CA2246374C - Apparatus and methods for surface contour measurement - Google Patents

Abstract

Apparatus and methods of measuring position information, typically the depth coordinate, of a point on the surface of an object. In one embodiment, the apparatus includes two sources of radiation (P1, P2) positioned to illuminate the point (Pa) on the surface of the object (10) with radiation from each of the sources. The radiation from each of the sources is coherent with respect to the radiation from the other source. A control system (32) changes the phase of the radiation from at least one of the sources relative to the phase of the radiation from the other source as measured at the point on the surface of the object. A detector (22) is positioned to receive radiation scattered by the point and a processor (28), in communication with the detector, calculates position information in response to the change in phase of the radiation from the source and the received radiation scattered by the point on the surface of the object.

Description

APPARATUS AND METHODS FOR SURFACE CONTOUR MEASUREMENT
Field of the Invention The invention relates to the field of surface measurement and, more specifically, to the field of non-contact surface measurement.
Bacl~~round of the Invention Dimensional metrology, the measurement of the size and shape of objects, is very important in today's manufacturing environment in which machines perform much of the fabrication and assembly of complex objects composed of many subassemblies.
The shape and size of each component in a complex assembly, such as an automobile, must be held to close tolerances to ensure that the components fit together properly.
Ideally such measurements of shape and size are accomplished without physical contact in order to save time in making the measurement. Many non-contact measurement methods make use of available machine vision systems. The measurement of surface contour information is an especially difficult problem in machine vision systems since depth information is often Iost or is difficult to interpret. To compensate for the loss of depth information and the difficulty in interpreting the information which is available, many machine vision systems utilize light to create moire patterns on the surface of the object in order to obtain contour information.
Interferometric methods have also been used when detailed measurements of the surface are needed. Although interferometric systems provide surface contour information, they axe sensitive to vibrations in both the object being measured and the WO 9712934! PCT/LTS97/01744

-2-source of illumination being used. The present invention is less sensitive to the vibration problem that has affected previous systems.
Summary of the Invention The invention relates to an apparatus for determining position information of a point on the surface of an object. In one embodiment, the apparatus includes two sources of radiation positioned to illuminate the point with the radiation from each of the sources.
The radiation from each of the sources is coherent with respect to the radiation from the other source. In one embodiment, the two sources of radiation are formed by splitting a coherent radiation beam, which may be accomplished with the use of a beam splitter or a fiber optic sputter.
A control system in communication with one or more of the sources changes the phase of the radiation from one of the sources relative to the phase of the radiation from the other source as measured at the point on the surface of the object. In one embodiment, in which the radiation source is a frequency tunable laser, the control system controls the tunable laser to emit coherent radiation of a changing frequency.
In another embodiment, in which one or more of the radiation sources is moveable, the phase of its radiation with respect to the phase of the radiation from the other source as measured at the point on the object, is varied by moving one or more of the sources.
A detector, in communication with a processor, is positioned to receive incident radiation scattered from the point on the surface of the object. In one embodiment, the -detector is an array of individual photodetector elements. In a further embodiment, the array of photodetector elements is a charge coupled device (CCD).

WO 97!29341 PCT/US97/01744

-3 -In a preferred embodiment, a focusing system is positioned between the detector and the surface of the object to focus the image of the surface of the object onto the image plane of the detector. A polarizing filter may be placed between the focusing system and the detector, with the optical axis of the polarizer aligned with the principal axis of polarization of the radiation emitted by the two sources, in order to remove any scattered light which has been depolarized and thus, which would degrade the signal to noise ratio of the system.
The processor, which may also be in communication with the controller, calculates position information in response to the relative change in phase of radiation from two sources as adjusted by the control system, which adjustment causes changes in intensity of the radiation scattered by the point on the surface of the object that is received by the detector. In one embodiment, the processor is a single processor operating on detector output signals associated with each of the photodetector elements of the detector array.
In another embodiment, the processor is a multiprocessor, with each photodetector 1 S element or some subset of elements of the photodetector array in communication with a respective processor or some subset of processors of the multiprocessor. In yet another embodiment utilizing a CCD array, a plurality of CCD elements is in communication with a respective processor of the multiprocessor. Use of the multiprocessor arrangements advantageously enhances signal processing speed.
The invention also relates to a method for determining position information of a ~ point on the surface of an object. The method includes the steps of providing two or more sources of radiation illuminating the point to be measured, changing the phase of the radiation from at least one of the sources relative to the phase of the radiation from the

-4-other source as measured at the point on the surface of the object, detecting radiation scattered by the point on the surface of the object, and calculating position information from the resulting changes in intensity of the radiation scattered by the point on the surface of the object. In one embodiment, the change in phase of radiation from one of the sources relative to the phase of radiation from the other source as measured at the point on the object is accomplished by using a frequency tunable source of radiation, such as a tunable laser. In another embodiment, in which one or more of the sources is moveable, the relative phase of radiation from the two sources is changed as measured at the point on the object by moving such source or sources.
In another embodiment, the apparatus for determining position information of a point on the surface of an object includes two sources of radiation positioned to illuminate the point on the object to be measured with the radiation from each of the sources. A
control system, in communication with at least one of the sources of radiation, changes the phase of the radiation from one source relative to the phase of radiation from the other source as measured at the point on the surface of the object. In one embodiment, the phase of radiation from one of the sources is changed relative to the phase of radiation of the other source as measured at the point on the object by adjusting a frequency tunable laser. In another embodiment, the phase of radiation from one of the sources is changed relative to the phase of radiation from the other source as measured at the point on the object by moving one or more moveable laser sources. A detector positioned at the point on the surface ofthe object to be measured receives the radiation illuminating the point.
In one embodiment, the detector is positioned on the end of a spring arm which undergoes movement in accordance with the surface contour of the object as the spring arm is moved

-5-over the surface of the object. A processor, in communication with the detector, calculates position information of the point on the surface of the object in response to the change in phase of the radiation from the source as adjusted by the control system and the radiation received at the point on the surface of the object.
The invention also relates to method for determining, on an object having a surface, three-dimensional position information of a point on said surface of said object, said method comprising the steps of providing two sources of radiation separated by a distance, said sources being coherent with respect to one another; illuminating said point on said surface of said object with said radiation from each of said sources; moving each of said sources relative to each other; detecting radiation scattered by said point on said surface of said object; and calculating position information in response to said movement of said sources and said detected radiation scattered by said point on said surface of said object.
The invention further relates to an apparatus for determining, on an object having a ~,urface, position information of a point on said surface of said object, said apparatus comprising two sources of radiation separated by a distance, said sources being coherent with respect to one another; a control system moving each of said sources relative to each other; a detector positioned to receive radiation scattered from said point on said surface of said object;
and a processor receiving signals from, said detector, said processor calculating position information in response to said movement of said sources and said received radiation scattered from said point on said surface of said object.
Further, the invention relates to a method for determining, on an object having a surface, position information of a point on said surface of said object, said method comprising the steps of providing two sources of radiation separated by a distance, said sources being coherent with respect to one another; providing a detector at said point on said surface;

- 5(a) -illuminating said point on said surface of said object with said radiation from each of said sources; moving each of said sources relative to each other; detecting said radiation at said point on said surface of said object; and calculating position information in response to said movement of said sources and said radiation detected at said point on said surface of said obj ect.
The invention also relates to an apparatus for determining, on an object having a surface, a depth coordinate of a point on said surface of said object, said apparatus comprising two sources of radiation separated by a distance, said sources being coherent with respect to one another; a control system moving each of said sources relative to each other; a detector positioned at said point on said surface of said object to receive radiation illuminating said point on said surface of said object; and a processor receiving signals from said detector, said processor calculating position information of said point on said surface of said object in response to said movement of said sources and said radiation received at said point on said surface of said object.
Brief Description of the Drawings This invention is pointed out with particularity in the appended claims. The above and further advantages of this invention may be better understood by referring to the following description taken in conjunction with the accompanying drawings, in which:
Fig. 1 is a block diagram of an embodiment of the invention for making surface contour measurements;
Fig. 2 is a block diagram of an embodiment of the two sources of radiation shown in Fig. 1 ;

- 5(b) -Fig. 2a is a block diagram of another embodiment of the two sources of radiation ~~hown in Fig. l;
Fig. 2b is a block diagram of yet another embodiment of the two sources of radiation ~,hown in Fig. 1;
Fig. 3 is a block diagram of an embodiment of apparatus for supporting the two sources of radiation of Fig. 1 at a fixed distance relative to one another;
Fig. 4 is another embodiment of the imaging system in Fi~_ l .

-6-Fig. 5 is a block diagram of an alternate embodiment of the invention for making surface contour measurements;
Fig. 6 is a flow diagram of an embodiment of the steps utilized by the processor of , Figs. 1 and S in making surface contour measurements;
Fig. ba is one embodiment of a portion of the flow diagram of Fig. 6;
Fig. 6b is another embodiment of a portion of the flow diagram of Fig. 6;
Fig. 6c is yet another embodiment of a portion of the flow diagram of Fig. 6.
Fig. 7 is a block diagram of one embodiment of a detector and processor arrangement for use with the systems of Figs. 1 and 5;
Fig. 7a is a block diagram of an alternate embodiment of a detector and processor arrangement including a multiprocessor for use with the systems of Figs. I and 5;
Fig. 7b is a block diagram of another alternate embodiment of a detector and processor arrangement for use in the systems of Figs. l and S;
Fig. 8 is a block diagram of another embodiment of the invention for making surface contour measurements;
Fig. 9 shows one embodiment of photodetector elements positioned on the surface of an object in accordance with the embodiment of Fig. 8; and Fig. 9a shows another embodiment of photodetector elements positioned on spring arms for use in the embodiment of Fig. 8.

_ '7 _ Description of the Preferred Embodiment . While describing the embodiment of the invention, reference will be made to "sources" and "sources of radiation." These terms are meant to refer to any source of radiation, including highly localized sources of radiation.
Referring to Fig. 1, and in brief overview, two sources of radiation P1 and PZ
are separated by a fixed distance D and have spatial coordinates of {xl,y,,zl) and (x2xya,zz), respectively. The radiation from each of the sources P1 and P2 is coherent with respect to the radiation from the other one of the sources. Each source, P1 and P2, directs its respective divergent beam of radiation 12 and 14 toward a point Po on the surface of an object 10. The distance from each respective source of radiation, P, and P2, to the point on the surface Po is indicated by Rl and R2, respectively. W is the angle between the line extending from the origin to the point Po and the line extending between sources P, and Pz, 8$ is the angle between the z axis and the line extending between the sources PI and P2, and cc is the half angle subtended by the source points as viewed from Po.
Each beam 12, ' 14 is substantially polarized in the same direction as the other beam 14, 12 and may be independently scannable to simultaneously illuminate different regions on the object 10.
Alternatively, the entire object 10 may be illuminated simultaneously.
Light scattered 20 by the point Po is detected by a photodetector 22. In one embodiment, the photodetector 22 comprises an array of photodetector elements providing a two dimensional image of the object 10 to be measured. In a further embodiment, the array of photodetector elements is a charge coupled device (CCD). The _g_ detector 22 provides an output signal 26 comprising one or more individual signals, each one associated with a corresponding one of the photodetector elements of the detector 22.
In a preferred embodiment, a focusing element 24 is positioned between the point Po on the surface of the object 10, and the photodetector 22, so as to image the illuminated portion of the object including point Po onto the detector 22.
Because of the roughness of the surface of the object, and because the illuminating radiation is coherent, the focused image will be speckled. The output signal 26 from the photodetector 22 is the input signal to a processor unit 28.
A polarizer 30, in one embodiment, is placed between the focusing element 24 and I O the detector 22. Polarizer 30 is oriented in a direction to maximize its coincidence with the principal polarization component of the scattered light 20, so as to improve the speckle contrast. With this arrangement, the signal-to-noise ratio associated with light scattered from the surface of the object 10 is maximized.
In one embodiment, the processor 28 is a single processor which operates on 15 detector output signals 26 associated with each of the photodetector elements of the detector array 22. In another embodiment, the processor 28 is a multiprocessor having a plurality of individual processors and each photodetector element provides an input signal to a respective one of the processors. In yet another embodiment, in which the detector 22 is a CCD array, a plurality of the CCD elements provide an input signal to a respective 20 processor of a multiprocessor. With the multiprocessor arrangements, computations on signals from a plurality of individual photoelements occur substantially simultaneously, thereby enhancing the signal processing speed.

WO 9712934! PCTlUS97/01744 _9_ A control unit 32 controls the operation of the sources of radiation, PI and P2, so as to change the phase of the radiation of one of the sources relative to the phase of the radiation from the other source as measured at the point Pa on the surface of the object 10.
The processor 28 may be in communication with control unit 32 via signal line, or bus 34.
For example, in certain applications it may be desirable for the processor 28 to process signals from the detector 22 at specific times relative to the scanning of the sources P1 and P2 over the surface of the object 10 or relative to the rate at which the frequency of the radiation from the sources is swept. Since such scanning and frequency sweeping operations are controlled by control unit 32, communication between the control unit 32 and the processor 28 is desirable in these circumstances. It will be appreciated that the control unit 32 and the processor 28 may be physically separate units or, alternatively, may be implemented by a single processing system.
Referring now to Fig. 2, in one embodiment the sources of radiation P, and Pz are formed from the radiation emitted from a tunable laser 40. The radiation beam 44 emitted I5 by the tunable laser 40 is split by a beam splitter 48. The radiation beam 50 reflected by the beam sputter 48 is caused to diverge by a lens 52. The divergent beam is then reflected by a moveable aiming mirror 54. The radiation beam reflected by the aiming mirror 54 provides one of the sources of coherent radiation, P I . Similarly, the radiation beam 46 passing through the beam splitter 48 is caused to diverge by a lens 58 which directs the divergent beam to a second moveable aiming mirror 60. The radiation beam reflected by mirror 60 provides the second source of radiation, P2. Aiming mirrors 54 and 62 may be pivotable to selectively illuminate the surface of object 10. They may also be moveable to vary the positions of sources P, and P2.

-IO-Referring to Fig. 2a, another embodiment of the sources of radiation P 1 and P2 is shown to include a tunable Iaser source 40 providing a beam of radiation 44.
The radiation beam 44 passes through a lens 62 which causes the beam to diverge, providing divergent beam 64. Divergent beam 64 is then reflected by beam sputter 48 to provide a first beam 66. A second beam 68 passes through the beam sputter 48, as shown.
Moveable aiming mirrors 54 and 60 reflect beams 66 and 68 to provide sources P1 and P2, respectively.
Referring to Fig. 2b, in another embodiment the sources of radiation P, and P2 are split from the radiation emitted from a tunable laser 40 using a fiber optic sputter 56.
Fibers may have beam-forming elements at their end to control or set the divergence angle of the two beams, and in one embodiment the beam-forming elements may be lenses.
Sources P ~ and P2 may alternatively be formed from a pair of tunable lasers which are frequency locked together. Other suitable embodiments of radiation sources include any sources which generate a wave having a controllable phase, such as microwaves and sonic waves.
In one embodiment, the sources of radiation P1 and P2 are maintained at a fixed distance D from one another by attaching each source to one end of a bar comprised of a material having a small coefficient of expansion. In another embodiment, the sources of radiation Pl and Pa are not held at a fixed distance but instead the distance between them, D, is known to a high degree of accuracy.
One illustrative bar 70 for supporting radiation sources Pi and P2 at a fixed distance D relative to one another is shown in Fig. 3. A bar 70 is provided with sockets 74 at opposite ends thereof. A ball joint 76 is pivotally positioned within each of the sockets 74, as shown. Each of the ball joints 76 has an end of a fiber from a fiber optic sputter 56 (shown in Fig. 2b) positioned therein and an aperture 80 through which divergent radiation passes. Fibers may have beam-forming elements at their end to control or set the divergence angle of the two beams and in one embodiment the beam forming elements are lenses. In operation, the ball joints 76 are pivotable as shown by arrows 78 within the respective socket 74 and may be under the control of control unit 32 (shown in Fig. I). With this arrangement, the divergent beams 12 and 14 provided by the sources PI
and P~ at the ends of the fibers can be directed as desired to illuminate all, or a portion, of the object 10 including the point Po to be processed, while maintaining a fixed separation distance D.
The coordinates of point Po on the surface of object I O are (xxy,z). Although the x and y coordinates of Pa are generally directly determinable from the geometry of the detector 22 and the object 10, taking into account any magnification by intervening focusing element 24, the depth coordinate z, where the z axis is defined as being parallel to the optical axis of the imaging system, is not directly obtainable. The depth coordinate, z, however can be measured by first considering the difference in path length s-R2-Rl~ S~ (1) from the radiation sources P1 and P2 to the point Po on the surface of the object I0. The quantity Sa is included to account for any path length difference in the beams that may occur before they reach points P1 and P2.

WO 97/29341 PCTlUS97/OI744 If s is non-zero, then changing the frequency of the radiation emitted from sources Pt and P2 will result in the phase of the radiation from one source, as measured at point Po, changing with respect to the other source. This phase change results in a modulation of intensity of the radiation at point Po. The change in frequency, Ov, required to complete one cycle of a change in intensity is given by the expression:
dv = -c (2) S
where c is the speed of light. Thus, by measuring the change in laser frequency, w, needed to cause one oscillation of intensity, the path difference s may be determined. The measurement of z is then based on determining the value of s for each value of x and y, as discussed below.
Improved accuracy in the determination of s is obtained by measuring dv over many oscillation cycles. In practice it is convenient to work in terms of the number of oscillation cycles N (not necessarily a whole number) induced by a total change in frequency B.
I 5 N is given in terms of w and B as N _ Q~
_ _B (3 ) Elimination of w from Eq. (3) using Eq. (2) yields the following expression for s in terms of N: ' s __ c N {4) B

WO 97!29341 PCT/LTS97/01744 An uncertainty 01V in the measurement of N, corresponds to an uncertainty 0s in s of - s~
Equation (5) indicates that if the uncertainty ~1V to which a single oscillation cycle can be determined remains constant, the uncertainty ds in s is reduced by a factor equal to the number of cycles N that are measured. There are numerous methods for determining N to various levels of resolution ~N that are known to those skilled in the art. Examples of methods yielding a resolution of roughly one oscillation-cycle count (~N=1) are to perform a fast Fourier transform (F'FT) on the data sequence or to count zero crossings of the high-pass filtered signal. Improved resolution of a fraction of an oscillation-cycle count (dN< 1 ) can be achieved, for example, by finding the argument of the discrete Fourier transform (DFT) where the magnitude of the DFT is maximized or by inspecting the phase of the oscillation cycle at the ends of the frequency scan. One technique known to those skilled in the art for accurate inspection of the phase is to insert a phase modulator in one leg of the beam path, i.e., between the beam sputter or fiber-optic splitter and one of the sources PI or P2 in Figures 2, 2(a), and 2(b).
If h, I2, and I3 are signal intensities corresponding to phase shifts induced by the phase modulator of -90°, 0°, and 90°, respectively, then the phase ~ of the oscillation cycle is given by:
= tan 2~~ - I1 - ~' 6 () For a typical frequency scan of B=lSTHz for a tunable diode laser, and for an uncertainty of ~N = 1 cycle, an uncertainty of ~l.r = 20 ~,m is provided. An uncertainty of ON = 0.1 cycle would improve the uncertainty in s to ~.s = 2.0 ~,m, provided that the spread in s over the lateral resolution is smaller than this quantity. If the spread in s over the lateral resolution on the surface of the object is larger than ds, then the improved resolution in the measurement of s may still result in an improved estimate of an average or representative value of s over that lateral resolution.
In terms of the coordinate system:
s= ~x-x2)2 +(Y-Y2)2 +(z-z2) - (x-x~)2 +~Y-Y~)2 +(z-zy2 To make the calculation simpler, assume that the two sources PI and PZ are located symmetrically about the origin at {xlzyi,zi) and (-xl, yt,-zi). Then Eq. (7) becomes, in terms of (xl,yl,zl):
s-'V1x+x1)2 +(y+Y~)z +tz+zl)2 -.~~x-xi)2 +~y-Yi)2 +(z-ziJ . {8) Solving for z, Eq. {8) becomes:
4{W +YYi )z~ ~ 2 16(W ~' YYi )2 + (s2 - 4zi )(s2 - D2 - 4x2 - 4y2 ) IS z = (9) s2 - 4zi where D is the distance between the two sources Pi and Pa. Thus z is determined to within an ambiguity due to the existence of the positive and negative roots of Eq. (9).
Qne way to avoid this ambiguity is by illuminating the object 10 so that the s=0 Iine WO 97129341 PCTlUS97/OI744 (labeled 16 in Fig. 1 for the case so=0) does not bisect the region of the object to be imaged. One way of moving the s=0 Iine is to vary so in Eq. (1).
y The sensitivity of the system to changes in s is shown by the ratio of dsldz, where dz is the uncertainty in z introduced by an uncertainty Ds in the value of s.
This ratio ranges between zero, for a system lacking any practical range sensitivity and two, for a theoretically maximal system. A value of two is impractical to achieve because the surface of the object 10 would have to Iie between the two point sources P1 and P~ and only one side of the surface could be illuminated from each beam. The ratio ~s/dz is calculated by taking the partial derivative of s with respect to z, from which the following expression for the range resolution is obtained:
4z=ds z+zi z-zl 1 10 DZ f Da Ro+RoDcosyr+ 4 Ro-RoDcosyr+
In equation (10), Ro is the distance from the origin to Po and ~ is the angle between the Line extending from the origin to the point Po and the line extending from point PI to point Pi as shown in Fig. 1. A useful configuration that provides good range resolution is to set ~=~90° for which the expression for ~z simplifies to __ 4s ~ 2sinacos8s _(11) WO 97!29341 PCTlUS97l01744 where 8$ and a. are as shown in Fig. 1. In terms of Ro and D, tanoc=D/(2Ro).
Equation (I 1) shows that the range resolution improves as the angle a. increases and the angle 8t decreases. For values of ~S~ixn, a=IO°, and 6s=45°, the range resolution is ~z=20p,m.
Uncertainties (~lx, ~y) in the lateral position {x, y) of the observation point P also affect the range resolution ~z. If the two source points lie in the x-z plane, then the measurement of z is insensitive to uncertainties ~y. For ~=90°, uncertainties L1x in x cause an uncertainty 4z = dx tan 8s ( 12) in the measurement of z. Therefore, angles near 6s 0° offer the best immunity to uncertainty in the lateral position of point Po.
Because the depth of focus decreases as the lateral resolution of the optical system improves, there is a tradeoffbetween lateral resolution and maximum object depth. One method for reducing this limitation in object depth is to sequentially focus on different range planes and use only those pixels that are within the depth of focus. For example, a 100p.m lateral resolution would limit the depth of field to the order of 1 cm, and an object with a 10 cm range could be imaged at full resolution by focusing sequentially at ten different ranges. To minimize the effects of depth of field, the z axis can be defined in a direction that minimizes the range extent, i.e., normal to the average plane of the surface of the object. To increase the lateral area that can be imaged without losing lateral resolution, multiple cameras {i.e., detector arrays 22) can be used to cover the entire area of interest of the object 10 or individual cameras can be used for inspecting regions of interest. Alternatively, the focal plane of single lenses can be populated with a plurality of detector arrays. These arrays can be translated independently to inspect various regions of the object at high resolution. Translation of individual detector arrays along the z axis or tilting of the detector arrays can achieve simultaneous focusing for regions of the object at different depths to increase the allowable object depth.
A potential di~cuIty with the optical imaging system in Fig. 1 is that the bistatic angle between the sources and the detector may introduce shadowing effects.
These effects can be reduced by placing the Iens closer to the sources as in Fig. 4 and using the lens in an off axis configuration where the detector is offset laterally in the image plane. If the lens is designed for this purpose or has a sufficiently large field of view, then aberrations resulting from ofd axis imaging can be minimized.
Refernng to Fig. 5, an alternate embodiment of the present invention includes a moveable radiation source P1 and a stationary radiation source Pa, each providing a divergent beam 150 and 154 and having a path length labeled R~ and R2 between such radiation source and a point Pa on the surface of an object 10, respectively.
The sources P1 and PZ may be generated by any suitable source of coherent radiation, such as a monochromatic laser, which is split to provide the two point sources P1 and P2.
Moreover, various techniques are suitable for splitting the radiation from the coherent radiation source, such as the beam splitter embodiments of Figs. 2 and 2a and the fiber optic splitter embodiment of Fig. 2b.
The divergent beams 150 and I54 are directed toward a surface of an object 10 on which a point Pe is located having position information which is to be measured.
Illumination scattered by the surface of the object 10 is focused by a focusing element, or WO 97!29341 PCT/LTS97/OI744 lens 158 to impinge on a detector array 22. The lens can be used in an off-axis configuration as illustrated in Fig. 4 to reduce shadowing effects due to the bistatic angle.
An optional poIarizer (not shown) of the type described above in conjunction with Fig. 1 may be positioned between the focusing element 158 and the detector array 22 in order to improve the contrast of the speckle image incident on the detector array 22.
The detector array 22 is in communication with a processor unit 28 for processing the image incident on the detector, as will be described. A control unit 32 is in communication with at least the moveable source P 1 for moving the source P, along an axis 160. As noted above, the control unit 32 and the processor unit 28 may be implemented by separate devices or alternatively, may be part of a single system.
Additionally, the control unit 32 and the processor unit 28 may communicate with each other, as may be desirable in certain applications.
As described above in conjunction with Fig. 1, the depth coordinate z associated with a point Po on the surface of the object 10 can be determined as a function of the difference, RZ-R~, between the path lengths Rl and Ra of beams 150 and 154, from sources P1 and PZ respectively, to point Po. In the embodiment of Fig. 5, the phase of the radiation from moveable source PI is changed by moving the source P, along the axis 160 under the control of the control unit 32. With this arrangement, oscillations in the intensity at point Po are produced.
The instantaneous coordinates of moveable point source P1 are x~ = a ~ s~ Yt = ~2g~ and z! = ans ( 13) WO 97!29341 PCT/US97/OI744 where a represents the magnitude of translation of point source P1, and 2 8, nzS, and nB are direction cosines representing the direction of translation with respect to the x, y and z axes, respectively. The phase difference of the radiation from the sources P1 and P2 as measured after propagation to point Po is given by:
~ _ ~ (RZ - R~) + ~ o ( 14) where ~o represents a constant phase offset that may exist between the two coherent sources P, and P2. As P1 translates along axis 160, the value of Ri changes, causing ~ to vary as a function of a.
The number ofintensity oscillations that occur at point Po as point source P1 I O moves away from the origin is given by:
__ ~{a) - ~(Q) __ Ro. - Ri __ _1 a z z 2 - z z IV 2~ ~ ~~ x +y +z - (x-a2s) +(y-ams) +(z-an~.) ~ (15) where Ro is the distance between point Pa and the origin of the coordinate system, ~ (a) is the angle of translation measured at a, and ~(0) is the angle of translation measured at 0.
Consideration of Eq. (I S) reveals that the number of intensity oscillations, N, resulting from movement from source P1 is independent of the location of the stationary source P2.
This independence permits the sources P, and PZ to be positioned in close proximity to one another. With this arrangement, the divergent beams 150 and 154 from respective sources Py and P2 experience common disturbances, such as air turbulence and vibrations.
In this way, the effects of such disturbances are minimized. Additionally, beams 150 and 154 reach the surface of the object 10 with substantially identical polarization.

WO 97!29341 PCT/US97/01744 Since the magnitude of translation a of point source Pi is relatively small as compared to the values ofx, y and z, Eq. (15) can be approximated to second order in alRo as follows:

N = ~ Ccos yr - ~ sin2 yr ~ _ ~ cos yr sin2 r~ ~2 ( 16) where W is the angle between the line extending from the origin to the point Po and the line defined by the direction of translation of P1.
Eq. (16) indicates that to lowest order in a/R-0, knowledge ofN allows the angle ~r to be determined. Given knowledge of yr from three or more locations, the (x,y,z) coordinates of Po could be determined through triangulation. We now describe an embodiment similar to the one corresponding to Fig. 1, where the x and y coordinates are determined from location of the image point in the detector array.
The measurement of z for a given (xy) location can be made either by counting the number of intensity oscillation cycles N that occur as P1 moves over a distance a or by measuring the rate at which such intensity oscillations occur. Consider first a 1 S measurement of z based on counting the number of cycles N. With N known, all of the variables in Eq. (15} are known except for z. Solving Eq. (1 S) for z yields the following expression:
fl~t.q t p A'' =(PZ Zrzs )(x2 +y2) I7 ( ) P _ ns where A=x.~s+ynzs+~(pz-1) (l g) and p = a (19) Equation (I9) defines a dimensionless parameter having a magnitude ranging between S zero and unity that represents the average modulation rate of the speckle intensity in terms of oscillation cycles N per wavelength unit traveled by PI. For values of a approaching zero, Eq. (I7) can be approximated as:
z - (x~s + yms)ns ~ p (x2s + ym,.)z -(pz -ns ~~xz +yzl (20) Pz -nz s The expressions for z in Eqs. 17 and 20 can be simplified by setting ns = 0, so that I0 the translation of source P1 is confined to the x y plane. This arrangement represents a good practical choice for translation of source P1, as described below. The resulting expression for z can be written as follows:
~z -xz -yz (21) where the distance Ro from the scattering point Po to the origin of the x, y coordinate - 15 system is given by the exact expression:
x~s+yms+~tp'--1) (22) P

WO 97/29341 PCTlLTS97/O1?44 When a is small, RQ can be approximated as:
~ y x2s + yms 23 ( ) Consider now the measurement of z based on knowledge of the instantaneous rate at which the intensity oscillations occur. The instantaneous oscillation rate p can be S expressed in a manner similar to the average oscillation rate in Eq. (19}, as follows:
(24) Substituting the expression for the number of intensity oscillations, N, from Eq. (15) into Eq. (24) yields:
p - xPs + ynZs + zn,. - cr - 25 {x-cr~s~2 +(y-ams}a +~z-ans?z where the relation:
~a + msa = ;isa = 1 (26) has been used to simplify the numerator. For small values of a, p can be approximated as:
p ~ cos qr - sine ~r ~ - ~ cos yr sin Z yr ~2 (27) Solving, Eq. (25) for z yields:
z= ~t Pa -~a (2g) s where O = ~x~s + yms +a(pz -1)I~S (29) and ~ =P a(Pz -1)~2{x~s + yms)-a(2s +ms )~+{x~s +yms)2 -(p2 -ns )(x2 +y2) (30) When ras = 0, Eq. (29) can be written in the form of Eq. (21 ), with:
a{p2 1)~2{x~s +yms}-a-+~x2~r +ymSJ2 31 Ipl ( ) For small values ofa, Eqs. {28) and (31) can be approximated by Eqs. (20) and (23), respectively, with p replaced by p.
in order to estimate range resolution, consider the uncertainty dz in the measurement ofz that would be introduced by an uncertainty (ON or ~.p) in the quantity being measured. For simplicity, this calculation is based on the approximate expression for N given by Eq. (16}. To find ~z, we take the partial derivative of N {or p) with respect to z and equate this derivative to the ratio ONl~lz (or dp/dz), to yield:
~z = G R° ~,dN = GR°OP (32) a where G = - 1 (33}
ns. -{k'~s. +~xms. +nns)n sinBcosBs~sinB-cosBcos~~-~5~~

WO 97!29341 PCT/LTS97/01744 is a geometrical factor that accounts for the direction of translation and the direction to the scattering point. In the first form for G, .~ °xlRa, m y/Ra, and zz z/Ro (34) .
are direction cosines for the Point Po. In the second form for G, 8 and c~ are the polar and azimuthal angles, respectively, representing the direction from the origin to P° in a spherical-polar coordinate system. Likewise, the direction of translation of the source point is given by 8$ and ~8.
Consideration of Eq. (32) reveals that range resolution degrades with increasing object distance Ro and improves with increasing magnitude of translation a of source P I.
Consideration of Eq. (33} reveals that the geometrical factor G ranges between unity and infinity, where unity corresponds to the best range resolution achievable.
The optimal direction of translation of source P~ for a given scattering-point direction is obtained from Eq. (33} by choosing 2 s, m8, and n8 such that G is minimized for the given values of.2 , m and n. Application of this constraint yields:
~~'s,nl~.,rzs~ ~~ , zzm ~ 1-~z (35) 1-Yt2 1-f22 which implies that the optimal translation direction is orthogonal to the line extending from the origin to the scattering point Po (fir=90°) and lies in the plane of incidence formed by said line and the z axis (~s c~). Substitution of the values in Eq. (35) into Eq. (33) , results in:

WO 97!29341 PCT/iJS97/01744 G - I _ 1 __ 1 - (36) 1-n2 f2 +m2 sing From Eq. (3 6), it is observed that the best achievable G value of unity occurs when n = 0 (6 = 90°), which implies that the scattering point lies in the x y plane. It is also observed that the resolution degrades such that G approaches infinity for scattering points lying on the z axis. For example, G = 2 for 8 = 30° and G = 5.76 for 8 =
10°. Although it is not possible to satisfy Eq. (35) for every point in the image without changing the translation direction fox each point, the condition for optimal resolution can be approximated by satisfying Eq. (35) for a representative image point.
By Eqs. (25) and (27), the instantaneous modulation rate p vanes as a function of the offset magnitude q of the translating point. For techniques based on measuring p, it is desirable for p to vary as little as possible during the scan so that there is nearly a one-to-one correspondence between values of p and z. Then standard spectral-analysis techniques can be applied to estimate the value of p and determine z. To quantify the degree of nonuniformity in p that occurs during a scan, we define:
x - P(g) - P(~) - P(a> - cos yr (37) p(0) cos yr Substitution of the approximate form for p from Eq. (27) into Eq. (37) and keeping only the lowest order term containing a, yields:
sine yr a x ~ cosqr Ro _ (38) Equation {38) states that the modulation nonuniformity increases linearly in the ratio a/Ro of scan length to object distance. Furthermore, the nonuniformity vanishes when yf=0°
and increases without bound when ~r=90°. We observe, however, that there is no range resolution when 4r=0° because all points on the yr=0° Iine have the same modulation rate, regardless of range, i.e., Goo in Eq. (33). Therefore, there is a tradeof~between minimizing the nonuniformity and obtaining optimal range resolution.
A reasonable measurement configuration that simultaneously provides good range resolution and reduced modulation nonuniformity is to set fis 0 and to use an off-axis optical system with the offset in the ~$ direction, i.e., ~=~8. Then Eq. (33) for G reduces to:
-2 { ) G = 39 sin(28) As an illustrative example of the measurement technique, suppose it is desired to image an object that is 200 mm by 200 mm in the x y plane from a distance of Ro=lm using a laser of wavelength ~.=0.7~m. If ns 0 and the center of the object is located at 6=30° and c~=~S, then, by Eq. (39), the geometric factor G wilt vary between 2.1 and 2.6 over the field of view. By Eq. (32), a translation of cr=Smm will produce a range uncertainty of ~80~n (in the middle of the image) for an uncertainty in the number of oscillations of one-quarter count, i.e., ~N=0.25. The total number of oscillation counts _ for the entire scan is N=3600 by Eq. (16). To estimate the modulation nonuniformity at the center of the image, we set ~=60° in Eq. {38) and obtain x=0.0075 so that there is less than a 1% nonuniformity over the scan. This nonuniformity could be reduced further by WO 97!29341 PCTJUS97/01744 introducing slight variations in the scan rate during the scan to compensate for any change in frequency during the measurement.
Figure 6 depicts an illustrative series of steps to be executed by the processor 28 of Figs. I and 5 to determine the depth coordinate z at each point {x, y) on the object. The processor begins (step I00) by measuring a parameter of the intensity of the radiation scattered by a plurality of illuminated points on the object surface (step 108). From this information, the z coordinate for each measured point is calculated (step I
12).
An optional filtering process may be performed in step 1 I6. Suitable filters known to those skilled in the art include, but are not limited to, smoothing filters, median filters and curve-fitting filters. Thereafter, the mapped points can be displayed or output in anyway known to one of ordinary skill in the art, following which the process is terminated in step 124, as shown. In one embodiment, the mapped points are plotted as a function of the computed z information on a mesh plot in step I20.
Referring also to Fig. 6a, one embodiment of steps 108 and I 12, suitable for use with the embodiment of Fig. 1 is shown. In step 108', the intensity of the scattered illumination is measured as a function of laser frequency ofl'set and N is measured using one of the methods known to those skilled in the art. Thereafter, s is calculated for each location (xy) in step 110' using Eq. (4) and z is calculated for each location (xy) in step 112' using Eq. (9).
An alternate embodiment of process steps 108 and 112 for use in conjunction with the embodiment of Fig. 5 is shown in Fig. 6b. In this case, the parameter of the intensity measured in step 108" is the number of times, N (not necessarily a whole number), that the intensity cycles as the moveable source Pi (Fig. 5) translates. Once N has been determined in step 108" through one of the methods known to be skilled in the art it is converted to p by Eq. (19) in step I IO". z is then calculated in step 112"
with the use Eqs. (17) and (18). Another embodiment of process steps 108 and 1 I2 for use in conjunction with the embodiment of Fig. S is shown in Fig. 6C. Here, the parameter of the intensity measured in step I08" is the instantaneous oscillation rate p at which oscillations occur as the source point P1 translates. p is converted to z in step 112"' through Eqs. (28)-(30).
Various arrangements of detector 22 and processor 28 are possible. In one embodiment, shown in Fig. 7, the photodetector elements 221,1 to 22~,m of the detector array 22 are read out serially. The serial output 36 of detector array 22 provides an input to a processor 28. Processor 28 may comprise a single processor or alternatively, may be a multiprocessor comprising a plurality of processors.
Referring also to Fig. 7a, an alternate detector and processor arrangement is shown. In this embodiment, the processor 28 is a multiprocessor comprising a plurality of processors 281,, to 28~m. Each of the photodetector elements 221,1 to 22~,,m of the detector array 22 provides a respective output signal 38 to a corresponding one of the processors 281,1 to 281,,",. With this arrangement, each of the processors 281,1 to 28~,,m is able to operate substantially simultaneously, in order to provide substantial performance advantages. More particularly, each processor 28,,1 to 28~,m in the multiprocessor unit 28 is responsible for making the z coordinate calculation based upon the data received from the corresponding element 221,1 to 22~m of the photodetector array 22. Thus, the z coordinate for each location of the surface of the object 10 may be determined rapidly.

Fig. 7b shows a further alternate embodiment of the detector and processor components for use in the systems of Figs. l and 5 in the form of a unitary detector and processor array 25. The array 25 is fabricated on, and supported by, a common substrate or is fabricated as a Multi-Chip Module (MCM) or with Surface Mount Technology (SMT). The detector portion ofthe array 25 includes photodetector elements 221,1 to 22~,~ and the multiprocessor portion of the array includes processors 281,, 'to 28",m. More particularly, each of the detectors 221,1 to 22",m is associated with, and positioned adjacent to, a respective one of the processors 281,1 to 28",~, and provides an input signal to the respective processor, as shown. The processors 281,1 to 28",m process the information from the respective detectors 221,1 to 22",m substantially simultaneously to provide the determined depth coordinates.
Referring to Fig. 8, another embodiment of the invention includes an array of detectors 22' placed against the surface of the object 10 whose surface contour is to be measured. With this arrangement, rather than observing the Light scattered from the point Po on the surface of the object 10 to determine z, the measurement of the phase shifting of the light is performed directly at the surface of the object. Although not shown, the system of Fig. 8 includes a control unit 28 for controlling sources P, and P2 and a processor 28 for processing radiation incident on detector 22' as shown and described above in conjunction with Figs. 1, 5 and 6.
The arrangement and mechanism for locating the photodetector elements 23 on the surface of the object 10 may vary. In one embodiment shown in Fig. 9, a plurality of individual photodetector elements 23 of array 22' are positioned on the surface of the object 10 in the area of interest.

WO 97!29341 PCT/LTS97l01744 In another embodiment, shown in Fig. 9a, individual photodetector elements 23 of the array 22' are mounted on spring arms 84 cantilevered from a support and control unit 88. The spring arms 84 are moved over the surface of the object 10 by the control unit 88 in order to contact specific points, or regions of interest. The cantilevered support of spring arms 84 causes each individual detector 23 to remain in contact with a location on the surface of the object 10 as the arms 84 are moved thereover. That is, as the contour of the object surface varies, the spring arms 84 move up and down accordingly.
It will be appreciated that three or more additional radiation sources may be used in apparatus and methods of the present invention. For example, an additional source or sources can be used to determine xy coordinate information regarding the object or a portion thereof. Additionally, extra radiation sources may be used to reduce any processing inaccuracies or ambiguities attributable to shadowing of a region of interest.
It will also be appreciated that other variations to the embodiment involving moving source points may be used. For example, the two points may both move with opposing motion, they may both move in the same direction with constant separation, they may rotate about a common center point, or motion may be simulated by using an array of source points that can be switched on and offby the control system.
Having described and shown the preferred embodiments of the invention, it will now become apparent to one of skill in the art that other embodiments incorporating the concepts may be used and that many variations are possible which will still be within the scope and spirit of the claimed invention. It is felt, therefore, that these embodiments should not be limited to disclosed embodiments but rather should be limited only by the spirit and scope of the following claims.

Claims (50)

What is claimed is:

1. A method for determining, on an object having a surface, three-dimensional position information of a point on said surface of said object, said method comprising the steps of:
providing two sources of radiation, said sources being coherent with respect to one another;
illuminating said point with said radiation from each of said sources;
changing the phase of said radiation from one of said sources relative to the phase of said radiation from the other of said sources as measured at said point on said surface of said object;
detecting radiation scattered by said point on said surface of said object;
and calculating three-dimensional position information in response to said change in phase of said radiation from said sources and said detected radiation scattered by said point on said surface of said object.

2. The method of claim 1 further comprising the step of imaging said radiation scattered by said point on said surface of said object prior to detecting said radiation scattered by said point on said surface of said object.

3. The method of claim 1 wherein said step of providing two sources of radiation comprises the steps of:
providing a single source of coherent radiation; and dividing said coherent radiation from said single source into two beams of coherent radiation.

4. The method recited in claim 3 wherein said radiation dividing step comprises the step of dividing said coherent radiation with a fiber optic splitter.

5. The method of claim 3 wherein said step of dividing said coherent radiation from said single source into two beams of coherent radiation comprises the step of placing a beam splitter in the path of said coherent radiation from said single source.

6. The method of claim 5 further comprising a pair of mirrors, a first one positioned to reflect a first beam of radiation reflected by said beam splitter and a second one positioned to reflect a second beam of radiation passing through said beam splitter.

7. The method of claim 6 further comprising a focusing element positioned between said source of said coherent radiation and said beam splitter.

8. The method of claim 6 further comprising a first focusing element positioned between said beam splitter and said first mirror and a second focusing element positioned between said beam splitter and said second mirror.

9. The method of claim I wherein said step of changing the phase of said radiation from one of said sources relative to the phase of said radiation from the other of said sources as measured at said point on said surface of said object comprises the step of changing the frequency of said sources of radiation.

10. The method of claim 9 wherein said sources of radiation are provided by a tunable laser.

11. The method of claim 1 wherein at least one of said sources of radiation is moveable.

12. The method of claim 9 wherein said sources are tunable and at least one of said sources is moveable.

13. Apparatus for determining, on an object having a surface, position information of a point on said surface of said object, said apparatus comprising:
two sources of radiation positioned to illuminate said point with said radiation from each of said sources, said radiation from a first one of said sources being coherent with respect to said radiation from said other one of said sources;

a control system changing the phase of at least one of said sources of radiation relative to the phase of said radiation from the other of said sources as measured at said point on said surface of said object;
a detector positioned to receive radiation scattered from said point on said surface of said object; and a processor receiving signals from said detector, said processor calculating depth coordinate information in response to said change in phase of said radiation sources and said received radiation scattered from said point on said surface of said object.

14. The apparatus of claim 13 further comprising an imaging system positioned between said detector and said point on said surface of said object, said imaging system imaging said point on said detector.

15. The apparatus of claim 13 wherein said imaging system comprises a lens.

16. The apparatus of claim 15 wherein said detector comprises a CCD array.

I7. The apparatus of claim 13 wherein said detector comprises an array of photodetectors.

18. The apparatus of claim 17 wherein said processor comprises a multiprocessor system having a plurality of processing units and said array of photodetectors comprises a plurality of photodetectors, each of said photodetectors sending signals to a respective one of said plurality of processing units.

19. The apparatus of claim 13 wherein said detector comprises a CCD array.

20. The apparatus of claim 16 wherein said processor comprises a multiprocessor system having a plurality of processing units and said CCD array comprises a plurality of CCD elements, wherein a plurality of said CCD elements send signals to a respective one of said plurality of processing units.

21. The apparatus of claim 13 wherein said two sources of coherent radiation comprises a laser providing a beam of coherent radiation and a beam splitter positioned to divide said beam of coherent radiation into two beams of coherent radiation.

22. The apparatus of claim 21 further comprising a pair of mirrors, a first one positioned to reflect a first beam of radiation reflected by said beam splitter and a second one positioned to reflect a second beam of radiation passing through said beam splitter.

23. The apparatus of claim 22 further comprising a focusing element positioned between said source of said coherent radiation and said beam splitter.

24. The apparatus of claim 22 further comprising a first focusing element positioned between said beam splitter and said first mirror and a second focusing element positioned between said beam splitter and said second mirror.

25, The apparatus of claim 21 wherein said laser is frequency tunable.

26. The apparatus of claim 21 wherein said beam splitter comprises an optical fiber arranged to divide said beam of coherent radiation into two beams of coherent radiation.

27. The apparatus of claim 13 further comprising a polarizing filter positioned between said point on said surface of said object and said detector, said polarizing filter oriented to permit radiation scattered by said point on said surface of said object having the same polarization as the coherent radiation illuminating said point on said surface of said object to reach said detector.

28. The apparatus of claim 13 wherein said two sources of coherent radiation comprise two frequency locked lasers.

29. The apparatus of claim 28 wherein each of said frequency locked lasers is frequency tunable.

30. The apparatus of claim 13 wherein said two sources of coherent radiation are located at a fixed distance from one another.

31. The apparatus of claim 13 wherein said two sources of coherent radiation are located at a known distance from one another.

32. A method for determining, on an object having a surface, position information of a plurality of points on said surface of said object, said method comprising the steps of:
providing two sources of radiation, with the radiation from said sources being coherent with respect to each other;
providing a detector at each of said points on said surface;
illuminating said plurality of points with said radiation from each of said sources;
changing the phase of said radiation from one of said sources relative to the phase of said radiation from the other of said sources as measured at said plurality of points on said surface of said object;
detecting said radiation at said plurality of points on said surface of said object; and calculating position information in response to said change in phase of said radiation from said sources and said radiation detected at said plurality of points on said surface of said object

33. The method of claim 32 wherein said step of providing two sources of coherent radiation comprises the steps of:
providing a single source of coherent radiation; and dividing said coherent radiation from said single source into two beams of coherent radiation.

34. The method recited in claim 33 wherein said radiation dividing step comprises the step of dividing said coherent radiation with a fiber optic splitter.

35. The method of claim 33 wherein said step of dividing said coherent radiation from said single source into two beams of coherent radiation comprises the step of placing a beam splitter in the path of said coherent radiation from said single source.

36. The method of claim 32 wherein said step of changing the phase of said coherent radiation from one of said sources relative to the phase of said coherent radiation from the other of said sources as measured at said plurality of points on said surface of said object, comprises the step of changing the frequency of said two sources of coherent radiation.

37. The method of claim 36 wherein said sources of radiation are tunable lasers.

38. The method of claim 32 wherein at least one of said sources of radiation is moveable.

39. The method of claim 36 wherein said sources are tunable and at least one of said sources is moveable

40. Apparatus for determining, on an object having a surface, a depth coordinate of a plurality of points on said surface of said object, said apparatus comprising:
two sources of radiation positioned to illuminate said plurality of points with said radiation from each of said sources, said radiation from each of said sources being coherent with respect to said radiation from the other one of said sources;
a control system changing the phase of said radiation from one of said sources relative to the phase of said radiation from the other of said sources as measured at said plurality of points on said surface of said object;
a detector positioned at each of said plurality of points on said surface of said object to receive radiation illuminating said plurality of points on said surface of said object; and a processor receiving signals from said detectors, said processor calculating depth information of said plurality of points on said surface of said object in response to said change in phase of said radiation from said source and said radiation received at said plurality of points on said surface of said object.

41. The apparatus of claim 40 wherein said two sources of coherent radiation comprises a laser providing a beam of coherent radiation and a beam splitter positioned to divide said beam of coherent radiation into two beams of coherent radiation.

42. The apparatus of claim 41 further comprising a pair of mirrors, a first one positioned to reflect a first beam of radiation reflected by said beam splitter and a second one positioned to reflect a second beam of radiation passing through said beam splitter.

43. The apparatus of claim 42 further comprising a focusing element positioned between said source of said coherent radiation and said beam splitter.

44. The apparatus of claim 42 further comprising a first focusing element positioned between said beam splitter and said first mirror and a second focusing element positioned between said beam splitter and said second mirror.

45. The apparatus of claim 41 wherein said laser is frequency tunable.

46. The apparatus of claim 41 wherein said beam splitter comprises a optical fiber arranged to divide said beam of coherent radiation into two beams of coherent radiation.

47. The apparatus of claim 40 wherein said two sources of coherent radiation comprises two frequency locked lasers.

48. The apparatus of claim 47 wherein each of said frequency locked lasers is frequency tunable.

49. The apparatus of claim 40 wherein said two sources of coherent radiation are located at a fixed distance from one another.

50. The apparatus of claim 40 wherein said two sources of coherent radiation are located at a known distance from one another.