Abstract:
An apparatus and method for compensating for measurement uncertainty due to atmospheric effects. In one embodiment the apparatus includes two sources separated by a predetermined distance and two target locations separated by a predetermined distance. The radiation at the target locations is combined to form an interference pattern onto a detector which generates a signal corresponding to the measurement having a substantially reduced error due to atmospheric effects such as temperature variations. In another embodiment the radiation from the sources crosses somewhere in the measurement environment as it propagates toward the target locations. In yet another embodiment the separation between the two sources is substantially the same as the separation between the two target locations.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a divisional of U.S. patent application Ser. No. 09/325,177 which was filed on Jun. 3, 1999 now U.S. Pat. No. 6,229,619, which was a continuation-in-part of U.S. patent application Ser. No. 09/241,354 which was filed on Feb. 2, 1999 now U.S. Pat. No. 6,031,612 which was a continuation-in-part of U.S. patent application Ser. No. 08/600,216 which was filed on Feb. 12, 1996 now U.S. Pat. No. 5,870,191, and claims priority to provisional U.S. patent application Ser. No. 60/087,960 which was filed Jun. 4, 1998. 
    
    
     GOVERNMENT SUPPORT 
     Work described herein was supported by Federal Contract No. F19628-95-L-002, awarded by the United States Air Force. The Government may have certain rights in the invention. 
    
    
     FIELD OF THE INVENTION 
     The invention relates to the field of metrology, and more specifically to optical non-contact position measurement. 
     BACKGROUND OF THE INVENTION 
     Precise non-contact measurement of the deformation or displacement of objects is critical when active or passive compensation for the deformation or displacement is required. For example, large parabolic dishes used for communication, radar or telescopes are susceptible to many natural influences which distort the shape of the dishes. Some of these natural influences include wind, gravitational forces which can cause the dish to sag depending on the orientation of the dish, and temperature variations in the dish which can distort the shape of the dish, etc. To compensate for these effects, several solutions may be employed. One such solution is to use mechanical actuators on a segmented dish with an array of detectors mounted in different locations on the dish. As the detectors sense a change in the shape of the dish, the actuators respond by moving segments of the dish to correct the shape of the dish. A large disadvantage of this technique is the amount of cables and other electrical components which need to be mounted to the dish to perform the compensation. Another solution involves using mathematical techniques to correct the received signal as reflected by the deformed surface. This technique is of limited usefulness since the distortions in the dish are typically not uniform and cannot always be accurately modeled. These measurements can also be applied to other structures where deformations are studied, such as building surfaces, airplane surfaces, space shuttle surfaces, and surfaces of automobiles. In each case, the amount of surface deformation must be accurately measured so that the correct amount of compensation can be applied. These measurements can effect the design of wings on an airplane or the rear spoiler on a racing car, for example. 
     Other techniques for measuring deformations in the surface of objects include using laser range finders which provide precise measurement information. Typically it is cost prohibitive to use many of these laser range finders to simultaneously measure many points on the surface. More practically these range finders are used for individual sequential measurements at different points on the surface of the object. Hence, the measurements are not made simultaneously. Also, these laser range finders are sensitive to temperature changes in the atmosphere along the z-axis in FIG.  1 . Temperature variations between the left side of FIG.  1  and the right side of FIG. 1, or bulk temperature changes along the entire path L can severely compromise the measurement accuracy of these laser trackers. 
     Another measurement technique projects a fringe pattern on a detector which is mounted to the surface to be measured. As the surface deforms, the detector moves and sweeps across the fringe pattern. By detecting the changes in the light intensity, the deformation of the surface can be determined. Although this technique provides a relatively inexpensive way to simultaneously detect relative displacement of a surface, it is sensitive to temperature variations in the atmosphere as well, but in the direction of the x-axis in FIG.  1 . In other words, the temperature sensitivity of this technique is to temperature variations in the direction displacements are being measured. Therefore, this measurement technique is sensitive to temperature gradients. Those gradients result in shifts in the fringe pattern at the detector independent of the relative motion of the detector and the fringe pattern. Note that this technique is not sensitive to temperature variations along the z-axis as shown in FIG.  1 . 
     These optical techniques are susceptible to temperature variations in the atmosphere because those temperature changes cause index of refraction variations. These index of refraction variations cause light traveling through the atmosphere to bend. The amount of this bending depends on the severity of the refractive index variations. The measurement techniques discussed above do not compensate for these refractive index variations. Thus, the measurement result is not as precise as it would be without refractive index variations in the atmosphere. Therefore, non-contact techniques for measuring surface deformation or distortion cannot identify whether the distortion is due to the wind, gravity, or atmospheric effects affecting the measurement equipment. 
     The present invention provides a method and apparatus for compensating for atmospheric effects that typically plague measurement equipment. The technique is useful in precise non-contact measurement of surface distortion without adding the uncertainty of refractive index changes in the atmosphere. The technique could also be used in precision land surveying, to aid in the building of a linear accelerator, or any situation where precise straightness measurements are required. The technique may be used to compensate for measurement uncertainty due to atmospheric refractive index effects. 
     SUMMARY OF THE INVENTION 
     The invention relates to an apparatus and method for compensating for measurement error due to refractive index variations in the measurement environment. In one embodiment the apparatus includes two sources separated by a predetermined distance and two target locations separated by a predetermined distance. The radiation at the target locations is combined to form an interference pattern onto a detector which generates a signal which corresponds to the measurement having substantially reduced error due to refractive index variations in the measurement environment. In another embodiment, the radiation from the two sources crosses somewhere in the measurement environment before reaching the two target locations. In yet another embodiment, the distance separating the two sources is substantially equal to the distance separating the two target locations. In yet another embodiment, the two sources are generated from a single source. The source(s) could be broadband or laser. 
     In one embodiment, the two sources and the two target locations are substantially adjacent to each other. The radiation from the sources is directed back from a target reflector towards the sources and is combined to form an interference pattern onto a detector which generates a signal which corresponds to the measurement having substantially reduced error due to refractive index variations in the measurement environment. 
    
    
     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 an embodiment of a measurement system which uses a measurement technique known to the prior art. 
     FIG. 2 is ray trace diagram of an embodiment of the invention. 
     FIG. 3 is a diagram of an embodiment of the present invention using polarization tracking. 
     FIG. 4 is a highly schematic diagram of another embodiment of the invention. 
     FIG. 5 is a highly schematic diagram of still another embodiment of the invention. 
     FIG. 6 a  is a highly schematic diagram of yet another embodiment of the invention. 
     FIG. 6 b  is a highly schematic diagram of still another embodiment of the invention. 
     FIG. 7 a  is a graph of simulated random temperature gradients in a simulated measurement environment. 
     FIG. 7 b  is a graph of simulated measurement error due to the simulated random temperature gradients of FIG. 7 a  for both conventional measurement and reduced error measurement according to the invention. 
     FIG. 8 is a graph of the measurement error for both conventional measurement and reduced error measurement using the embodiment of the invention shown in FIG.  5 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In brief overview and referring to FIG. 1, one prior art measurement technique for measuring displacement of the surface of an object is shown. In order to measure the displacement of a point  10  on the detector  12 , two sources of radiation  2 ,  2 ′ can be used. The two sources of radiation  2 ,  2 ′ must be coherent with respect to one another. As the two sources of radiation  2 ,  2 ′ irradiate the detector  12 , they produce an interference pattern  16  on the detector  12 . The interference pattern  16  consists of regions of varying intensity  18 ,  18 ′; each region represents a fringe period of the repeated interference pattern  16 . As the point  10  is displaced in the x direction, the detector  12  senses the movement of the point  10  with respect to the interference pattern  16 . The detector  12  is sensitive to the intensity variations of the fringes so it can detect movement within the same fringe period. The detector  12  can also detect movement through whole fringe periods so determining larger displacements of the point  10  is relatively straightforward and involves counting fringe periods as they pass over the detector  12 . 
     Although the technique described above can measure the relative displacement of the point  10 , it cannot identify whether the fringe shifts were due to the actual displacement of the detector or due to atmospheric effects, or a combination of the two. This is because as the temperature changes in the measurement environment (i.e. the atmosphere), so does the refractive index of the environment and the beams  4  and  6  will refract differently in the changing temperature environment. This beam refraction will shift the interference pattern  16  on the detector  12 . The detector  12  will thereby sense a false change in the displacement of point  10  and report that artificial displacement causing the measurement to be imprecise. 
     Turning again to FIG. 1, the optical path length difference between beams s 1  and s 2  is described by the equation: 
     
       
           OPD=s   2   −s   1   (1) 
       
     
     where s 1  and s 2  are optical path lengths that involve the integral of the path length multiplied by the refractive index at each point along the length. 
     In a uniform temperature gradient in the x direction, this can also be represented by the expansion: 
     
       
           OPD ≈( an   0   /L ) x   0 −( P   0   L /2) a+. . .    (2) 
       
     
     where P 0  represents the uniform temperature gradient described as the derivative of the refractive index (n) with respect to the x direction, substantially perpendicular to a bisector of the sources  2 ,  2 ′. The term P 0  is difficult to ascertain. The nominal refractive index in the measurement environment is n 0 . The distance between the sources  2 ,  2 ′ is a, and L is the distance between the z=0 plane of the sources  2 ,  2 ′ and the z=L plane of the target locations  20 ,  20 ′. The term x 0  is the unknown displacement. The higher order terms in equation 2 have negligible effect and can be ignored when a is much less than L. The first term in equation 2 consists of known quantities except for x 0  which is the unknown displacement. The second term in the equation includes the term P 0  which as discussed above is the derivative of n with respect to the x direction. That is, this term represents the change in refractive index with respect to x and is difficult to discern. 
     Referring now to FIG. 2, one embodiment of the invention includes two mutually coherent light sources  2 ,  2 ′ separated by a distance a which irradiate two target locations  20 ,  20 ′ located a distance L from the sources  2 ,  2 ′. The two light sources  2 ,  2 ′ may be generated from a single source and also may be broadband or laser sources. An optical combiner  24  in communication with the target locations  20 ,  20 ′, directs the beams  22 ,  22 ′ to the detector  12 . The beams  22 ,  22 ′ will combine at the location of the detector  12  to produce an interference pattern  16 . The detector  12  is sensitive to intensity variations of the fringes so it can detect movement within the same fringe period. The detector  12  can also detect movement through whole fringe periods so determining the displacement of the point  10  is relatively straightforward and involves simply the counting of the passing fringe periods. In another embodiment of the invention, multiple detectors may be used including array detectors. By choosing a suitable separation a of the two sources  2 ,  2 ′ and a suitable separation b of the two target locations  20 ,  20 ′, where b is defined as the difference between the x-value at target location  20  and the x-value at target location  20 ′ (for example, b is positive as drawn in FIG.  2 ), the atmospheric term in equation 2 above will be removed from the equation, for a=b. The equation for the OPD corresponding to FIG. 2 is: 
     
       
           OPD ≈( an   0   /L ) X   0 −( P   0   L /2) ( a−b )  (3) 
       
     
     In a uniform temperature gradient, the atmospheric refraction error being accumulated is proportional to the distance between two points at the same z-location on two beam paths  22  and  22 ′. For example, the distance between the sources is a, so the error being accumulated at z=0 is proportional to the distance a. The reason for this is that the change in refractive index is proportional to the linear temperature gradient. Moving towards the center, the distance between the points on the beam paths  22  and  22 ′ is less, and therefore the accumulated error is less. Reaching point  21  where the beams cross, the error is zero. Moving to the other side of the crossing point  21 , the upper beam now becomes the lower beam and the lower beam becomes the upper beam. The error is now being accumulated in the opposite direction since the beams are switched. Since the gradient is uniform across the length L, the error that was accumulated on the left side of crossing point  21  is exactly undone by the opposite accumulation on the right side of crossing point  21 . Therefore, when the separation a between the two sources  2 ,  2 ′ is equal to the separation b between the two target locations  20 ,  20 ′, the effect of the uniform temperature gradient is canceled. Hence, a is equal to b. In other words, this embodiment of the invention has effectively compensated for atmospheric effects due to a uniform temperature gradient  8  in the measurement environment. Therefore the OPD is given by the equation: 
     
       
           Opd ≈( an   0   /L ) x   0   (4) 
       
     
     in which all quantities except x 0  are known. 
     In certain circumstances it may be desirable to change the separation between the two sources  2 ,  2 ′ or the two target locations  20 ,  20 ′. This will actually allow for the tuning of the apparatus to more effectively compensate for non-uniform temperature gradients. 
     Because both sources may irradiate both target locations, a technique must be used to differentiate which beams came from which source. This is required since the invention takes advantage of the geometry of the sources and the detectors in order to compensate for the atmospheric temperature gradient. In one embodiment, radiation from source  2 ′ is directed at target  20 , while radiation from source  2  is directed at target  20 ′. In this embodiment, the beams cross somewhere in the measurement environment. In order to manipulate the beams according to the invention, the source and the target of the radiation must be known. 
     This beam tracking can be done in various ways. For example, in one embodiment of the invention, narrow coherent lasers may be used as sources  2 ,  2 ′, each could be directed to irradiate only one target location  20  or  20 ′. Another embodiment of the invention uses polarization as shown in FIG.  3 . Linear polarizers  28 ,  30  are orthogonal with respect to one another. Radiation from source  2  is directed into polarizer  28  allowing only linearly polarized light  22  to reach target location  20 ′. Radiation from source  2 ′ is directed into polarizer  30  allowing only linearly polarized light  22 ′ to reach target location  20 . The two beams  22  and  22 ′ are orthogonally polarized with respect to one another. Before these beams  22 ,  22 ′ interfere and reach the detector, they pass through analyzers  32 ,  34 , which are linearly polarized in the same orientation of the beams  22 ,  22 ′. This technique allows only the beam from the correct source  2 ,  2 ′ to reach each target location  20 ,  20 ′ and thereby produces a desired condition (for example, a=b) in equation 3. In this embodiment, analyzer  26  is positioned just before the detector  12  to cause the cross polarized beams to interfere. In another embodiment according to FIG. 4, calcite  38  or another suitable material can be used to separate the beams into orthogonally polarized components and recombine the beams from such components. 
     In FIG. 4, source  36 , which can be a broadband or laser source, directs radiation into crystal  38 , which can be calcite or any other suitable birefringent material. Birefringent crystal  38  divides and diverts the radiation internally and produces extraordinary beam  40  and ordinary beam  42 . Beams  40  and  42  become virtual sources  2  and  2 ′ as they leave the crystal  38 . Beams  22  and  22 ′ are orthogonally polarized with respect to one another. As beams  22  and  22 ′ propagate, they eventually encounter a detection birefringent crystal  44  which performs the function of recombining the beams in such a way as to produce an interference pattern on detector  12  after the beams traverse analyzer  26 . 
     FIG. 5 depicts another embodiment of the invention. Since it may not be desirable to mount electronics and cables on the surface of the object, FIG. 5 illustrates an embodiment of the invention that addresses that issue. Here the analyzer  26  and the detector  12  are substantially adjacent to the source  36 . A beam splitter  60  is positioned to allow radiation from source  36  to enter crystal  38  which generates beams  22  and  22 ′ as above. Crystal  38  is a birefringent crystal which splits the polarization between beams  40  and  42 . Beams  40  and  42  exit the crystal  38  and propagate as  22 ,  22 ′ until they reach a target reflector  50 . The target reflector in one embodiment is a polarizing beam splitter. Retro-reflectors  52  and  54 , which may be corner cubes, are used to direct the beams  58  and  56  back to beam splitter  60 . The separation b may be changed by adjusting retro-reflectors  52  and  54 . This allows tuning of the apparatus for non-uniform temperature gradients. Beam splitter  60  directs the beams  22 ,  22 ′ through the analyzer  26  to produce an interference pattern at detector  12 . The analyzer  26  in one embodiment is a polarizer. 
     FIGS. 6 a  and  6   b  illustrate alternate embodiments of the invention. Turning to FIG. 6 a,  it can be shown that by taking two measurements for different values of b (for example b=0 and b=−a), the P 0  term in equation 3 can be eliminated. The physical apparatus may include two sources, or a single source  36  and two virtual sources  2 ,  2 ′. The apparatus also includes optical combiner  24 , analyzer  26 , and a detector  12 . The apparatus may use multiple detectors including array detectors. Note that when using the polarization technique to track the beams as shown in the FIG. 6 a  embodiment of the invention, the axes of the analyzers  32  and  34  in FIG. 3 must be switched, since the beams are not crossed in the measurement environment. Moreover, with reference to FIG. 6 a,  depending on which beams from virtual sources  2  and  2 ′ are tracked, it is possible to achieve the desired result of atmospheric refraction compensation with or without crossing the beams in the measurement environment. 
     In the embodiment of the invention as shown in FIG. 6 a,  beams  22  and  22 ′ do not cross in the measurement environment. Source  36 , which could be a broadband source or a laser source, illuminates crystal  38 . Beams  22  and  22 ′ are orthogonally polarized with respect to one another. As beams  22  and  22 ′ propagate they are directed to optical combiner  24 . As optical combiner  24  combines the beams, they pass through analyzer  26 , which can be a polarizer, before producing an interference pattern on detector  12 . Alternatively, as shown in FIG. 6 b,  multiple crossing points may be desirable and can be achieved by using reflectors  66 ,  68 ,  70 , and  72  and crystal  44 . In this embodiment, beams  23  and  23 ′ propagate through the measurement environment and eventually reach crystal  44 , where they are recombined, pass through an analyzer  26 , and produce an interference pattern on detector  12 . Again, by manipulating the beams  23 ,  23 ′, the desired condition of atmospheric compensation can be satisfied. 
     FIG. 7 a  and  7   b  are graphs of the simulated results observed for random temperature gradients using the invention. The purpose of the simulation is to show that there is effective compensation even though the gradients in FIG. 7 a  are not exactly uniform. Simulated measurements were taken using a conventional measurement technique. The call-out numbers in FIG. 7 a  correspond to the realization numbers in FIG. 7 b.  The results of simulated conventional measurements are shown by the dotted lines in FIG. 7 b.  Simulated measurements using the invention are shown in FIG. 7 b  as solid lines. Note that the solid lines indicating the measurement error show less error in the measurement using the invention. 
     FIG. 8 shows actual measurement results for the embodiment of the invention shown in FIG.  5 . As indicated in the FIG. 8, when heaters were activated in the measurement environment the technique used by the invention showed dramatic improvement in measurement accuracy. 
     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.