Method and apparatus for comparing two objects having similar shapes and dimensions
United States Patent [191 Mathieu Apr. 22, 1975 METHOD AND APPARATUS FOR COMPARING TWO OBJECTS HAVING SIMILAR SHAPES AND DIMENSIONS [75] Inventor: Ernst Mathieu,Aulnay-Sous-Bois.  
 France [73] Assignee: Compagnie Electro-Mecanique. Paris, France 22 Filed: Dec. 20, 1973 21 Appl. No.: 426,436  
 [30] Foreign Application Priority Data Dec. 21. 1972 France 72.45722 [52] HS. C1. 356/168; 356/156; 356/162; 356/169; 356/171; l78/D1G. 36 [51] Int. Cl. G0lb l1/24 [58] Field of Search 356/156, 162, 168, 169, 356/171, 2; l78/D1G. 36; 250/237 G [56] References Cited UNITED STATES PATENTS 3.546.377 12/1970 Troll 356/168 X 3.619.064 11/1971 Brooks 356/156 X 3.663.107 5/1972 Denis et a1 250/237 G 3.814.521 6/1974 Frcc 356/156 Primary E.\&#39;aminerRonald L. Wibert Assislant E.\&#39;aminerPaul K. Godwin Attorney. Agent, or FirrnPierce, Scheffler &amp; Parker [57] ABSTRACT A method and apparatus for comparing two objects having similar shapes and dimensions wherein one of the objects is illuminated by a collimated light beam whose intensity is modulated in a direction transverse thereto at a spatial frequency l/p. The modulated light beam generates a lattice of alternately dark and light parallel bands similar to interference fringes having a pitch p on the illuminated surface of the object having a configuration in the manner of contour lines determined by the relief of the illuminated side of the object. The other object is thereafter substituted for the first one and in the same position, and is similarly illuminated by the same beam but wherein the intensity modulation has been phase-shifted by 180. The respective images of the two objects are optically superimposed and from which is then obtained only the differential component of the image which is modulated with a spatial frequency l/p.  
 Apparatus for carrying out the method can be either (1) photographic in which case the superimposed images of the two objects are established by development on a photo-sensitive negative which is light-transmissive in conjunction with a lens which transmits only the differential component in the negative to a photographic plate. or (2) electronically by means of a TV camera and related components by which the respective video images are added and passed to a visual display device such as a cathode ray tube via a filter having a pass band transmitting only the differential signal component which is modulated by the time frequency \&#39;/;1 wherein r constitutes the linear sweep speed of the TV camera.  
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 METHOD AND APPARATUS FOR COMPARING TWO OBJECTS HAVING SIMILAR SHAPES AND DIMENSIONS The present invention relates to a process and to an apparatus for comparing two objects closely related in shape and dimensions.  
  A process of a similar kind already is known, which consists essentially in super-posing images of the objects to be compared. in causing two light beams respectively emanating from each object to converge on a single image detector, the intensity of said beams being modulated in the same transverse direction and being of identical spatial period p but out of phase by 180, filtering being used to extract the differential component which is modulated by a spatial frequency Up, and said differential component being visually displayed. As regards this known process, the two object images are formed by spatially modulating the respective intensities of the light beam emitted from the objects, by traversing a plane grid located behind the objective of a camera and against the photosensitive plate with which latter is loaded and on which the two object images are recorded by superposition.  
  This known process mounts to modulate the superposed image of the two objects recorded in the photographic plate by the spatial frequency l/p, the plate being two-dimensional whereas the compared objects generally are three-dimensional. This visually displayed modulation of the differential image therefore cannot provide any information concerning the directions normal to the photographic plate (differences in relief). Thus this known process only allows comparing object dimensions in two directions; in order to compare the objects in the third, the comparison must be repeated following rotation of the objects in such manner as to position their third dimension essentially parallel to the photographic plate.  
  The process of the present invention allows comparison, in a single operation. of the shapes and dimensions of two three-dimensional objects.  
  The process of the present invention also consists essentially in superposing the images of two objects being compared, in causing two light beams respectively em anating from the two objects to converge on the same detector. said light beam intensities being each modulated at least in the same transverse direction and with the same spatial frequency 1/ but 180 out of phase, the differential component of the superposed image being extracted by means of a filter and being modulated by the spatial frequency l/p, said differential component being visually displayed, and being characterized in that the two object images are formed by illuminating each object with a different light beam of different intensity at least with respect to said transverse direction, of said spatial period p and out of phase by 180.  
  If for instance the intensities of the illuminating light beam vary in one or two transverse directions, a unidimensional network of alternately dark and light bands will appear, or else a two-dimensional network or lattice of alternately dark and light spots, which will be superposed to the image of each object formed by the process of this invention, the structure of said network, that is, notably the configuration of its parallel bands or the shape and array of its spots, being influenced by the structure of the corresponding object to the extend latters illuminated surface will not be truly plane, that is, to the extent it is endowed with relief. If the reliefs of the two objects are not rigorously identical, this will be shown to the same extent in the network structures of bands and spots that appear superposed to their images, these themselves being superposed, so that local differences in relief between the two objects are rendered on their superposed images obtained from the process of this invention by moire phenomena due to the superposition of two band or spot networks with slight local shifts. lt is these moire phenomena which provide additional information and indicate structural differences between the two objects being compared along their third dimension, whereas the differential image proper provides information about their structural differences in the other two dimensions. Therefore the process of the present invention allows conclusions as to the identities of two objects compared in all three dimensions. For identity of the two objects simultaneous absence of differential image and of moire patterns occurs.  
  From the structural differences in the two band or spot networks or lattices which show up superposed to the differential image, the zones with relief differences and also the size of the differences in the two compared objects may be easily inferred if use is made of the width of the periodic bands or spots as a standard. said width being easily determinable; obviously. the accuracy of such determination will be the higher, the finer the structure of the band or spot lattice. that is. the lesser the value of the spatial period p. Clearly the same standard may also be used to analyze the size of the zones of the two objects being compared in which latter differ in structure in the other two dimensions and corresponding to the differential image proper.  
  The process of the present invention lends itself to numerous applications,for instance comparing massproduced line objects with a standard object, or periodic monitoring of shape and dimensions of a part susceptible to wear or deterioration, or comparison of one part at one production stage with another part of another production stage, etc.  
  As regards a preferred embodiment of the process of the invention, a single light beam is used, which may be phase-modified by with respect to spatial, periodic and intensity variations in order to consecutively illuminate the first and second of the objects being compared, the first object being stored at least until it may be superposed on the second one. The memory used to this end may be either an optical kind such as a photographic plate or roll, or an electrical type as explained in further detail below; it is to be understood that with such an electric storing of the image of the standard object, the process of the invention will allow continuous monitoring and controlling the characteristics of shape and dimensions of, say, line-produced objects as they arrive at the end of the production line, that is, this may be done in real time, which cannot be done by photography.  
  This preferred embodiment or mode of the process besides allowing use ofa single light beam also provides the following additional advantage: the position of the first object may be located or indexed for instance with respect to the dark bands that the light beam causes to appear on the first object, the second objects position then being determined with respect to the light bands caused by said light beam after its spatial intensity variation phase has been shifted by 180 in order to simultaneously illuminate the two objects being compared: this illumination can be used for adjusting the two objects with respect to one another in such positions that their images will precisely superpose.  
  The present invention also applies to different embodiments of an apparatus for implementing the previously defined process. One advantageous embodiment of the apparatus of the present invention comprises a single illuminating light beam source used to illuminate consecutively the two objects being compared. said source comprising two point-sources each generating a monochromatic light beam coherent with the other. means causing the two beams to converge in such manner that the illuminated object is located in their interference field causing periodic interference fringes on the object. means for shifting one of the light beams from the point-sources by 180 at will, a TV camera with a linear sweep speed v. means for storing electric video signals from said camera when picking up the first object. means for adding the stored video signals to those obtained from the said camera when picking up the second object. and means for obtaining the components from their sum by means of a filter. said components being modulated by the time-frequency v/p. and for transmitting these components in the form of a video signal to a visual display device.  
  Several embodiments implementing the process of the present invention are illustratively and schematically shown in the attached drawings and described below:  
 FIG. 1 shows a first embodiment in schematic form:  
  FIGS. 2 and 3 are copies of photographs meant to explain operation of the apparatus of FIG. 1;  
 FIG. 4 is a schematic illustration of one embodiment of the light beam source of FIG. 1;  
  FIG. 5 schematically illustrates a variation of the light beam source of FIG. 4;  
  FIG. 6 schematically illustrates a second embodiment of the apparatus of the present invention.  
  Reference 0 in FIG. 1 denotes a three-dimensional first object to be compared with a second object of closely-related shape and dimensions. not shown in FIG. 1. but which subsequently will be denoted by 0 Reference numeral 1 denotes a generator or source providing a light beam 2 illuminating object 0,; two embodiments will be described for this generator I further below and in detail; said generator in conformity with the invention offers the following characteristics; its intensity varies as a function of the spatial period p which is precisely defined in a direction transverse to said beam 2. that is, latter will generate a lattice of parallel. straight, alternately dark and light bands similar to interference fringes. of pitch or denier p and independent of the screen position if the beam is collimated, where said illustrative screen is located in a plane normal to the axis 2a of beam 2 pointing towards object 0 To the extent the side of object 0 illuminated by light beam 2 deviates from the above mentioned transverse plane, a lattice of alternately dark and light. parallel bands 3 essentially of the same pitch p as those formed on the plane screen above mentioned will appear on that illuminated side of object 0 however, they are no longer straight because their respective positions are affected by the relief of the illuminated side of object 0 in the manner of contour lines. The source or generator for light beam 1 further comprises means to be specified below to shift by the variations with respect to space. period and intensity of light beam 2 it produces in regard to the transverse direction; such means may be controlled for instance from a double-throw switch la. the activation of which will invert the respective positions of the light and dark bands of the band lattices previously mentioned (3 on object O, that is. these lattices will shift by p/Z in the .v-direction.  
  Reference numeral 4 denotes a camera loaded with a photographic plate 5; light beam 6 emanating from object 0 and passing through objective 4a of camera 4 projects an image of said object 0 on said plate when. for instance. switch la of source 1 is in the position of O-degree phase-shift. Upon closing the shutter (not shown) of camera 4, switch la is turned to the 180 phase shift position and the second object 0 is substituted for the first object O with which it is to be compared. so that this second object 0 occupies essentially the same position as object O did previously with respect to objective 4a of camera 4. This substitution and positioning operation of the second object may be facilitated by using the index marks of the first object: the simplest of such index marks precisely are the alternately dark and light bands 3 caused to appear on the illuminated side of the object by beam 2&#39;. however. if the second object is substituted for the first after a shift of p/Z of the band lattice. and if for instance the position of the first object was indexed by making one of its details coincide with the edge of a light band. the positioning of the second object 0 must be effected in such manner that there be coincidence between its homologuous detail and the edge of a dark band that was substituted for said light band. When the shutter is then opened a second time. light beam 6 from the second object O- will therefore together with objective 4a form an image of object 0 precisely superposed on plate 5 on that of the previously recorded one of object 0 insufficiently accurate superposition of the images of two absolutely identical objects on the other hand would cause blurred zones likely to be erroneously attributed to differences in shape or dimensions of the two objects.  
  When the photographic plate 5 has been twice exposed in this manner, it will be withdrawn from camera 4 and developed by means of apparatus 7. FIG. 2 illustratively shows a photograph obtained from the process stages described so far and in conformity with the invention. object O being a rubber eraser slice seen in cross-section and slightly arched, object 0 being the same eraser. also seen in cross-section, but more arched; the references a1 and bl on one hand. and a2 and b2 on the other, denote the longitudinal sides of arched nature of objects O and 0 respectively; the crescent-shaped zones a1 a2 and bl b2 lacking overlapping of the two superposed images and therefore corresponding to differences in shape of the two objects O and 0 appear more clearly than the overlap zone in the middle, a2 bl; however no information at all may be inferred as to the differences in shape and dimensions of this eraser slice, which did arise from its deformation; one may only notice that the two lattices of parallel bands 3 and 3 are not precisely superposed in the median, overlap zone a2 bl, no precise data being available.  
  If in a plane normal to axis 2a of light beam 2 and es sentially at the level of the illuminated surface of object O and in that plane, one considers the xaxis to be (FIG. 1) parallel to the direction in which there is periodic variation of the intensity of the beam 2. and if further one assumes this variation is sinusoidal and of spatial period p, then one will observe that I which denotes the intensity distribution function along said xaxis oflight beam 6 reflected by said object if illuminated by a light beam of constant intensity. is given in a form expressing the exposure distribution of photo plate along the straight line parallel to the x-axis. i.e..  
  As regards object 0 on the other hand, the distribution function I (X) may locally differ from I (.v) and the exposure distribution along the same straight line of the image of object 0 on plate 5 is given by Therefore. following developing. the amplitude transmission of photo plate 5 will therefore vary, within a constant of proportionality. and along the straight line under consideration, as  
  The superposed image appearing on the developed plate 5a may therefore be considered as the resultant of the superposition of an additive image independent of the spatial period p and of a subtractive image the density of which is modulated by a sinusoidal function of argument (Z-nzv/p).  
 The developed photo plate 5a then is inserted into a photographic reproduction apparatus denoted by 8 in FIG. I and which may be of a known type; there is no need therefore to describe it in detail with respect to all of its components; it will suffice to indicate it comprises known means. notably at least one lens 9. for transforming the optical image of the developed plate 5a in a plane with a spatial filtering mask or stop 10. Latter is known per se and made to transmit exclusively the differential component of said optic image. which is modulated with a spatial frequency l/p; it is known that the optic image formed by lens 9 is the Fourier transform of the two-dimensional amplitude transmission of the developed plate 5a, and that the components of this spatial distribution corresponding to different spatial frequencies will be formed at different points in said plane. so that they may be filtered spatially by means of a suitable mask or stop 10.  
  The image formed by a second lens 11 behind stop on an unexposed photo plate 12 therefore corresponds only to the differential component modulated at the spatial frequency l/p, which alone was transmitted by the filtering stop 10; this second photo plate 12 then will be developed by means of apparatus 13.  
  FIG. 3 shows in schematic form the photograph obtained in the reproduction device equipped with optical filter 8 as described above and based on the photograph shown in FIG. 2 and previously described. The two crescent-shaped zones a1 a2 and b1 b2 corresponding to differences in shape of the two objects in a plane essentially normal to axis 2a of beam 2 appear with almost uniform exposure, whereas the moire patterns in the median overlap zone a2 bl, which are due to the superposition of two fringe lattices with slight local shifts, show alternately dark and light bands of fairly irregular shapes and contours; one may infer therefrom the shape variations of the eraser slice due to its deformation.  
  The embodiment of the apparatus of the present invention illustrated in FIG. 1 and that was described just above may be varied as follows: in lieu of providing a single light beam source for the consecutive illumination of the objects being compared in the manner described above. two independent analog sources may be provided allowing simultaneous illumination of the two object O, and O now assumed different respectively by means of two light beams of which the respective intensity variations in the transverse x-direction will be out of phase (FIG. 1). In this instance, optical means such as mirrors and beamsplitter must be provided in order to superpose the two light beams emanating from each of the objects at the entrance of the camera objective; this kind of embodiment however is less advantageous than the one illustrated by FIG. 1 to the extent it will require additional and fairly complex means for positioning the two objects being compared with respect to the photo objective and in such manner that their images will precisely superpose on the photo plate.  
  FIG. 4 shows in schematic form an embodiment mode of the light beam source 1 of FIG. 1 in the area enclosed by dashed lines. In this embodiment. reference numeral 14 denotes a monochromatic light source with a wavelength A; this may be a natural light source or else a coherent light source such as a laser: it generates a collimated beam 15 which is split by a semireflecting mirror 16 into a transmitted beam 15a and a reflected one 1511; latter is again reflected by a second mirror 17. and the two collimated beams 15a and 1512 thereupon will pass through a lens 18 to converge in its focal plane at two points 20a and 20b. which constitute two point sources separated by a distance d and each generating two divergent beams of monochromatic light, 21a and 21b. which are coherent with respect to each other. The double coherent beam 2111.211) superposition zone is shown shaded in FIG. 4 and constitutes their interference field, corresponding to a light beam 2 of which the intensity varies periodically in the xdirection and parallel to the focal plane 19 of lens while normal to the axis 2a of said beam 2 (this axis 2a coincides with that of lens 18 in the instance of the arrangement shown in FIG. 4).  
  For a given distance of object 0 from the focal plane 19 of lens 18. the pitch p of the interference fringes appearing on the surface illuminated by beam 2 depends only on the wavelength A of the monochromatic light generated by source 14 and on the distance 11 between the two point sources 20a and 20b; said distance d for the embodiment of FIG. 4 and for a given focal length of lens 18 only being a function of the angle 0 subtended between the initial light beam 15 and axis 2a of the system, it will be quite easy to vary the distance d and hence the pitch p of the fringes mentioned so as to adjust it optimally to the fineness of the superficial details of object 0 Reference numeral 22 in FIG. 4 denotes a glass plate, or a plate of any other transparent material intercepting one of the two beams 15a and 15b, and, as regards the case being illustrated, especially the reflected beam 15b; the plate thickness is so chosen that. allowance being made for the constituent index of refraction, the path of light beam 15b will be increased by an odd multiple of M2, and the plate is mounted in movable mannet. for instance translationally, when controlled from a displacement component such as switch la of FIG. 1,  
 so that said plate may be placed at will in the path of light beam b. Plate 22 also may be mounted in rotatable manner so as to allow adjusting the angle of incidence of beam 15b on the entrance side of the plate and therefore so as to allow adjusting the length of transparent material being tranversed by said beam 15b; provision for the latter allows utilizing a plate 22 of which the tolerance regarding thickness is much larger than M2. In this instance. achieving out-of-phase shifting of light beam 15b with respect to light beam 151: by an amount exactly equal to an odd multiple of M2 requires control by means of apparatus comprising a photoelectric detector 23 of which the entrance window width 23a is of the order of pitch p of the interference fringes, further comprising means 24 and for the amplification and the measurement resp. ofthe output current; photoelectric detector 23 is located near object O, in beam 2 so that for instance its output current by a minimum when plate 22 is not intercepting beam 15b; in this case, once said plate 22 has been interposed in beam 15h, the inclination of said rotating plate must be so adjusted that the output current of photoelectric detector 23 becomes a maximunm In a variation. means are provided for polarizing the two light beams 21a and 21b; this may be done either by using a polarized light source 14 or by interposing at least one polarizer in the path of the various light beams, as shown in FIG. 4&#39;. on the other hand a phaseshifter, especially one made of a birefringent material and of such dimensions as to cause a shift of 180, is so located as to intercept one of light beams 1511,15!) or 2111,2112. This phase-shifter for instance may be a birefringent half-wave plate.  
 Light beam source 1 shown in FIG. 4 may be part of the variation shown in highly schematic form in FIG. 5;  
 a third point source 200 is located in a plane normal to axis 2a of the system and at the third apex ofa right triangle formed with the other two point sources 200 and 20h, source 20c forming a third divergent light beam which is coherent with those from sources 20a and 20b: this may be achieved for instance by adding a semireflecting mirror similar to 16 and a second mirror similar to 17 to the source 1 in FIG. 4 so as to project a third collimated beam 150 (not shown) on lens 18; polarizers furthermore are placed in the paths of the three light beams generated by the three point sources 2011,2012 and 20c, which will cause linear polarizations as indicated by the arrows Pa, Pb and P0 in FIG. 4; these polarizers are so adjusted that. the three polarizations Pu through Pc&#39; being in the same plane as the right triangle with the three point sources 200 through 200 at its apexes, the polarizations Pa and Pc will be normal to each other, whereas Pb is normal neither to Pa nor to P(, but preferably at with respect to them; therefore interference may only take place between, on one hand, Pa and Pr, and on the other, the Pb components, in the respective directions of Pa and Pc, so that two fringe lattices will appear on a screen 26 located normally to axis 2a in the interference field of the three diverging light beams generated by the three point sources 201: through 20c, and especially by fringes 270 which are normal to the projection on screen 26 of the side 20u-20b of the right triangle, and fringes 27c which are normal to the projection on the screen of side 20b-20c of said triangle, these two fringe lattices therefore being normal to each other. Thus, a source of this kind will generate a light beam with an intensity periodically variable in the two orthogonal and ydirections which are normal to axis 2a of said beam, and with spatial periods that may differ if the two point sources 20a and 200 are not equidistant from point source 20b. It should be understood that such a beam causes to appear on the illuminated side of the object a two-dimensional network or lattice of alternately dark and light spots. of which the relative positions and shapes are affected by the surface relief of the object. Means such as previously described may also be provided to arbitrarily cause phase shifts in the light beam obtained from source 20b.  
  The light beam source(s) of the apparatus of the present invention also lend themselves to an embodiment comprising a source in the beam of which one places a screen or grid in perpendicular manner, such grid consisting of thin opaque wires or threads, or else of a one-dimensional or two-dimensional optical network modulating said beam in one or two transverse directions, there being one or two spatial periods depending on the pitch or denier of the grid, screen or network, and further comprising an objective for projecting an image of the grid, screen or network on the ojbect being illuminated.  
  By allowing the gird, screen or optic network to shift parallel to its own plane from a first to a second position in which latter its opaque elements are at the site of the transparent ones in the former, the periodic spatial intensity variations of the transmitted light beam may be shifted by 180.  
  The embodiment of the apparatus of the present invention shown in FIG. 6 differs from that shown in FIG. 1 only in that light beam 6 which is reflected from object O, illuminated by light source 1 is picked up by objective 28a of a TV camera 28, latter being of any suitable type, its video signals being transmitted via line 29 to the movable part 30 of a double throw switch. 31 and 32; contact 31 is connected to input 33a of a memory 33 which will store the totality of the video signals from camera 28, these corresponding to a complete image. Contact 32 of switch 30 is connected to the first input 34a of an adder circuit 34, of which the second input 34b is connected to output 33b of memory 33; output 340 of adder circuit 34 is connected by line 35a to the video input ofa video visual display device 36 of known type and comprising a cathode-ray tube with a screen referenced as 360, passing through an electric filter 35 with a suitable pass-band which preferably may be adjusted as to width and central frequency; latter will be set to be essentially equal to v/p, where v is the linear sweep speed of camera 28.  
  Switch 1a of the light beam source 1 being in its 0 position, and switch 30 making contact with its pole 31, the video signals of the image of object 0 picked up by camera 28 will be applied to input 330 of memory 33, which may be any suitable model, for instance an electronic storage tube or a circulating memory; proper operation ofa memory of this kind requires synchronizing it with the sweep of camera 28; this is shown in FIG. 6 by line 37 transmitting the sweep synchronization signals of camera 28 to a synchronization input 336 of memory 33.  
  Next, switch la of source I will be placed in its 180 position and switch 30 will make contact with pole 32, while object 0 replaces object 0 in the manner previously described. The following video signals from camera 28 and corresponding to the image of object 0 are applied to the first input 34a of adder circuit 34, of which the second input 34b receives the previously stored video signals from memory 33 and emitted at latters output 3312. this operation occurring in synchronism with the analysis of object on account of the transmission of the synchronizing signals from camera 28 via line 37 to input 336&#39; of said memory 33. Video signals corresponding to the super-position of the respective images from objects 0, and O. and successively picked up by camera 28 will therefore appear at the output 340 of adder circuit 34, these super-posed video signals being transmitted to the input of filter 35, of which the output will only transmit via line 3511 the components modulated by the time frequency r/p to the video input of the visual display device 36. Latters operation being synchronous with camera 28 on account of the synchronizing sweep signals emitted by said camera and received via line 38 extending line 37, a differential image spatially modulated at a frequency equal or proportional to Up depending on the linear sweep speed of screen 36a of the cathode ray tube being equal or proportional to the linear sweep speed v of camera 28 will appear on screen 360 of said visual device 36.  
  If the light beam source 1 comprises means allowing to vary the pitch or denier p of fringes 3, filter must be so designed as to allow setting the center frequency of its bandpass that it will always remain equal to v/p; clearly some coupling may be provided for these two settings.  
  The embodiment shown in FIG. 6 and described above is particularly well suited for consecutively comparing a large number of objects with a constant, first one serving as standard and of which the electrical image is permanently stored in memory 33, which is such as to allow reading without erasure, or else is provided with immediate restoring; such a device allows among others real time control and monitoring of production line processes.  
  The embodiment of FIG. 6 allows a variation which was not shown and which comprises two light beam sources for the simultaneous illumination of two objects being compared. two TV cameras with synchronized sweeps and identical linear sweep speeds and so positioned as to pick up each only one of the illuminated objects and to allow super-position of the images on a visual display device, further comprising means for adding the electric video signals from the two cameras and to obtain from their sum and by means of filtering the components modulated at a time-frequency r/p, said components alone being transmitted as a video signal to said visualization display.  
  I claim: 1. A method for comparing two three-dimensional objects having similar shapes and dimensions which comprises:  
 illuminating a side of one of the objects by a collimated light beam whose intensity is modulated in a direction transverse thereto at a spatial frequency of l/p, said modulated light beam generating on the illuminated side of the object a lattice of alternately dark and light parallel bands similar to interference fringes having a configuration in the manner of contour lines determined by the relief of the illuminated side of the object and pitch p,  
 similarly illuminating the other object but wherein the intensity modulation is phase-shifted by optically superimposing images of the illuminated sides of the two objects, and  
 obtaining from the superimposed images the differential component thereof which is modulated at the spatial frequency Up.  
  2. The method as defined in claim 1 for comparing two three-dimensional objects having similar shapes and dimensions wherein a single light beam is utilized which illuminates the two objects in succession and wherein the image of the first one of the objects to be illuminated is stored until the image of the second object to be illuminated has been obtained.  
  3. Apparatus for comparing two three-dimensional objects having similar shapes and dimensions which comprises:  
 means for illuminating a side of one of the objects by a collimated light beam whose intensity is modulated in a direction transverse thereto at a spatial frequency of l/p, said modulated light beam generating on the illuminated side of the object a lattice of alternately dark and light parallel bands similar to interference fringes having a pitch p and a configuration in the manner of contour lines determined by the relief of the illuminated side of the object.  
 means for similarly illuminating the same side of the other object and wherein the intensity modulation of the light beam in phase-shifted by 180,  
 means for superimposing images of the illuminated side of the two objects,  
 means including a filter for obtaining from the superimposed images the differential component thereof which is modulated at the spatial frequency 1/11. and  
 means for displaying said differential component.  
  4. Apparatus as defined in claim 3 for comparing two three-dimensional objects having similar shapes and dimensions wherein said means for illuminating a side of the object comprises at least one source of a collimated light beam whose intensity is modulated along at least one direction transverse thereto and the phase of which can be shifted by 180.  
  5. Apparatus as defined in claim 3 for comparing two three-dimensional objects having similar shapes and dimensions wherein said means for superimposing images of the illuminated side of the two objects comprises a TV camera having a linear sweep speed v, and means for indexing the position of a first one of the objects with respect to the objective of the TV camera and for the ensuing positioning of the other object at precisely the same location as said first object. wherein said means for obtaining the differential component from the superimposed images comprises means for storing the video signals obtained from said TV camera when picking up the illuminated side of said first object, means for adding the stored video signals to those produced by said TV camera when picking up the illuminated side of said other object, and a filter connected to the output of said adding means, said filter having a pass band the central frequency of which is substantially equal to v/p, said means for displaying the said differential component being connected to the output