Apparatus and method for performing electronic shearography

Abstract: Two shearograms, taken while an object (3) is unstressed and stressed, respectively, are compared electronically to yield a composite interference pattern. Each shearogram is formed by the interference of pairs of distinct rays of coherent light, reflected from different points on the object (3). The first ray of each pair is reflected from the object, strikes a mirror (5) spaced from the object (3), and is reflected from the mirror (5). At the same time, the second ray of each pair is reflected from the object (3) and strikes a beam splitter (7) positioned beside the mirror. The beam splitter directs a portion of the second ray in the same direction as the reflected first ray, and a portion of the first ray passes through the beam splitter without being reflected. Since the two rays have the same polarization angle, and are mutually parallel, the rays interfere. A detector (9) is positioned to receive light from the beam splitter (7). The detector observes the interference between pairs of rays over the entire field of view, and the result is an interference pattern, i.e. a shearogram, formed from the optical interference of the two laterally-displaced images of the object.

Description: This invention relates to the field of nondestructive testing. In particular, the invention relates to electronic shearography, which produces interference patterns generated by laterally-displaced images of an object. Early laser-based systems for nondestructive testing used holography, wherein a beam reflected from the test object is made to interfere with a reference beam. Examples of patents showing holographic systems for performing such testing are US-A-3 828 126 and US-A-4 304 458. The major disadvantage of holography is that, by definition, it requires an object beam and a reference beam. Because two beams are needed, the optical arrangement is relatively complex. Moreover, a holographic system is especially sensitive to vibration. For these reasons, holography has been used mainly in the laboratory, and has not been satisfactory as a technique for inspecting manufactured parts in a factory environment. The technique of shearing interferometry, or "shearography", has been recognized as a preferred alternative to holographic interferometry. In shearography, there is no separate reference beam. Instead, two images of the same object are made to interfere, forming a pattern that can be recorded. A first shearogram is taken while the object is in an unstressed condition, and another shearogram is taken while the object is stressed. Comparison of the two shearograms can reveal information about the strain concentrations (and hence the integrity) of the object. Various methods have been proposed for using shearography in non-destructive testing for example in US-A-4 139 302. In the latter patent, the shearing is accomplished by a wedge-shaped prism which lies along a portion of a lens. The light rays passing through the prism are displaced relative to those which do not pass through the prism. In this way, the system produces two laterally-sheared interfering images of the object. While the apparatus shown in the latter patent (and in other comparable references) does produce acceptable results, it is not capable of generating interference patterns in "real time". The interference patterns generated must be recorded on a high-resolution photographic film. To obtain a shearogram, one must wait to develop the film, and must use film-developing chemicals. Thus, a film-based system cannot be used to view the instantaneous status of a test object. In order to view a "real time" image, one must record the interference pattern with a video camera, or its equivalent. Until recently, it was virtually impossible to use a video camera to record shearograms because the spatial frequency of the interference patterns generated by the then known shearography methods was great, far beyond the resolving power of available video cameras. The only known workable system for practicing shearography in "real time" is that described in US-A-4 887 899. The latter patent is a remarkable advance over the prior art discussed above. It discloses a device which forms a shearogram by passing light, reflected from a test object, through a birefringent material and a polarizer. The birefringent material, which can be a calcite crystal, splits a light ray, reflected from a point on the object, into two rays, and the polarizer makes it possible for these rays to interfere with each other. It turns out that the spatial frequency of the shearogram produced with this arrangement is relatively low, because the effective angles between the interfering rays are small. Each shearogram is stored in a computer, or in an equivalent device, and the shearograms taken before and after deformation of the object are compared electronically. The result is a composite interference pattern which graphically shows the condition of the object. By storing the shearogram of the object in its initial, unstressed condition, and by comparing that shearogram, almost instantaneously, by computer, with further shearograms taken under varying levels of stress, a "real time" image of the resultant strains on the object can be observed. As used herein, the term "electronic shearography" means a shearography process in which the interference patterns are recorded and analyzed electronically, as by a video camera, in contrast to processes which record patterns on photographic film. US-A-4 887 899 describes the first known commercially viable method for electronic shearography. Although the system shown in US-A-4 887 899 is commercially useful, it still has certain disadvantages. One of these is the inability to control easily the amount of shear (also called the "angle of shear"). The amount of shear is fixed, and is determined mainly by the optical properties of the birefringent material. But it is sometimes necessary to vary the amount of shear; a smaller angle of shear (i.e. in a system having reduced sensitivity) is preferable in a high-vibration environment, and conversely. To change the amount of shear, in the above-described system, one must replace the optical element. Also, the above-described system is subject to losses caused by attenuation of the light beams as they pass through the birefringent material and polarizer. Non prepublished EP-A-0 485 728 having designated states DE, GB, FR, IT discloses an apparatus for determining changes of path lengths of beams. The apparatus comprises two mirrors arranged close to a beam splitter to receive and reflect light reflected by or transmitted from the beam splitter. The light reflected from both mirrors towards the beam splitter is then reflected by or transmitted from the beam splitter to be directed into a video camera or a CCD-array. The beams directed towards the camera interfere between the beam splitter and the camera. Received images of a stressed and unstressed object are stored in a memory and are subtracted from each other. In view of that document, separate claims were filed for DE, GB, FR, IT. It is therefore the object of the present invention to provide an improved apparatus for performing electronic shearography and an improved method for generating an interference pattern. This object is achieved according to the invention by the appartus and method claimed in the independent apparatus and method claims respectively. Embodiments of the invention are claimed in the independent claims. The present invention provides a new apparatus of practicing electronic shearography. The invention solves some of the problems mentioned above. In particular, the invention makes it easier to adjust the amount of shearing, and thereby to adjust the sensitivity of the system, without replacing the optical element. The invention also tends to reduce the power requirements for the illuminating laser, because it minimizes the attenuation of the light reflected from the test object. The invention also allows the reflected beam to be phase-stepped, thereby providing more detailed information about the imaged strains. According to the present invention, a source of coherent light, such as a laser, illuminates a test object. A mirror is positioned near the object, such that light reflected from the object is also reflected by the mirror. A beam splitter is positioned near the mirror, such that light reflected from the mirror passes through the beam splitter, and such that a portion of the light reflected from the object can also be reflected by the beam splitter. The arrangement of the mirror and beam splitter is such that beams reflected from pairs of distinct points on the object are reflected, respectively, by the mirror and beam splitter, and are thus directed into parallel paths. A detector in form of a video camera or equivalent, is placed in the path of the light emerging from the beam splitter. The beams emerging from the beam splitter interfere with each other, virtually to the maximum degree possible, because they are parallel and have the same polarization angle. An interference pattern is observed at the video camera, and stored in a computer, or its equivalent, for further processing. The amount of shearing can be varied simply by adjusting the orientation of the mirror. When the orientation of the mirror is changed, a different set of distinct beams will interfere with each other. The latter is equivalent to a statement that the amount of shearing is varied. Varying the amount of shearing varies the sensitivity of measurement. In an alternative embodiment, the light exiting the beam splitter is directed by a second mirror into the detector. This arrangement permits the detector to be positioned in the most convenient orientation. In a further apparatus, there are two mirrors, disposed at or near adjacent sides of the beam splitter. One mirror may be linearly movable and the other mirror may be rotatable. Linear motion of the first mirror varies the phase angle of the interference pattern. Rotation of the second mirror varies the angle of shear. The path lengths of the beams reflected from the mirrors are the same, or nearly the same, so that the coherent length requirement is reduced. An interference pattern, made with any of the embodiments described above, is formed twice, once while the object is in an unstressed condition, and once while the object is stressed. The patterns could also be made at two different levels of stress. Each interference pattern is stored in a memory of a computer, or equivalent. The two interference patterns are then compared electronically to obtain a composite interference pattern. If the comparisons are performed by computer, the result can be a "real time" image showing changes in the object due to applied stress. In a method according to the present invention, one obtains shearograms, for the stressed and unstressed conditions, in the manner described above, for each of a plurality of linear positions of the mirror. The mirror can be moved to these positions by a piezoelectric phase-stepping mirror driver which changes the beam path length by fractions of a wavelength. The composite interference patterns obtained for each phase-shifted position are then averaged, to obtain a final composite interference pattern giving greater detail. The invention provides a method and an apparatus for performing electronic shearography and an optical apparatus for generating a shearogram. With the method and the apparatus according to the invention non-destructively analyzing of manufactured parts is possible. Furthermore, the amount of shearing can be easily and continuously varied. An embodiment of the invention which is claimed for all designated states except DE, FR, GB, IT provides a two-mirror apparatus for electronic shearography, which apparatus reduces the coherent length requirement of the laser used to illuminate the test object. Further, the power losses in an electronic shearography system are reduced, and thereby the required laser power is minimized. According to the apparatus and the method of the invention the detector can be oriented in the most convenient direction. The method according to the invention provides patterns representing the average of a plurality of shearograms, taken at various phase angles. Advantages of the invention will be apparent to those skilled in the art, from a reading of the following brief description of the drawings, the detailed description of the invention, and the appended claims. Figure 1 is a schematic diagram illustrating the preferred embodiment of the apparatus according to the present invention, and showing the paths of travel of four sample rays of light. Figure 2 is a schematic diagram showing the optical components of the apparatus according to the invention, and showing an approximate determination of the intensity of the interference pattern at the detector. Figure 3 is a schematic diagram showing an alternative embodiment of the invention. Figure 4 is a schematic diagram showing another apparatus according to the invention, in which there are two mirrors disposed near adjacent sides of a beam splitter. This apparatus is claimed for all designated states except DE, FR, GB, IT. Referring to Figure 1, a Laser 1 directs coherent light onto the surface of a test object 3. A beam expander 2 causes the light beam from laser 1 to diverge in the direction of the object. Mirror 5 and beam splitter 7 are positioned near the object, so as to receive light reflected therefrom. Detector 9 is positioned near the beam splitter, as shown. The detector is preferably a video camera, but it can be any device which is capable of detecting an image, virtually instantaneously, for subsequent digital processing. The detector includes lens 11 and a photosensitive screen 13. Images are focused onto the screen by the lens. Detector 9 is connected to computer 15 which stores the images formed on the screen, and processes the stored images digitally. Figure 1 shows how the apparatus performs shearography. Consider the light reflected from two nearby points on the object, the points being labeled A and B. The first beam reflected from point A strikes mirror 5, and is reflected by the mirror to generate a mirror output beam that is directed towards beam splitter 7 defining a first input beam. One portion of the first input beam is transmitted through the beam splitter, towards detector 9, defining a first output beam, and the other portion, labeled A', is reflected, as indicated by the dotted line. The beam splitter is represented symbolically as two prisms, joined along a common surface. In the preferred embodiment, the beam splitter is actually constructed of two prisms. However, the beam splitter is not limited to a particular structure; any device which transmits a portion of an incident ray, and reflects the remainder, can be used. Note also that, in general, the mirror is not parallel to the plane of the common surface of the prisms (which is the plane at which incident beams are reflected.) A second input beam reflected from a second point B strikes beam splitter 7. One portion of that beam is reflected from the beam splitter, defining a second output beam, and the remainder, labeled B', is transmitted through the beam splitter, also indicated by a dotted line. The beams illustrated by the dotted lines (A' and B') are not of concern here. The reflected beam originating from point B is reflected in a direction which coincides with that of the beam originating from point A. Thus, the reflected beams from points A and B are directed into the same path. Because both beams have been reflected exactly once, after being reflected from the object itself, both beams will have the same angle of polarization. Since their polarization angle is the same, and the wavefronts are parallel, these rays interfere with each other to the maximum extent possible. The interfering beams pass through lens 11 which focuses the beams onto screen 13. If the interfering beams did not have the same angle of polarization, they would interfere only partially. Partial interference reduces the signal-to-noise ratio, as the components of the beams which do not interfere would add to the background signal, causing loss of sensitivity. It is absolutely necessary that the beams have polarization angles which are not mutually orthogonal; otherwise, the beams would not interfere at all. It is preferable that the polarization angles be equal, to maximize the signal-to-noise ratio. A similar analysis applies in the case of beams reflected from points C and D. Figure 1 therefore shows that the detector can observe a wide range of points on the object, and is not limited to only a small area. From Figure 1 and the above description, it follows that the image at each point on the detector screen comprises the superposition of two beams reflected from two distinct points on the object. Therefore, what is observed at the detector is an interference pattern equivalent to the superposition of two laterally-displaced ("sheared") images of the object. In other words, the apparatus of Figure 1 generates a shearogram. The images formed in the detector are stored and processed by computer 15, or its equivalent. The processing steps can be essentially the same as those described in US-A-4 887 899. As in the above patent, shearograms can be taken while the object is in an unstressed condition, and while the object is stressed. Stressing the object at, say, point A, changes the effective path length of the beam reflected from point A, thus changing the phase angle between the beams reflected from points A and B. This change perturbs the interference pattern, and the change can be observed by comparing the two patterns. Note that the shearograms can also be taken while the object is at two different conditions of stress, and is not limited to the special case in which the object is unstressed initially. It is therefore understood, throughout this specification, that wherever it is stated that shearograms are taken with the object in the "stressed" and "unstressed" conditions, the same could be done while the object is subjected to two distinct levels of stress. Another useful procedure, described above with respect to US-A-4 887 899, is to store the shearogram formed while the object is unstressed, and then to form and store shearograms repeatedly, in "real time", as the object is stressed. The computer is programmed to compare electronically the original shearogram (representing the unstressed condition) with the shearogram taken at a given instant, and to display the instantaneous results of the comparison on a video screen. Thus, the apparatus can display a "real time" image representing the current state of the object. Two major advantages of the described apparatus and the described method arise from its ability to adjust the position of the mirror both rotationally and linearly. Changing the amount of shearing can be done simply by rotating mirror 5 slightly. In Figure 1, mirror adjuster 17 can be used to vary the position of the mirror. It is understood that adjuster 17, as shown in the figure, represents any device or devices capable of causing linear and/or rotational movements of the mirror. Rotational movements cause variations in the amount of shearing; linear movements vary the path length of the light ray reflected from the mirror, and are useful in the technique described below. The mirror adjuster can be a motor, a piezoelectric crystal, or any other device capable of causing large or small movements. Rotation of the mirror changes the set of points "A" which give rise to light which interferes with light from points "B". This is equivalent to saying that the relative displacement of the sheared images is varied. Note that the amount of shearing can thus be continuously varied. The ability to vary the linear position of mirror 5 can also be exploited in the following method of analyzing interference patterns. The following method reduces "noise" in the final pattern, due to laser speckle. A piezoelectric crystal, or equivalent device, is used to move the mirror linearly, shifting the phase of the light from set of points "A". Each movement of the mirror is extremely small, i.e. a fraction of a wavelength. In one example, the mirror is "stepped" through four discrete positions, representing changes in the path length of one-quarter wave length. A shearogram is obtained for the first position, and is stored electronically. Then, a shearogram is obtained for each of the four quarter-wave positions while the object is stressed. Thus, four shearograms are taken for each of the four linear positions of the mirror, each shearogram being stored as before. Then, each phase-shifted shearogram is compared with the initial shearogram. That is, the shearogram taken in the first position, while the object is deformed, is compared with the stored shearogram taken while the object is undeformed. The same is done for each of the other mirror positions. Finally, the four composite patterns are averaged, or otherwise compared, or image processed, to obtain a final pattern. The final pattern has reduced speckle "noise". The method used to compare the shearograms taken for each given phase angle can simply be the subtraction of one pattern from the other, or it can be any of the other methods described in US-A-4 887 899, or other methods. In the above-described method, the number of times the mirror is "stepped" can be varied. In general, the greater the number of "steps", the greater the sensitivity of the method, and the longer the time required to perform the analysis. In the above-described method, a piezoelectric crystal is preferred because it enables one to move the mirror by very small distances, of the order of a fraction of a wavelength, in a controlled manner, according to a voltage applied to the crystal. Piezoelectric crystals have been used, in other fields, for producing small controlled movements of objects, and such devices are therefore commercially available. However, the above-described method is not limited to the use of a piezoelectric crystal. Another advantage of the described apparatus, and the described method is that they make efficient use of the radiation reflected from the object. This point is more clearly illustrated in Figure 2. For the sake of simplicity, Figure 2 shows only two beams, labeled A and B, reflected from the object (not shown in Figure 2). Also, the source of coherent radiation is omitted from Figure 2. In Figure 2, beam A enters beam splitter 7. Approximately one half of beam A is transmitted and the other half is reflected. The reflected beam travels towards lens 11, which focuses the beam onto screen 13. At the same time, beam B is reflected from mirror 5, onto the beam splitter. Approximately one half of beam B is reflected by the beam splitter, and the other half is transmitted through the beam splitter, towards lens 11, and eventually onto screen 13. Thus, the intensity of the radiation received at the screen can be represented as 0,5A +0,5B. Since A and B are approximately equal (both beams originate from the same source, and are reflected from the same object), it follows that the intensity observed at a point on the screen, due to the interference of two beams, is approximately equal to the intensity of one of the beams reflected from the object. Thus, the amplitude at the detector is of comparable magnitude to the amplitude of the light reflected from the object. The above-described feature contrasts with the shearography systems of the prior art, wherein the optical elements inherently cause substantial attenuation of the light reflected from the object. By reducing the attenuation, one reduces the power requirements of the laser. One requirement of the above-described embodiments is that the laser have a relatively long coherent length. In general, the difference between the path lengths of the reflected beams must be no greater than the coherent length of the laser, i.e. the length beyond which the laser beam ceases to be coherent. Figure 3 shows an alternative embodiment. In this embodiment, laser 21 illuminates object 23. Coherent light is reflected from the object onto mirror 25 and also onto beam splitter 27. A second mirror 29 directs light exiting the beam splitter into detector 31. This embodiment permits the detector to be "pointed" towards the object. This embodiment may be used where the available space is such that it is more convenient for the detector to be pointed towards the object. Also, mirror 29 can be oriented in other ways, depending on the position of the detector. Thus, Figure 3 shows that the detector can be positioned in whatever manner is most convenient, the mirror 29 being used to direct the light from the beam splitter into the detector. Figure 4 illustrates another apparatus with two mirrors 39 and 41. Mirror 39 is linearly movable, as indicated by the arrows. Mirror 41 is rotatable, also as indicated by arrows. Beam splitter 33, lens 35, and screen 37 are similar to the corresponding components of the other embodiments. Figure 4 shows beams A and B, which originate from nearby points on the object. (For purposes of simplicity, neither the laser nor the object is shown in Figure 4.) A portion of a first input beam from point A is reflected from the beam splitter onto a second mirror 41. A portion of the reflected beam then passes through the beam splitter producing a first output beam directed towards the lens and screen. At the same time, a portion of a second input beam from point B passes through the beam splitter, and is reflected from a first mirror 39. A portion of the reflected beam is then reflected by the beam splitter producing a second output beam directed towards the lens and screen. Thus, the arrangement of Figure 4 generates a shearogram. Note that Figure 4 does not show the portions of the beams that are lost due to the beam splitter. In the apparatus of Figure 4, linear displacement of mirror 39 causes phase shifts, so that the apparatus of Figure 4 can be used to practice the method, described above, wherein shearograms are taken at different phase angles. Rotation of mirror 41 changes the angle of shear, i.e. the amount of separation of the laterally-displaced images. As mentioned above, varying the amount of shear varies the sensitivity of measurement. Note that the apparatus of Figure 4 can be used even where the mirrors are not moved or rotated. Also, it is possible to make mirror 39 rotatable and mirror 41 linearly movable. It is also possible to make both mirrors linearly movable or to make them both rotatable. The advantage of the apparatus of Figure 4, whether or not the mirrors are movable or rotatable, is that the path lengths of the rays originating from points A and B are the same, or almost the same. Therefore, the coherent length requirement of the laser is substantially reduced, as compared with the other embodiments. The described apparatus discussed above can be modified in many ways. The particular means of analyzing the shearograms generated by the apparatus can be varied. It is also possible to vary the order in which shearograms for stressed and unstressed conditions of the object are taken. Additional mirrors can be added, to direct the interfering beams to the detector, if it is necessary to change the position of the detector, subject to the requirement that the total path length of the beams be less than the coherent length of the laser. As discussed above, it is possible to make some or all of the mirrors of the apparatus adjustable, either by rotation or linear displacement, or both. Except for the coherent length requirement, the described apparatus and the described method are not limited by the type of laser used, or by the particular structure of the beam splitter. The apparatus can be used to test virtually any kind of opaque or translucent object. The invention is not limited to a particular type of detector.

Claims: Claims for the following Contracting States : AT, BE, CH, DK, ES, GR, LI, LU, NL, PT, SE Apparatus for performing electronic shearography, comprising: a) means (1) for generating a beam of coherent light, the beam generating means being positioned to direct the coherent light onto a test object (3), b) a mirror (5) located sufficiently near the object (3) and to receive a first beam reflected from a first point (A) on the object (3) and to reflect it to produce a corresponding mirror output beam, c) a beam splitter (7) arranged to receive directly a second input beam reflected from a second point (B) close to the first point (A) on the object (3), and a first input beam which is the mirror output beam received directly from the mirror (5), d) the beam splitter (7) being arranged to produce a second output beam by reflecting the second input beam and a first output beam parallel to the second output beam by transmitting the first input beam, and e) a video camera (9) positioned to receive the first and second output beams from the beam splitter (7). The apparatus of Claim 1, characterized in that the apparatus includes means (2) for expanding the beam of light produced by the generating means (1), before the beam reaches the object (3). The apparatus of Claim 1 or 2, further characterized in that the apparatus includes image processing means (15) for storing an image formed in the video camera (9), wherein an interference pattern is formed in the camera (9), and wherein the image processing means (15) comprises means for comparing electronically two interference patterns so formed. The apparatus of any of Claims 1-3, characterized in that the apparatus includes means (17) for adjusting the position of the mirror (5). The apparatus of any of Claims 1-4, characterized in that the apparatus also includes a second mirror (29) disposed to receive the ouput beams from the beam splitter (7) and to reflect the light beams received from the beam splitter (7) into the video camera (9). Apparatus for performing electronic shearography, comprising: a) means (1) for generating a beam of coherent light, the beam generating means being positioned to direct the coherent light onto a test object (3), b) a beam splitter (33) arranged to receive directly a first input beam reflected from a first point (A) on the object (3) and a second input beam reflected from a second point (B), close to the first point(A), on the object (3), c) first and second mirrors (39, 41) located near the beam splitter (33), wherein the first input beam reflected from first point (A) is reflected by the beam splitter (33) onto the second mirror (41) and reflected by the second mirror (41) and transmitted through the beam splitter (33) to produce a first output beam, d) wherein the second input beam reflected from the second point (B) is transmitted through the beam splitter (33) onto the first mirror (39) and reflected by the first mirror (39) and then reflected by the beam splitter (33) to produce a second output beam parallel to the first output beam, and e) a video camera (37) positioned to receive the first and second output beams from the beam splitter (33). The apparatus of Claim 6, wherein at least one of the mirrors (41) is rotatable, and wherein at least one of the mirrors (39) is linearly movable. A method of generating an interference pattern from two laterally-displaced images of a test object (3), the method comprising the steps of: a) illuminating the object (3) with coherent light, b) reflecting a first beam of said coherent light directly from a first point (A) on the object (3) to a mirror (5), and transmitting said reflected first beam directly through a beam splitter (7) and into a video camera (9), c) reflecting a second beam of said coherent light directly from a second point (B) on the object (3), the first and second points (A, B) being close to each other, said second beam being reflected directly to the beam splitter (7), a portion of said second beam being reflected by the beam splitter (7) into the video camera (9) such that said second beam is substantially parallel to said first beam, and such that said first and second beams interfere with each other, and d) observing with the video camera (9) an interference pattern caused by said first and second beams generated for various pairs of points on the object (3), whereby the interference pattern represents the interference between two laterally-displaced images of the object (3). The method of Claim 8, characterized in that the illuminating step comprises the step of providing a source (1) of coherent radiation and a beam expander (2) positioned adjacent the source, wherein the beam which reaches the object (3) is an expanded beam. The method of Claim 8 or 9, further characterized in that the method includes the step of adjusting the linear position of the mirror (5) so as to vary the length of the path of said first beam reflected from the mirror (5). The method of Claim 10, wherein the mirror (5) is adjusted so as to record an interference pattern for a plurality of linear positions of the mirror (5), wherein interference patterns are obtained, for each of said linear positions, while the object (3) is in both of two conditions of stress, the method further comprising the steps of deriving composite interference patterns from a comparison of patterns taken while the object (3) is in two conditions of stress, for each given linear position of the mirror (5), and averaging the composite interference patterns so derived, to form a final interference pattern. Claims for the following Contracting States : DE, FR, GB, IT Apparatus for performing electronic shearography, comprising: a) means (1) for generating a beam of coherent light, the beam generating means being positioned to direct the coherent light onto a test object (3), b) a mirror (5) located sufficiently near the object (3) to receive a first beam reflected from a first point (A) on the object (3) and to reflect it to produce a corresponding mirror output beam, c) a beam splitter (7) arranged to receive directly a second input beam reflected from a second point (B) close to the first point (A) on the object (3), and a first input beam which is the mirror output beam received directly from the mirror (5), d) the beam splitter (7) being arranged to produce a second output beam by reflecting the second input beam and a first output beam parallel to the second output beam by transmitting the first input beam, and e) a video camera (9) positioned to receive the first and second output beams from the beam splitter (7). The apparatus of Claim 1, characterized in that the apparatus includes means (2) for expanding the beam of light produced by the generating means (1), before the beam reaches the object (3). The apparatus of Claim 1 or 2, further characterized in that the apparatus includes image processing means (15) for storing an image formed in the video camera (9), wherein an interference pattern is formed in the camera (9), and wherein the image processing means (15) comprises means for comparing electronically two interference patterns so formed. The apparatus of any of Claims 1-3, characterized in that the apparatus includes means (17) for adjusting the position of the mirror (5). The apparatus of any of Claims 1-4, characterized in that the apparatus also includes a second mirror (29) disposed to receive the output beams from the beam splitter (7) and to reflect the beams received from the beam splitter (7) into the video camera (9). A method of generating an interference pattern from two laterally-displaced images of a test object (3), the method comprising the steps of: a) illuminating the object (3) with coherent light, b) reflecting a first beam of said coherent light directly from a first point (A) on the object (3) to a mirror (5), and transmitting said reflected first beam directly through a beam splitter (7) and into a video camera (9), c) reflecting a second beam of said coherent light directly from a second point (B) on the object (3), the first and second points (A, B) being close to each other, said second beam being reflected directly to the beam splitter (7), a portion of said second beam being reflected by the beam splitter (7) into the video camera (9) such that said second beam is substantially parallel to said first beam, and such that said first and second beams interfere with each other, and d) observing with the video camera (9) an interference pattern caused by said first and second beams generated for various pairs of points on the object (3), whereby the interference pattern represents the interference between two laterally-displaced images of the object (3). The method of Claim 6, characterized in that the illuminating step comprises the step of providing a source (1) of coherent radiation and a beam expander (2) positioned adjacent the source, wherein the beam which reaches the object (3) is an expanded beam. The method of Claim 6 or 7, further characterized in that the method includes the step of adjusting the linear position of the mirror (5) so as to vary the length of the path of said first beam reflected from the mirror (5). The method of Claim 8, wherein the mirror (5) is adjusted so as to record an interference pattern for a plurality of linear positions of the mirror (5), wherein interference patterns are obtained, for each of said linear positions, while the object (3) is in both of two conditions of stress, the method further comprising the steps of deriving composite interference patterns from a comparison of patterns taken while the object (3) is in two conditions of stress, for each given linear position of the mirror (5), and averaging the composite interference patterns so derived, to form a final interference pattern.

Inventors: NEWMAN JOHN W, TYSON JOHN II, NEWMAN, JOHN W., TYSON II, JOHN
Applicants: LASER TECHNOLOGY INC, LASER TECHNOLOGY, INC.(A CORPORATION OF PENNSYLVANIA)
Application year: 1992
Main classification: G01L1
IPC codes: G01B11, G01L1