Abstract:
Presented is a speckle interferometer for measuring displacement of an optically irregular surface of a specimen having, a laser for producing a beam of coherent radiation, an adjustable polarizing beam splitter for dividing the radiation into a first portion of the radiation and into a second portion of the radiation. The reflected radiation is then passed through a pair of collimating lenses onto the optically rough surface of the specimen. The reflection off of the specimen is captured by an imaging system that processes the reflection to measure strains on the surface.

Description:
This application claims benefit of Provisional Application Ser. No. 60/100,936 filed Sep. 18, 1998. 
    
    
     BACKGROUND AND SUMMARY OF THE INVENTION 
     This invention relates to interferometry, and more particularly to a speckle interferometer apparatus and method that utilizes a scattering reference plate that can incorporate phase shifting. 
     Optical interferometers are known which make use of the interference phenomena known as the “speckle effect,” the speckled pattern seen when laser light is used to illuminate a rough surface. This invention utilizes the speckle effect, but it offers significant cost and performance improvements over conventional apparatus and methods. 
     U.S. Pat. No. 4,850,693 teaches a compact and portable moiré interferometer for determining surface deformations of an object; and U.S. Pat. No. 4,794,550 teaches a method of extending the measurement range of the moiré reference beam techniques by constraining the reconstruction of a surface contour based on a prior knowledge about the surface. These moire methods require that some form of a grating be created or projected onto the surface of the specimen, perhaps by the use of coherent laser light. 
     The physics of this invention are distinctly different from moire techniques. This invention measures deformations, displacements, and strains of an object, but it does not employ the “moire effect,” in that no grating is created on the specimen or in the optical system. Only the “speckle effect” is used. 
     Speckle interferometry is known for use in measuring strain in structural members and mechanical components. U.S. Pat. No. 4,591,996 teaches a method and apparatus for measuring strain in structural members utilizing a laser beam to illuminate a surface being analyzed and an optical data digitizer to sense a signal provided by the light beam reflected from the illuminated surface. The optical data digitizer is used to compare the signal received from the surface in a reference condition to subsequent signals received from the surface after surface deformation. 
     As in known in the art, data from the interference speckle can be used in several ways. While the specimen is stretched, the speckles translate indicating in-plane displacement and also vary in intensity indicating out-of-plane displacement. Due to the nature of materials, it can be assumed that changes from one speckle to an adjacent one are small and therefore linear. Because of this, contour maps of displacements and strains, both in-plane and out-of-plane can be constructed. The mathematical theorems and explanations of the recombination of object and reference beams are known in the art and are further described in a publication of the inventor,  Optical Methods of Engineering Analysis , Cambridge University Press 1995, Gary Cloud, which is expressly incorporated herein by reference. 
     The speckle is itself an interference phenomenon. The formation of speckles in imaging systems can be described at any image region as the superimposition result of the coherent point spread functions for adjacent object points. The speckle created by imaging optics is referred to as a “subjective” speckle. The nature of the illuminated surface gives rise to two different classes of speckle patterns. One class is called the “fully developed” speckle pattern; it develops only from interference of light that is all polarized in the same manner. The speckle field itself will then be similarly polarized. Surfaces at which polarized light is singly scattered, such as matte finished metal, generally give rise to polarized speckle fields as do lightly scattering transmission elements such as ground glass. Matte white paint surfaces or opal glass, into which the light penetrates and is multiply scattered, depolarize the light and thus do not generate a fully developed speckle pattern. The brightness distributions of the two classes of speckle patterns differ substantially, but this difference is not important in the functioning of speckle interferometry systems. 
     The mixed speckle pattern is recorded by the imaging system. The specimen is then subjected to a load, which causes displacement of the specimen&#39;s surface. This displacement causes changes in location and brightness of the various speckles. These newly changed speckles are again recorded by the imaging system. A computer connected to the camera captures the images and calculates displacements and strains on the object&#39;s surface based on the changes in the speckle pattern. The actual displacement and strain components that are calculated depend on the configuration of apparatus used, as outlined below. 
     In one embodiment of the current invention, a uniformly bright field of coherent radiation, which is the so-called reference beam, is added to the speckle field. The addition of the reference field will affect both the size and the brightness distribution of the speckle field. When a reference beam is introduced, the size of a speckle will approximately double. The reason for this involves the interference effect of adding a uniform strong wave to the speckle pattern in the direction of the optical axis. 
     In this embodiment of the current invention, a beam of coherent light is split. The first portion of the beam is projected onto the surface of the specimen. An imaging lens then collects an image of the speckle pattern formed by the reflection off of the specimen surface. This image is then combined in a beam combiner with the second portion of the beam. The combined image is then captured by a camera, and the computer calculates out-of-plane displacement and strain. This embodiment is used to measure out-of-plane displacement and strain. The data are independent of in-plane displacement as long as the angles of illumination and viewing of the specimen are held in the range 0-10 degrees. 
     In another embodiment of the invention, a beam of coherent light is split. The first portion of the beam is projected onto the surface of the specimen at a certain angle from the line of viewing. The second portion of the beam is projected onto the surface of the specimen at the opposite angle from the line of viewing. Thus, two beams are used to illuminate the specimen at equal and opposite angles of illumination. The resultant speckle pattern is recorded by the imaging system for before-load and after-load states. The computer calculates in-plane displacement and strain. In this embodiment, the changes in the speckle pattern depend only on in-plane displacement and strain for any angles of incidence, as long as the setup is symmetric with respect to the viewing axis. 
     The disclosed speckle interferometer system is thus very good at measuring both in-plane and out-of-plane displacements. The setup and function of the two embodiments of the system are different, but both embodiments utilize changes of brightness of individual speckles as the specimen is deformed. 
     Optionally included is a capability that allows for the regulated change in phase of one of the divided beams. This allows the imaging system to take additional brightness data for a given speckle and calculate precisely the displacement for that speckle without the necessity of creating fringe patterns. As such, the current system provides an efficient non-contacting system that can measure both in-plane and out-of-plane translation of the surface of a specimen. 
     In view of the above description it is an object of this invention to provide a method and apparatus for measurement of strains in structures of all kinds. 
     It is further an object of the present invention to measure the relative magnitude of displacements from an original position on different points on a surface of an object under stress. 
     It is yet another object of the present invention to provide an improved technique and apparatus for performing electronic speckle pattern interferometer in the analysis of motion, strain, and deformation of all kinds of structures, components, bodies and materials. 
     It is yet another object of the present invention to provide a speckle interferometer which will be useful in engineering, manufacturing, medicine, and natural science to provide precise measurements without the necessity of heavy investment in equipment; and, in its commercial form, can be used by untrained persons in field and industrial environments. 
     It is yet another object of the present invention to provide an interferometer which is greatly simplified in comparison to traditional setups and is more resistant to vibration and other noises which tend to contaminate the result produced by an electronic speckle pattern interferometer. 
     It is yet another object of the present invention to provide an interferometer apparatus which affords the capability of placing the illumination sources, (the fiber terminations) on a testing machine or even attaching them to the specimen; while the laser and other components are at a separate locations. In this way, remote measurement of strain in hostile environments can be conducted. 
     It is yet another object of the present invention to provide an interferometer apparatus which is set up to measure in-plane displacement or strains. 
     It is yet another object of the present invention to provide an interferometer apparatus that optionally contains collimating lenses for more accurately measuring in-plane or out-of-plane displacement. 
     It is yet another object of the present invention to provide an interferometer apparatus that contains a phase shifter along one of the fiber optic cables for changing the phase of one of the laser beams to improve strain measurements in the systems. 
     It is yet another object of the present invention to provide a method and apparatus for the control, analysis and calibration of the phase shifting, the development of displacement maps, determination of specimen displacements, extraction of signal noise from the signal, calculation of strain from displacements maps, and display of displacement and strain maps performed by a computer. 
     It is yet another object of the present invention to provide an interferometer apparatus utilizing the speckle effect having a phase shifter disposed before the specimen. The speckle interferometer includes a laser, an integrated monobloc adjustable beam splitter and fiber optic coupler, a second adjustable fiber optic splitter, a phase shifter, and a recording medium. 
     The foregoing as well as other objects, features and advantages of the present invention will become more apparent from the following detailed description when taken in conjunction with the appended drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic diagram of the apparatus of the invention; 
     FIG. 2 is a schematic diagram of an alternate embodiment of the apparatus of the invention; 
     FIG. 3 is further an alternative embodiment of the apparatus of the invention; and 
     FIG. 4 is further an alternative embodiment of the apparatus of the invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention uses the techniques of the speckle interferometer method. In accordance with this method, all necessary information to compute displacements and strains in an arbitrary shaped and oriented surface is determined by recording two or more images of an illuminated surface. 
     In one embodiment, fringes occur upon making a pair of exposures of the interference patterns made before and after deformation of a rough surface. The relative magnitude of the displacements and strains from the original position at different points on the surface can be determined from the position of, movement, and changes in intensity of the fringes. 
     In another embodiment, phase shifting is used to precisely determine the displacements and strains. In this case, several images are captured before and after deformation of the specimen surface, each image being taken with an arbitrary phase shift. 
     FIG. 1 shows a schematic representation of an electronic speckle pattern interferometer being one embodiment of this invention. From a functional point of view, the system comprises the following components: an illumination system  10 ; a commercially available adjustable polarizing beam splitter fiber coupler  12 ; a pair of fiber optics  16 ,  18  coupled with collimating lenses  24 ,  26 . The illumination system can be a standard HeNe laser, an ion laser, compact solid-state laser, or a laser diode. The system further comprises a CCD camera  35  having a lens  34  coupled to a computer and frame grabber  60  having a display monitor  70 . Of course, other imaging systems utilizing traditional film cameras or video equipment are usable and equivalent. 
     As best seen in FIG. 1, the laser  10  produces a beam which is split into two object beams by the beam splitter fiber coupler  12  and transmitted through fibers  16  and  18 . The beams then leave the fibers  16 ,  18  and pass through optional filters  20 ,  22  and preferably enter collimating lenses  24 ,  26 . It has been observed that the addition of collimating lenses improves the accuracy of the system by 20% in comparison to a system without collimating lenses. The beams  23 ,  27  leave the collimating lenses  24 ,  26  and are overlappingly projected onto the test specimen&#39;s  25  surface  28 . The combined light beams reflect off of surface  28  forming a speckled pattern. The lens  34  with CCD camera  35  captures an image of the speckle pattern. This image is received and processed by the computer and accompanying frame grabber  60 . The image is then stored within the computer  60  for use in strain calculations. In operation, the use of a second object beam as carried by fiber  18  when allowed to overlap the illuminated portion of the beam as carried by fiber  16  will allow the computation of in-plane displacement of a specimen  25  while the specimen  25  is subject to loading. In this configuration, the critical changes of speckle brightness are not affected by out-of-plane displacement. 
     The specimen is then subject to loading. This causes deformation of the specimen which leads to the movement of the specimen&#39;s surface  28 . This movement causes variation of the individual speckles by in-plane displacement. The imaging system then captures the image of the modified speckle pattern. The imaging system then analyzes the images and calculates the change in brightness of each individual speckle. A calculation is made which converts the change of brightness into a map of the strain on the specimen&#39;s surface  28 . In this case the specimen surface displacement is represented by a system of fringes. 
     As best seen in FIG. 2, a PZT phase shifter  32 , or optionally  32 ′, can be utilized to stretch a length of fiber on one branch of the optical path. As known in the art, at least three images using the phase shifter are taken before the specimen is subjected to stress and the same number of images are taken after. This information is used by the computer to calculate a precise contour map of the displacements on the specimen  25  using methods well known in the art. 
     As previously mentioned, the addition of collimating lenses  24  and  26  allow for an accurate reading over the whole field as opposed to the described system without collimating lenses. This is particularly true in systems utilizing a phase shifter  32 . It is also possible to control the phase shift of one of the beams in the fiber by using a heat source to control the length of the fiber. 
     Yet a further embodiment of the current invention can be seen in FIG.  3 . In the current configuration of the interferometer, out-of-plane displacements and strains can be measured with great accuracy. As shown, a laser  10  projects a light beam into the commercially available beam splitting fiber coupler  12 . The beam is split into object and reference beams and sent down fiber optic cables  16  and  18 . Light traveling through cable  16  exits the fiber and is collimated by collimating lens  24  to be dispersed on the surface  28  of specimen  25 . The light is reflected off of surface  28  in the form of a speckle pattern and is captured by lens  34  and projected through a second beam splitter  36  into CCD camera  35 . The reference beam  27  from the second fiber optic cable  18  is likewise projected through the beam combiner  36  and combined with the light reflected off of surface  28  into camera  35 . In this configuration, the speckle brightnesses are affected primarily by out-of-plane motion of the specimen surface  28 . 
     In operation, the speckle pattern reflected off of surface  28  and captured by CCD camera  35  is stored in computer  60 . The specimen  25  is subject to loading causing a change in the speckle pattern. This new pattern is recorded by the CCD camera. The computer then compares the first and second stored images to calculate the resultant out-of-plane displacements and strains of the surface  28 . Again, the displacements are represented by the computer as a system of fringes. 
     The addition of an optional PZT phase shifter  32  coupled to second fiber optic cable  18  carrying the reference beam  27  can be used to produce a precise contour map showing the out-of-plane displacement of the specimen  25  while the specimen is being subjected to loads. 
     What is envisioned as a commercially feasible embodiment of the current invention can be seen in FIG.  4 . In the current configuration of the interferometer, in-plane as well as out-of-plane displacements and strains can be measured with great accuracy. This particular embodiment combines the advantages and features of the interferometers shown in FIGS. 2 and 3 with an additional beam splitter  12 ′. The beam splitter contains a mechanism  38  that allows light traveling along fiber  18  to be transmitted to either the beam splitter  36  or collimating lens  26 . This improvement allows a single speckle interferometer system to be used to measure both in-plane and out-of-plane displacements. This is a particularly important improvement in as much the system must only be set-up and aligned once, saving significant time and effort. This improvement also allows for an interferometer in a single transportable package, which can measure in-plane and out-of-plane displacements. 
     When measuring in-plane displacements, mechanism  38  directs the light toward collimating lens  26 . The function of the system is identical to the function of the interferometer as described in FIG.  2 . To measure out-of-plane displacements, mechanism  38  directs the light away from the collimating lens  26  toward beam splitter  36 . The system then functions as described in the description of FIG.  3 . 
     The foregoing discussion discloses and describes exemplary embodiments of the present invention. One skilled in the art will readily recognize from such discussion, and from the accompanying drawings and claims, that various changes, modifications and variations can be made therein without departing from the spirit and scope of the invention as defined in the following claims.