determining the phase
.phi., the angle .DELTA..theta. can be derived and triangulation
calculations can be performed based on the angle
.theta.-.DELTA..theta. to determine the distance z.sub.P. More
specifically, if a rational quantity N represents the total integer
and fractional number of fringes between the points P and P.sub.0,
the corresponding phase .phi. is given as .phi.=2.pi.N. The
quantity N, hereafter referred to as the "fringe number," is given
by
N = ( a .lamda. ) ( x R ) , ##EQU00001##
where a is the source separation distance described above, .lamda.
is the optical wavelength of the sources 14A and 14B, x is the
physical distance along the x-axis between the points P and
P.sub.o, and R is the physical distance between the projector 14
and the point P.sub.o as measured along the central axis 46 of the
projector 14 from the midpoint of the sources 14A and 14B and the
point P.sub.o. The equation above is generally valid provided that
the source separation distance a and the distance x are relatively
small compared to the distance R. Since the parameters a and
.lamda. are known a priori, observing the phase .phi. in terms of
the fringe number N from the acquired images of the fringe pattern
50 provides information relating to the parameters x and R which
relate to the angle .DELTA..theta. and allow a determination of the
distance z.sub.P by triangulation.
[0032] The accuracy with which the distance z.sub.P can be
determined is related in part to the spatial frequency of the
fringe pattern 50. Profile changes in the surface contour 34 along
the z direction correspond to changes in observed intensity of the
fringe pattern 50. The resolution of the surface contour
measurement increases as the change in intensity for a given change
in distance along the z direction increases. Stated differently, as
the slope of the individual fringes of the fringe pattern 50
increases, more accurate measurements of the distance z.sub.P can
be made. This situation corresponds to "finer" fringes or,
equivalently, an increased spatial frequency of the fringe pattern
50 on the object surface 34.
[0033] Although the interferometric fringe pattern 50 has an
infinite field of focus relative to the projector 14, images of the
projected fringe pattern 50 are limited by practical constraints.
For example, imaging a fine fringe pattern over a large range in
the z direction requires an imaging system having a large depth of
focus and high spatial resolution. These requirements are generally
difficult to achieve simultaneously and a particular application
typically requires a tradeoff between the two parameters.
Accordingly, in some projector implementations, the grating pitch d
and the source separation distance a for a given single channel
interferometer are selected based, at least in part, on a
compromise between high measurement resolution and camera depth of
field.
[0034] FIG. 3 depicts an image 62 of the object surface acquired by
the digital camera. Image data is obtained using a two-dimensional
matrix of pixels in the camera image plane. Each pixel is
associated with an intensity value I based on the light scattered
from the object surface and incident on the pixel. One such pixel
66 is shown as containing the point of interest P. The image 62
includes the alternating bright and dark lines of the fringe
pattern in which the fringes are indicated with shading and are
marked for reference with the designations N.sub.+3 to N.sub.-3
moving from left to right in the figure. It should be appreciated
that the intensity values of pixels in a row of the image 62 reveal
the continuous wave nature of the fringe pattern 50 shown in FIG.
2.
[0035] By analyzing the image 62 using the intensity I detected at
the pixel 66 to determine the phase .phi. at the point P, the
distance z.sub.P to the point P can be calculated. Additionally,
the x and y coordinates of the point P can be determined from
knowledge of the geometry and optical properties of the camera.
Performing the analysis and three-dimensional coordinate
calculation for each pixel in the image 62 allows a detailed
profile, or map, of the object surface to be determined.
[0036] The phase .phi. is calculated by (i) determining the
"fractional phase" .phi..sub.frac of the intensity detected for the
point P relative to its nearest maximum intensity peak, that is,
the center of the fringe N.sub.-1 and (ii) identifying in the image
62 the location of the zero degree reference phase corresponding to
the intensity peak 54 in FIG. 2 so as to determine the portion of
the total phase .phi. that corresponds to an integer number of
fringe peaks. A known phase shift technique can be utilized to
determine the fractional phase .phi..sub.frac. Referring again to
FIG. 2, the projector 14 is configured so that the fringe pattern
50 can be phase-shifted by two or more known values relative to its
nominal position. For example, the projector 14 can be configured
to shift the fringe pattern 50 by shifting the diffraction grating
so that the maximum intensity peak 54 of the center fringe N.sub.o
is shifted either to the right or left of the point P.sub.o by a
known amount (e.g., .+-.120.degree., or .+-.2/3.pi.).
[0037] Referring again to FIG. 3, the fractional phase
.phi..sub.frac can be unambiguously calculated if an image of the
fringe pattern 50 is acquired at each of three different known
phase shifts (e.g., +120.degree., 0.degree. and -120.degree.). For
each image 62, the intensity I detected by the pixel 66 is
determined (e.g., I.sub.+120, I.sub.0 and I.sub.-120). While phase
shifts of .+-.120' and the reference position of 0.degree. are used
in the present example, it should be appreciated that other numbers
of phase shifts and other values of phase shift can be utilized in
the technique.
[0038] The location of the zero degree reference phase in the image
62 is determined so that the portion of the total phase .phi. that
includes the integer number of fringes can be determined. Although
the zero degree reference phase is indicated in the image 62, the
corresponding center fringe N.sub.o is otherwise not readily
apparent as the fringes N.sub.+3 to N.sub.-3 appear identical. This
fringe similarity results in a "2.pi. ambiguity because there is no
analysis of a single image that yields the integer number of fringe
peaks from the 0.degree. phase fringe and the point P. Resolving
the 2.pi. ambiguity to determine the integer number of peaks
contributing to the phase .phi. is referred to in the art as
"fringe unwrapping."
[0039] Fringe unwrapping can be described with reference to FIG. 4
in which an image 62' of a different fringe pattern projected onto
the object surface is acquired. In this instance the projector
irradiates the object surface with a coarse (i.e., lower spatial
frequency) fringe pattern. The coarse fringe pattern includes only
one fringe within the camera field of view, namely the central
fringe N.sub.o, wherein the fringe pattern is calibrated to have
the same zero degree reference phase as the fringe pattern 50 shown
in FIG. 2 and FIG. 3. Using only the single central fringe N.sub.o,
the phase .phi. corresponding to the point of interest P can be
determined directly using the phase shift technique discussed
above. Ambiguity is avoided because the phase .phi. variation
across the image 62' is less than 2.pi.. It should be appreciated,
however, that the smaller slope of the coarse fringe pattern
results in a lower resolution and accuracy for the determination of
the phase .phi. and the distance z.sub.P. Thus the accuracy of the
calculated distance z.sub.P can be unacceptable for many
applications.
[0040] In view of the above limitations, an iterative technique can
be employed in which the projector is configured to alternately
project coarse and fine fringe patterns onto the object surface
wherein the 0.degree. reference phases for the coarse and fine
fringe patterns are precisely calibrated to have a known
relationship. For each projection of a coarse or fine fringe
pattern, three or more images are acquired to implement the phase
shift technique described above. In one example utilizing only
three phase positions, a coarse fringe pattern is projected,
imaged, phase shifted once, imaged again, phase shifted a second
time, and imaged again to acquire a total of three images. The same
three image procedure is implemented for the fine fringe
pattern.
[0041] Based on the acquisition of the three fine fringe images, a
higher resolution determination of the fractional phase
.phi..sub.frac can be determined. Using a known phase relationship
between the coarse and fine fringe patterns, a higher resolution
determination of the phase .phi. (a "fine .phi.") can be determined
using the coarse .phi. and the fractional phase .phi..sub.frac. In
this manner, the coarse phase .phi. is used to "unwrap" the fringe
number for the fractional phase .phi..sub.frac to permit
determination of the fine phase .phi.. Finally, a higher resolution
determination of the distance z.sub.P is calculated based on the
fine phase .phi.. The foregoing unwrapping procedure is performed
for each pixel to determine respective x, y and z coordinates and
to generate a comprehensive higher resolution profile, or map, of
the surface contour.
[0042] While in some instances the iterative unwrapping technique
permits the phase .phi. to be determined with sufficient
resolution, generally it is difficult to generate coarse fringe
patterns in which only one central fringe is projected in the field
of view. To generate coarse fringe patterns using an interferometer
employing a diffraction grating, a large grating pitch d is
required. However, as the grating pitch d is increased, the grating
generally becomes less efficient, that is, less optical power is
diffracted into the non-zero order modes. Consequently, the
available laser power can limit the grating pitch d if sufficient
laser power is not otherwise present in the first order modes to
generate fringe pattern intensities sufficient for imaging.
[0043] One conventional technique for simulating a coarse fringe
pattern employs two less coarse (i.e., higher spatial frequency)
diffraction gratings having slightly different grating pitches d.
Consequently, the fringe patterns generated by such gratings have
similar but not identical spatial frequencies. Images of the two
fringe patterns are analyzed to determine a beat frequency between
the similar spatial frequencies to synthesize a significantly
coarser fringe pattern for the determination of the phase .phi.
without any 2.pi. ambiguity. Although the synthesized coarse fringe
pattern requires an additional diffraction grating, the optical
power efficiency limitation typically associated with a single
coarse diffraction grating is eliminated.
[0044] In some implementations of the above iterative measurement
technique, the diffraction gratings are sequentially moved into the
path of a laser beam to generate the different fringe patterns. In
particular, each diffraction grating generates a set of diffracted
beams which are focused by an objective and spatially filtered to
isolate the pair of first order beams to generate the fringe
pattern. The distance between the apertures of the spatial filter
and the size of the apertures are selected to accommodate the
different source separation distances (e.g., a.sub.fine and
a.sub.coarse) between the focused first order beams. Thus, a single
channel interferometer projector is implemented in which different
diffraction gratings are multiplexed into the beam path to generate
different fringe patterns.
[0045] One aspect of the present invention is directed to a
multiple channel interferometer projector that can be employed as a
replacement for the projector 14 in the measurement system 10
depicted in FIG. 1 and FIG. 2. The multiple channel interferometer
projector includes two or more interferometer channels that are
spatially separated from each other. The multiple channel
interferometer projector achieves a significant improvement in
system performance and measurement accuracy in comparison to the
single channel interferometer projectors discussed above.
[0046] Advantageously, as an optical component set is provided for
each of the interferometer channels, the need to alternately move
different diffraction gratings into the coherent beam path is
eliminated. Instead, the only grating movement is a significantly
smaller grating displacement for each diffraction grating relative
to a reference phase position to implement the phase shifting
technique described above. The major reduction in the grating
displacement yields a significant decrease in the mechanical
complexity, weight and size of the projector, an improvement in
overall projector stability, and a significant increase in
measurement speed and accuracy. Another advantage is the higher
system reliability provided by the projector. If one of the
interferometer channels fails to operate properly, surface contour
measurements may still be possible using the other channels.
Moreover, in some instances, if data obtained using one of the
channels is inconsistent with data obtained from other channels,
the suspect data can be ignored and the user can be alerted to the
discrepancy.
[0047] The multiple, spatially diverse interferometer channels
provide additional advantages. Averaging the measurements obtained
from multiple channels results in a decrease in the effect of
optical noise on the calculated surface contour data. For example,
measurement error introduced by photon shot noise and speckle
decreases by an amount proportional to the square root of the
number of channels averaged. In particular, speckle error is
significantly reduced by analyzing multiple images having
uncorrelated speckle due to the spatial separation and associated
angle diversity between the two fine interferometer channels. As a
result, the z coordinates of an imaged surface contour can be
determined with high accuracy over a large field of view. In
various embodiments described below, the z coordinates can be
determined with an accuracy of better than 10 micrometers over a
0.5 meter field of view.
[0048] A further advantage of multiple channels is the reduction of
the effects of projector optical aberrations on measurement
accuracy. Each interferometer channel exhibits inaccuracies due to
specific optical aberrations caused by the optical components in
the channel. However, multiple channel measurements are averaged
together to generate a final measurement result and the effect of
individual channel aberration errors are decreased due to the
uncorrelated nature of the aberrations between channels.
[0049] Based at least in part on the noise reduction advantages
provided by a multiple channel interferometer projector according
to various embodiments of the present invention, the spatial
frequency of the fine fringe patterns can be reduced in many
applications while maintaining sufficient measurement resolution.
As previously described, utilizing fine fringe patterns of high
spatial frequency for increased measurement resolution can limit
the practical imaging depth of field of the camera, therefore there
can be a compromise required between measurement resolution and
imaging depth of field. The substantial noise reduction in the
measurement process achieved with the multiple channel
interferometer of the present invention can, in many instances,
provide a significant improvement in signal-to-noise ratio to allow
a reduction in the spatial frequency of the fine fringe patterns
without sacrificing a desired measurement resolution or a greater
imaging distance between the camera and the object surface.
[0050] FIG. 5A and FIG. 5B illustrate front and rear perspective
views, respectively, of a multiple channel interferometer projector
70 constructed in accordance with the present invention. In the
illustrated embodiment, the projector 70 has an approximately
cylindrical form, a length of approximately 140 millimeters, a
diameter of approximately 65 millimeters and a weight of less than
1 kilogram. The projector 70 includes four interferometer channels
although in other embodiments different numbers of channels are
employed. The projector 70 includes a front end cap 74 having four
passageways 78A, 78B, 78C and 78D (generally 78) through which pass
the optical radiation from the interferometer channels. A rear end
cap 82 includes four optical couplers 86A, 86B, 86C and 86D
(generally 86) to provide for coupling an optical light source to
each of the four interferometer channels.
[0051] The interferometer channels can be sourced by a single laser
beam that is multiplexed in space and optionally in time. In one
exemplary implementation, a single laser source is separated into
four optical signals and each optical signal is coupled to a
respective one of the four optical couplers 86, for example, by an
optical fiber. Alternatively, one or more interferometer channels
are sourced by a dedicated laser source such that at least two
laser sources are utilized with the projector 70. According to one
implementation, a laser source used for the multiple channel
interferometer projector 70 of the present disclosure has a
wavelength of 658 nanometers and generates approximately 50 to 100
milliwatts of optical power.
[0052] In another embodiment, different interferometer channels of
the projector 70 utilize different wavelengths of radiation. The
fringe patterns generated by two or more spatially-separated
different-wavelength interferometer channels are projected
simultaneously onto an object surface, imaged simultaneously by a
camera, and processed separately according to the wavelengths. For
example, image data generated at the camera can be separated
according to color and processed independently. Alternatively,
fringe patterns projected by two or more spatially-separated
different-wavelength channels can be temporally multiplexed and
synchronized with data acquisition by the camera for separate
processing.
[0053] FIG. 6A and FIG. 6B illustrate front and rear exploded
perspective views, respectively, of the projector 70 shown in FIG.
5A and FIG. 5B. According to the illustrated embodiment, a number
of separate modules are mechanically coupled to form the projector
70. Each module includes one or more components for each
interferometer channel so that a total of four component sets are
employed per module. Each component set of a given module comprises
a particular functional portion of a respective one of the four
interferometer channels.
[0054] More specifically, the projector 70 includes a collimator
lens module 90 including four fiber couplers 94A, 94B, 94C and 94D
and four collimator lenses 98A, 98B, 98C and 98D (generally 98).
Each coupler-lens pair receives laser light in a respective
interferometer channel. The laser light is supplied to the
projector 70 through optical fibers secured by the couplers 86 of
the rear end cap 82. The collimator lens module 90 is configured
such that an end face of the optical fiber for each channel is
aligned to a corresponding collimator lens 98.
[0055] The projector 70 also includes a diffraction grating module
102 that includes four diffraction gratings. In one embodiment, the
diffraction gratings are integral to a common substrate disposed
over four clear apertures 108A, 108B, 108C and 108D so that each
grating is irradiated with collimated laser light from a
corresponding lens 98 in the collimator lens module 90. In another
embodiment, the four diffraction gratings, 110B, 110C and 110 D
(generally 110) are individually fabricated and then attached to a
common platform 106 as shown in FIG. 7. In one exemplary
implementation, the module 102 has an approximate diameter D of 67
millimeters and an approximate thickness T of 19 millimeters. The
common substrate 106 (or platform) can be laterally displaced along
a direction perpendicular to the central axis of the projector 70
such that the fringe patterns projected on an object surface are
translated along a direction parallel to the x-axis of the
coordinate system 30 (see FIG. 3). The diffraction grating module
102 can include a single axis translation stage coupled to the
common substrate 106 (or platform), one or more actuators (e.g.,
piezoelectric transducers) and one or more high accuracy position
sensors (e.g., capacitive probes) to provide closed-loop precision
shifting (i.e., "micro-positioning") for implementing the phase
shift technique described above.
[0056] Preferably only the first diffraction orders generated by
the diffraction gratings 110 are used to generate the two sources
of radiation used for each interferometer channel. In a preferred
embodiment, the diffraction gratings 110 are transmissive optical
components configured for high efficiency of the first diffraction
orders. For example, to achieve a near-optimum first order
efficiency, the diffraction gratings are configured as 50% duty
cycle binary phase gratings having a phase depth of one-half the
wavelength .lamda. of the laser light or, equivalently, the
physical depth .delta. of the grating thickness variation is
.lamda./2(n-1) where n is the index of refraction of the grating
substrate 106. Diffraction gratings fabricated with these
properties can transmit approximately 81% of the incident laser
light in the first order beams. In an exemplary embodiment, the
diffraction gratings 110 are fabricated on a fused silica substrate
having an index of refraction n of 1.457 at a wavelength .lamda. of
658 nanometers. Based on these parameters, the physical depth
.delta. of the gratings is 0.72 micrometers and the first order
beams include approximately 79% of the optical power in the
incident laser beam. In another embodiment, the diffraction
gratings 110 are fabricated on a fused silica substrate having a
thickness of approximately 2.5 millimeters. Four grating patterns
are written on a photomask covering the substrate using electron
beam lithography and ion etching is used to form the patterns in
the fused silica. Each diffraction grating 110 is rectangular with
dimensions of approximately 8 millimeters by 10 millimeters. Once
the diffraction gratings are formed, both sides of the grating
substrate are anti-reflection (AR) coated. For example, a
multi-layer dielectric AR coating can be applied to limit the
reflectivity per surface to be less than 0.5% at the operating
wavelength .lamda..
[0057] FIG. 8A and FIG. 8B illustrate detailed rear and front
views, respectively, of the diffraction grating module 102. The
grating substrate is not shown so that other components of the
module 102 can be viewed. A centrally located translation stage 150
provides movement along a single axis parallel to the bottom flat
edge of the module 102. The translation stage 150 includes small
threaded bores 174 for attachment of the grating substrate 106
using screws. Alternatively, any of a variety of adhesives can be
used to attach the grating substrate 106.
[0058] The movement of the stage 150 is facilitated by two
symmetrically disposed flexure assemblies 154. An electrically
controlled actuator 158 (e.g., a piezoelectric transducer) coupled
to the translation stage 150 provides precise linear motion by
exerting a force on the stage 150 that deflects the flexure
assemblies 154. A precision position sensor 162 such as a
capacitive probe is coupled to the stage 150 to detect motion.
[0059] In one embodiment, the actuator 158 and the position sensor
162 are part of a closed-loop servo control system for positioning
the stage 150. For example, the actuator 158 is controlled by
electrical signals transmitted over wires 164 and a feedback signal
from the position sensor 162 is transmitted over cable 166. A wire
170 is coupled to a reference voltage (i.e., ground, not shown) and
a ground plane of the module 102. In one exemplary implementation,
the translation stage 150 and control components are configured to
facilitate stage movement over a range of approximately 250
micrometers, with a positional resolution of approximately 30
nanometers. A translation stage assembly and associated control
components similar to those described above are available from
Dynamic Structures and Materials, LLC, of Franklin, Tenn.
[0060] In one embodiment, at least one of the actuator 158 and the
position sensor 162 are coupled to a processor (e.g., processor 30
in FIG. 1) through the wires 164 and cable 166), and the processor
can be configured to implement a servo control loop for precision
movement of the translation stage 150. Other control components can
be employed in addition to or in place of the processor to
facilitate control loop signal conditioning and amplification.
[0061] Referring again to FIG. 6A and FIG. 6B, the projector 70
also includes an objective and spatial filter module 114 to
optically process the laser light from the diffraction gratings.
The module 114 includes four projection lenses 118A, 118B, 118C and
118D (generally 118). Each lens 118 focuses the first order
diffracted beams from a respective diffraction grating. Preferably,
the focal length of each lens 118 if manufactured to a tolerance to
ensure that the focused beams are aligned with the apertures of a
respective spatial filter. The focal length can vary according to
implementation. In one exemplary implementation, each lens has an
effective focal length of 8 millimeters. The numerical aperture of
each projection lens determines how quickly the fringe pattern
expands as it exist the projector 70 and, therefore, can be
selected to accommodate the required standoff distance between the
projector 70 and the object surface. In one implementation the
numerical aperture is 0.45 and the angle of the first order beams
with respect to the channel axis is approximately .+-.0.2.degree..
Each projection lens 118 is adapted to operate over a finite
wavelength range about the wavelength .lamda. for a given channel
(e.g., 20 nanometers) to accommodate small changes in wavelength,
for example, due to changes in operating temperature. Preferably,
each projection lens 118 has an AR coating such as a multiple-layer
dielectric coating for improved optical transmission.
[0062] Each diffraction grating is positioned relative to a
respective lens 118 such that the plane of the diffraction grating
is imaged onto the object surface. Consequently, any error in the
pointing angle of the laser illumination of a grating does not
result in a significant change in the position of the N.sub.0
fringe location at the object plane.
[0063] The objective and spatial filter module 114 also includes
four spatial filters 122A, 122B, 122C and 122D (generally 122).
Each spatial filter 122 includes two spaced apart pinhole apertures
to pass the focused beams of the first diffracted orders. In one
exemplary implementation, each spatial filter 122 is constructed of
a thin fused silica substrate coated with an opaque metal layer
such as a chromium layer. The metal layer is etched using a
photolithographic technique to create two transparent pinholes. The
fused silica substrate has an AR coating on both sides for improved
transmission through the pinhole apertures.
[0064] The spatial filters 122 provide a significant advantage over
spatial filters employed in conventional interferometer projectors.
More specifically, conventional projectors utilize a spatial filter
having multiple pairs of pinhole apertures with each pair matched
to a particular diffraction grating. Thus only one pair of pinhole
apertures provides the desired beams. Other pairs of pinhole
apertures can pass coherent light which can create other less
intense fringe patterns having different spatial frequencies on the
object surface. As a consequence, the surface contour data is less
accurate and can exhibit harmonic variations.
[0065] The optical reflectivity of the surface of an object can
vary with position. As a result, an image of the object surface can
include significant variations in intensity independent of the
fringe intensity variations. Such intensity variations can prevent
observation of the fringe pattern in portions of the image or can
result in image saturation in other portions of the image.
[0066] In one embodiment of the present invention, optical
processing methods and devices compensate for intensity variations
in the fringe patterns due to varying surface reflectivity. In one
exemplary implementation, the compensation is based on modifying
the intensity profile of a fringe pattern (i.e., "intensity
shaping") at the projector. In one embodiment, the projector 70'
includes an intensity shaping module 124 disposed between the front
end cap 74 and the objective and spatial filter module 114 as shown
in FIG. 9. The intensity shaping module 124 includes four optical
processors 128A, 128B, 128C and 128D (generally 128). In a
preferred embodiment, the intensity shaping module 124 is
positioned in the collimated beams between the collimator lens
module 90 and the diffraction grating module 102. In another
embodiment, the intensity shaping module 124 includes a single
optical processor that performs optical processing for two or more
channels.
[0067] The optical processors 128 are configured to selectively
attenuate portions of one or more fringe patterns projected from
the focusing objective and spatial filter module 114. In one
exemplary implementation, one or more of the optical processors 128
are provided as high resolution liquid crystal displays (LCDs)
wherein the optical transmissivity of individual pixels of each LCD
are individually controlled, for example, by a processor such as
that depicted in FIG. 1. In this manner, intensity shaping of the
projected fringe pattern can be utilized to compensate for a
variety of complex surface reflectivities.
[0068] Although the embodiments described above relate primarily to
a projector for a multiple channel interferometric surface
measurement system, the invention also contemplates system
embodiments having a crossbar that provides improved structural,
thermal and load stability. FIG. 10 is a cross-sectional block
diagram depicting a crossbar 126 for rigidly coupling a camera 22
to a projector 14 according to an embodiment of the invention and
FIG. 11 shows a perspective view of an assembled crossbar 126. The
crossbar 126 is configured as a sleeve-like structure that includes
an outer tube 130 coupled to an inner tube 134 through two O-rings
138. The O-rings 138 are located along the length of the inner tube
134 at two positions determined to significantly reduce changes in
the orientation of the camera 22 relative to the projector 14 and
to significantly reduce changes in the translation of the camera 22
and the projector 14 relative to a mounting plate 142 secured to
the outer tube 130. The changes are due primarily to gravity and
typically occur when the crossbar 126 is reoriented. The body
weight of the inner tube 134 compensates for angular changes caused
by the two loads (i.e., the camera 22 and the projector 14). The
O-rings 138 reduce local stresses and distribute the weight of the
inner tube 134 over a significant surface area of the inner surface
of the outer tube 130. In one embodiment, the inner tube 134 is
kinematically constrained for axial and radial movement.
Preferably, the outer tube 130, inner tube 134 and mounting plate
142 are fabricated from identical materials or materials for which
the coefficients of thermal expansion are closely matched so that
thermal changes in the measurement environment do not significantly
affect measurement accuracy. In one embodiment, the outer tube 130,
inner tube 134 and mounting plate 142 are fabricated from
aluminum.
[0069] While the invention has been shown and described with
reference to specific embodiments, it should be understood by those
skilled in the art that various changes in form and detail may be
made therein without departing from the spirit and scope of the
invention.
* * * * *