GB2118736A - Method of and interferometer for holographic measurement of components of a displacement vector - Google Patents

Abstract

The invention concerns a method and an interferometer for holographic measurement makes use of a pulsed light source (1) for generating a coherent light beam, of an optical system dividing the coherent light beam into an object beam and a second beam, for dividing the second beam into at least four part beams, of means for projecting the object beam after scattering and the part beams into respective holocameras and of a control unit for controlling the work of the pulsed light source (1). The interferometer comprises also a continuous light source for adjusting the optical system. The control unit is capable of performing the start of the pulsed light source (1) at two predetermined time moments. The time moments can be determined also dependent on the conditions of the movement of the moving surface. The proposed method and interferometer can be applied in an industrial environment. They are particularly useful in measuring when displacements or mechanical tensions of objects of great extents are to be determined. <IMAGE>

Description

SPECIFICATION Method of and interferometer for holographic measurement of components of a displacement vector The present invention relates to a method and an interferometer making use of the principles of holography for the measurement of a displacement vector. The proposed method includes the known steps of generating a coherent light beam by means of a pulsed light source, dividing said coherent light beam into an object beam and a reference beam, projecting the object beam onto a moving surface, scattering it thereon and producing a holographic interferogram on the basis of the scattered object beam and the reference beam.The proposed interferometer includes the known features of a pulsed light source for generating a coherent light beam, a continuous light source for generating a coherent adjusting beam, an optical system for transforming the coherent light beam into an object beam and a reference beam and for forming the adjusting beam, and sensing means for receiving a holographic interferogram on the basis of the object beam scattered on a moving surface and of the reference beam. By means of the method and interferometer according to the invention it is also an industrial conditions possible to compute with high accuracy and components of a displacement vector associated with the movement and deformation of a moving surface, and to determine the direction of this movement.

There are known some holographic methods for determining the amplitude of a displacement vector. For example A. E. Ennos (J. Sci. Instrum., 1968, 1, 731) proposed a method based on computing the number of the interference fringes, and relating on the fringe of so-called zero range.

The last should be assigned to a non-moving point of the surface to be analyzed. The method as proposed is capable of measuring this component of the displacement vector which lies in the direction of sensitivity vector (the sensitivity vector is determined by the bisector of the angle between the direction of observation and that of illumination). The proposed method can be used only in a limited range of problems due to the above limitation and to the fact that any nonmoving point is very difficult to find even in a laboratory environment prepared with high care.

A. B. Alexhandrov and A. M. Bonch-Bruevich proposed first (Zhurnal Teknischeskoj Fiziki, 1967, 37, 360) the use of an interferogram for determining a displacement. The method as proposed is based also on computing fringes. For this purpose the holographic interferogram should be observed from different directions and during observation the number of fringes which seem to be moving over a chosen point should be determined. The determination of the three components of a displacement vector requires preparing at least three interferograms at the same time by devices working independently of each other; however, in every case the required accuracy can be achieved only for the component lying parallel to the holographic plate.

The known methods, like that listed above, are capable of solving measurement problems only in laboratories. The interferometer needs to be adjusted with high accuracy from problem to problem and so requires the presence of specialists skilled in the field of optics and of interferometry.

There are known devices for the industrial realization of the scientific principles. For example, the firm Rottenkolber AG in West Germany produces devices comprising a ruby crystal laser, i.e. a pulsed light source for preparing a unique interferogram. The components of a displacement vector can not be determined generally with the required accuracy, though it is possible to determine one of them.

The invention aims to avoid the above mentioned disadvantages.

Thus an object of the invention is to provide a method and an interferometer-especiallyfor realizing the method-on the basis of which holographic interferograms representing the components of a displacement vector with high accuracy can be made also in an industrial environment and when investigating large work pieces.

The invention is based on the recognition that the components of a displacement vector can be determined with high accuracy, by fixing the direction of movement at the same time while using a common coherent light beam for preparing at least four holographic interferograms of the moving surface at two time moments and analysing in a numerical way the interferograms.

According to this invention there has been proposed a holographic method of determining components of a displacement vector characterizing the movement and/or deformation of a moving surface to be analysed, comprising the steps of generating a coherent light beam by means of a pulsed light source, dividing the coherent light beam into an object beam and a reference beam, projecting the object beam onto the moving surface and producing a holographic interferogram on the basis of the object beam scattered by the moving surface and the reference beam, wherein the method further included the steps of starting the pulsed light source at two time moments following one another according to the movement of the moving surface, dividing the reference beam into at least four partial beams and producing the holographic interferogram in form of at least four holographic pictures performed on the basis of the partial beams and relating to the time interval determined by the two time moments.

It is particularly advantageous to start the pulsed light source at two time moments determined synchronously with the movement of the moving surface because in this way the displacement vector characterizing an oscillating surface can be determined well.

The proposed method can be realized also in an environment wherein different vibrations and other movements make preparing holographic interferograms according to the known principles impossible.

In order to achieve the object mentioned above there is further provided a holographic interferometer which is capable also of realizing the method as described above and can be used in an industrial environment, comprising a pulsed light source for generating a coherent light beam, a continuous light source for generating a coherent adjusting beam, an optical system for transforming the coherent light beam into an object beam and a reference beam and for forming the adjusting beam, and sensing means for receiving an interferogram on the basis of the object beam scattered on a moving surface and of the reference beam, wherein the optical system comprises optical elements for dividing always one of the coherent beams of the pulsed light source or of the continuous light source into at least five parts, and the pulsed light source is connected to a control unit for controlling the work of this pulsed light source, and the sensing means comprise at least four holocameras operated synchronously with the pulsed light source.

When the vibration of the moving surface is to be analysed it is advantageous to connect to the control unit, for example across a selector switch, a sensing indicator for following the movement of the moving surface and vibrator means, the vibrator means via its input forwarding signals.

The optical system of the interferometer is advantageously constructed such that it comprises a first splitter for producing an object beam and a second beam, second splitters for producing as a holographic reference beam at least four partial beams from the second beam, mirrors for adjusting the length of travel of the second beam between the first and second splitters, beam expander means for expanding the object beam, expander units for expanding the reference beams and mirrors for directing the expanded partial beams into respective holocameras. The path of the optical beam determined by the elements of the optical system can be designed, especially, if optical system contains a penta prism before the first splitter, to realize the measurement and the adjustment as well.

Because the length of the travel of the object beam is altering from measurement to measurement it is therefore advantageous to arrange the mirrors for adjusting length of the travel to be movable along the path of the second beam.

The beam expander means is advantageously a concave dispersing lens and as expander units spatial filter beam expanders producing plane wave have been found to be especially advantageous.

The flat elements of the optical system can be advantageously clamped into a positioning unit having adjusting screws, moving element, and a clamping plate and ground plate connected to each other by means of the adjusting screws and plane shaped resilient return members.

The holocameras are advantageously built up so that they may be used in daylight, and for this purpose they comprise respective high-pass light filters and closing mechanisms with movable lamellae, wherein the characteristics wavelength of the light filter lies below the wavelength of the light of the pulsed light source and the closing mechanism is controlled synchronously to the pulsed light source.

The control unit may be advantageously equipped with a detector for sensing the work of the pulsed light source, because in this way a double exposition control is possible which is required when observing vibration movements while taking in to account the phase positions.

In industrial equipment it is advantageous to construct the interferometer according to the present invention on a carriage which allows the interferometer to be displaced and to be fixed in a predetermined spatial position and ensures damping of any ground vibration. The interferometer on the basis of the carriage can be used in environments where a high level of vibration is present.

By means of the method and interferometer as proposed in the present invention holographic interferograms can be made also in industrial conditions and it is possible to make interferograms of surfaces of great extent which interferograms carry highly reliable information about the displacement and tension state of the surfaces to be analysed.

The invention will be further described in more detail by way of example and with reference to preferred embodiments illustrated in the accompanying drawings, wherein: Figure 1 shows the front view of the interferometer according to the invention; Figure 2 shows the side view of the interferometer according to the invention; Figure 3 shows the top view of the interferometer according to the invention; Figure 4 is a schematical view of the ray path of the interferometer according to the invention; Figure 5 is a cross-section of a positioner used for clamping the flat optical elements of the interferometer according to the invention; Figure 6 is a top view of a ground plate of the positioner illustrated in Figure 5; Figure 7 is a cross-section of a holocamera used in the interferometer according to the invention; ; Figure 8 shows the arrangement of the interferometer according to the invention during adjustment; Figure 9 shows another arrangement of the interferometer according to the invention used also during adjustment; Figure 1 0 shows the ray path of the interferometer according to the invention when it is made for measurement; and Figure 11 shows a block diagram of the control unit used in the interferogram according to the invention.

When realising the method for holographic measurement of components of a displacement vector, a pulsed light source 1 adjusted with high accuracy, e.g. by means of a continuous light source 33, is forced to emit two pulses of a coherent light beam. The coherent light beams are transmitted into an optical system adjusted with high accuracy, e.g. also by means of the continuous light source 33. For transmitting, a mirror 10 is used (Figure 4). The coherent beam on the output of the mirror 10 is divided into two parts one of which is projected as the object beam by means of a concave dispersing lens 12 or similar beam expander means (e.g. a spatial filter comprising a pin hole) on a moving surface to be inspected. The other part, which can be called a second beam, is projected by means of adjusting mirrors 13 to second splitters 14.The adjusting mirrors 13 are necessary in order to take into account the limited coherence length of the pulsed light source, e.g. a ruby crystal laser, i.e. the distance between the interferometer and the moving surface, which distance changes from measurement to measurement. The second splitters 14 should divide the second beam into at least four partial beams which will be used as parts of a reference beam for producing holographic interferograms. If necessary a fifth reference beam 1 6 is produced as well. In the path of the fifth reference beam it is advantageous to include a device which takes up a hologram performs a quick quality analysis to determine whether the conditions of the measurement are acceptable.In this way, for example by use of a special checking camera 9 with thermoplastic light sensitive film, the adjustment of the optical system can be checked quickly and if necessary the changes therein can be carried out. The partial beams are after the splitters 14 expanded by means of beam expander units, for example by spatial filter beam expanders 49 producing a plane wave. After expanding the partial beams are projected by mirrors 1 5 to places wherein the interference between them as reference beams and the object beam scattered by the moving surface can be observed, and, if necessary, fixed. To make an interferogram in at least four parts the pulsed light source 1 is forced to emit two coherent light beams in a predetermined time interval.The duration of this interval between the moments in time when the two coherent light beams are initiated should be determined according to the movement of the moving surface to be inspected, and in the case of vibration synchronously with it.

The interferogram consists of at least four parts and by numerical analysis of the different parts the components of the displacement vector, the direction of the displacement, can be determined.

The parts of the interferogram are generally fixed on a light sensitive material, and to make the interferogram holocameras 4 are used. The holocameras 4 should be operated synchronously with the operation of the pulsed light source 1. In this way the holographic interferogram can be fixed also in daylight. Without holocameras 4 the method should be performed in darkness. As hotocameras, videocameras can be also used if they can respond to the quality conditions (to analyse the hologram, a resolving power value of 2000 to 3000 line/mm range is necessary).

An interferometer realizing the method described above based on the principles of holography comprises (Figures 1, 2 and 3) an interferometric unit which is fixed during operation to avoid any displacement. This interferometric unit is built up generally on a frame 2 bounded by a system 3 of arms. The interferometric unit is advantageously mounted by the frame 2 on a carriage. This is desired to ensure high reliability in industrial use. The carriage comprises wheels 8 in housing 6. The carriage and with it the interferometric unit can easily be displaced. The housing should ensure the possibility of fixing the wheels 8 and damping the ground vibration; this can be done by well known solution. On the housing 6 there is a column 5 including a hydraulic work cylinder to which a panoramic head 7 is connected.The panoramic head can be rotated in a horizontal plane, and lowered and raised by means of the hydraulic work cylinder 5. The panoramic head 7 comprises an axle wherein the interferometric unit can be fixed and therearound rotated.

According to the known solution, the frame 2 can be fixed in a desired spatial position. The frame 2 consists of elements being advantageously rigidly connected with each other. On the frame the main parts of the interferometric unit can be fixed at predetermined steady distances.

In the interferometric unit there are a pulsed light source 1, an optical system as shown in Figure 4 and realized by elements fixed on the frame 2, at least four holocameras 4 mounted on the system 3 of arms, and a checking holocamera 9. On the frame 2 a continuous light source 33 is arranged which can be included in the ray path of the optical system.

The main flat elements of the optical system of the interferometric unit are clamped in positioners (Figures 5 and 6). The positioners comprise a ground plate 1 7 with an interior orifice of the required diameter for the light. This ground plate 1 7 is fixed in a steady position according to the light source. The ground plate 17 is equipped with an adjusting screw 24 and is connected to a moving part 19 via a clamping plate 20 abutting against it and by an adjusting screw 47. Like the ground plate 17, the moving part 19 comprises an interior orifice of the necessary diameter. The ground plate 17 and the moving part 19 are connected to one another also by means of plane shaped resilient return members 18, e.g. plate springs which are arranged in planes crossing one another, and by a spring 25.The adjusting screw 24 bears up against a lever plate 21 connected to the ground plate in the plane of the ground plate.

The lever plate 21 is connected by plane shaped resilient return members 22 and 23, e.g. plate springs, to the ground plate 17 and also to the moving part 19. The clamping plate 20 is connected to the moving part 19 also by plane shaped resilient return members arranged in planes crossing one another (not shown in the drawings). In this way the elements of the optical system can be adjusted with high accuracy-the adjusting screw 24 can not force the movement of the moving part 19 along the whole length belonging to a given number of screw-threads, because the rotation of the adjusting screw 47 is transmitted to the moving part 19 by means of the lever plate 21. The plate springs arranged in planes crossing one another are capable of ensuring that the position is held with high accuracy.

Each holocamera 4 comprises (Figure 7) a holographic plate 26 arranged on a ground plate 27. In front of the holographic plate, in the direction designated by the arrow showing the light beam, there are lamellae 29 which can be moved by a swing back 30 to free the path of the light beam. Behind the lamellae 29 there is a high-pass light filter 28 clamped by a carrier 31 to which the swing back 30 is also fixed. The carrier 31 is connected to the ground plate 27 by rubber springs 32.

To operate the interferometer a control unit 37 is very important (Figure 11). An output 44 of the control unit 37 gives the signals required for operation of the pulsed light source 1. The control unit 37 comprises first and second control inputs.

To the first control input 48 a selector switch may be connected, to the outputs of which switch a sensing indicator 39 and an output 38 of a vibrator are connected. The sensing indicator 39 and the output 38 can, however, be joined also directly to the control unit 37. The sensing indicator 39 is attached to the moving surface which is vibrating and it comprises e.g. a piezoelectric crystal for sensing vibration. The vibrator is not a part of the interferometer according to the invention, it is e.g. an acoustic generator or a joggling machine, and it is only important that it should have an output 38 connected to the control unit 37, directly or via the selector switch. In this way also the pulses of the sensing indicator 39 or the pulses forwarded by the output 38 of a vibrator can be processed in a control unit 37, practically at the same time as the vibration takes place.The control unit 37 is built up advantageously as a matching trigger network and therefore it comprises a series member including a filter unit and an analogous amplifier connected to the first control output 44 from which the work of the pulsed light source 1 can be controlled. The input of the series member is the first control input 48 of the control unit. The second control input of the control unit 37 is connected to a detector 45 which detects the light pulses of the pulsed light source 1. The parameters of the filter unit of the matching trigger network can be adjusted by a unit 40. The control unit 37 can be equipped with an oscillator 41 for observation of the filtered signals. The characteristic parameters of the observed signals can be adjusted by units 42 and 43.

The control unit 37 should be built up in such a manner that starting the pulsed light source 1 will be possible on the basis of individual commands or of two automatically generated commands.

Accordingly the control unit assures the repeated pulse operation mode and the double pulse operation and excited state of the ruby crystal laser two pulses are generated. The time interval between them is so short that the conditions in this operation mode are generally not suitable to analyse vibration with a period substantially longer than 1 ms-which corresponds to frequency higher than 1 kHz. The upperfrequency limit of analysis is in about the 10 kHz range. The other operation mode, which can be called the repeated pulse operation mode, is particularly advantageous if the period of vibrations is longer than the duration of the excited state of the ruby crystal laser, and generally if its value is under 1 ms. The starting pulses to the pulsed light source should be generated according to the phase position of the oscillating moving surface.

Before the measurements the pulsed light source 1 should be adjusted. To assure the adjustment the continuous light source 33 is used (Figure 8 and 9). In the ray path of this light source deflecting mirrors 34 and 46, an aperture 35 and a penta prism 36 are arranged. By using the above mentioned optical elements it is possible to adjust with high accuracy the continuous light source 33 and the pulsed light source 1 as well.

In the interferometer according to the invention the pulsed light source 1 is advantageously a rubin crystal laser including an oscillator and an amplifier. The wavelength of the emitted light is 649.3 nm. The energy of the light pulses is about 200 and 300 mJ, the power is sometimes about ten mW. The short light pulses of about a few ns duration are generated in Q-coupled operation mode, and if using repeated pulse operation mode the time interval between two pulses lies in the range of 1 to 800 ,us.

The continuous light source 33 emits advantageously light having a wavelength near to that of the pulsed light source. It can be e.g. a He Ne gaseous laser, which emits a continuous light beam with a wavelength of 632.8 nm. The power of this light source is about 10 mW.

The interferometer according to the invention operates in the following way: At the site of the measurement the portable and movable device should be set up in the desired position. Thereafter the light sources and the elements of the optical system should be adjusted. To do this the continuous light source 33 should be switched in and its light directed by means of the mirrors 34 (Figure 8, 9 and 10) into the aperture 35 and therethrough to the penta prism 36. The penta prism 36 is in a position wherein its light beam is suitable for adjusting the elements of the pulsed light source 1. Thereafter the penta prism 36 should be taken out of the system and the oscillator of the pulsed light source 1 switched in. Before the amplifier a lightsensitive screen is to be arranged and thereon a spot is to be formed.The penta prism 36 should reset in a position turned by 900 in comparison to that shown in Figure 8 (as shown in Figure 9) and turned into a position wherein the light beam of the continuous light source 33 illuminates the spot of the light-sensitive screen. After removing the light-sensitive screen the light beam should be traced through the amplifier of the pulsed light source 1. This beam serves as the basis for adjusting all elements of the optical system, i.e.

splitters 11, 14, mirrors 13, 1 5 etc. Removing the penta prism 36, the interferometer is ready for measurements. Using the checking holocamera 9 on a thermoplastic film a quick exposure can be made. The thermoplastic film can be developed in 20 to 30 seconds and therefore it is highly suitable to determine whether the conditions of the measurement correspond to the requirements: whether the resolution of the interferogram is high enough; whether the load is not higher than a permitted value; whether the amplitude of the vibration is not higher. If the requirements are not met, the conditions should be changed: e.g. the distance between the interferometer and the surface to be inspected or the arrangement of the optical system, especially by use of the mirrors 1 3 etc., should be changed.

In order to make exposures the oscillator of the pulsed light source 1 should be inserted in place, the continuous light source 33 switched out and the holocameras 4 should be filled with holographic plates 26.

The operation mode of the interferometer prepared for the measurements depends on the object of the measurement. When investigating static deformation the laser of the pulsed light source 1 should be started at time moments determined according to the speed of deformation. For this purpose the control unit 37 can be connected to a hand actuated starter.

When investigating surfaces of vibrating objects the double pulse operation mode can be advantageously used in the range of 1 to 10 kHz, applying a delay of 1 to 800 jbS between the two pulses. If the frequency of vibration is lower than 1 kHz, the repeated pulse operation mode is advantageous, wherein the light pulses follow one another in a time interval depending on the frequency value.

When investigating the surface of vibrating objects the laser should be operated at a given phase of the vibration. This is assured by the control unit 37 on the basis of information sent via the first control input 48. At the moment of starting the pulsed light source 1 the control unit 37 causes the holocameras 4 to open the lamellae 29 in order to free the light path. This solution combined with the light filter 28 is very advantageous because in such a manner it is possible to make exposures also in daylight. The high-pass light filter 28 does not pass the most disturbing part of the daylight and therefore the outer light causes only disturbance of negligible extent on the holographic plate. After the pulses of the pulsed light source 1 the lamellae 29 have to be closed as soon as possible.As mentioned the holocameras 4 can be electronic units if they correspond to the quality requirements (resolving power in the 2000 to 3000 line/mm range).

The interferograms can be evaluated according to the known methods, e.g. by numerical computing.

As follows from the above mentioned features, the method and interferogram as proposed according to the invention arc suitable for preparing holographic interferograms in an industrial environment and by daylight. The elements of the light sources and optical system can be adjusted with high accuracy, the disturbing influence of vibration can be avoided and therefore the analysis of the displacement vector is possible in all cases when the surface to be inspected is under a corresponding angle of view, i.e. it can be seen from the interferometer.

From the above description it should be understood that interferometers equivalent to those given above will be within the scope of the claimed invention and such interferometers will have optical system and sensing means dependent on the field of application and the given circumstances.

Claims (24)

Claims

1. Method of holographic measurement of components of a displacement vector, comprising the steps of generating a coherent light beam by means of a pulsed light source, dividing said coherent light beam into an object beam and a reference beam, dividing said reference beam into at least four partial beams, projecting said object beam onto a moving surface of a displaced object to be measured and producing at least four halographical interferograms on the basis of said object beam scattered on said moving surface and of said partial beams, wherein the pulsed light source is started to emit coherent light beam at two determined time moments following one another according to the movement of the movement surface.

2. A method according to claim 1, wherein said pulsed light source is forced to emit coherent light beam at two time moments chosen synchronously to the movement of the moving surface.

3. Interferometer for holographic measurement of components of a displacement vector, particularly for industrial use, comprising a pulsed light source for generating a coherent light beam, a continuous light source for generating a coherent adjusting beam, an optical system which is arranged along a ray path common for both said light sources and which is for transforming said coherent light beam into an object beam and a second beam and for forming said adjusting beam by means of optical elements common for both of said light sources and forming from said second beam at least four partial beams forming a holographic reference beam, sensing means for receiving at least four halographic interferograms in corresponding holocameras on the basis of said object beam scattered on a moving surface belonging to a displacing object and of said partial beams, and a control unit for controlling the work of said pulsed light source.

4. An interferometer according to claim 3, wherein said control unit is connected to a sensing indicator for following the movement of said moving surface via a first control input.

5. An interferometer according to claim 4, wherein said sensing indicator comprises a piezoelectric crystal.

6. An interferometer according to claim 4 or 5, wherein said control unit is connected via said input to an output forwarding signals of vibrator means.

7. An interferometer according to claim 6, wherein said control unit is connected to said output across a selector switch connected to said sensing indicator.

8. An interferometer according to any of claims 3 to 7, wherein said optical system comprises mirrors for adjusting length of travel between a first splitter for dividing said coherent light beam into said object beam and second beams and second splitters for producing partial beams as holographic beams on the basis of the second beam, beam expander means for expanding said object beam, and mirrors for directing said partial beams after expansion into said holocameras.

9. An interferometer according to claim 8, wherein the mirrors for adjusting length of travel are guided movably along the ray path of said second beam to be transformed into said partial beams.

10. An interferometer according to claim 8 or 9, wherein said expander means comprise a dispersing concave lens.

11. An interferometer according to any of claims 8 to 10, wherein said optical system comprises spatial filter beam expanders for producing a plane wave arranged between said directing mirrors and said second splitters for producing reference beams.

12. An interferometer according to any of claims 8 to 11, wherein a penta prism is arranged before said first splitter for producing the object beam.

1 3. An interferometer according to any of claims 4 to 12, wherein said control unit comprises a matching trigger network including filter means connected between said control input of said control unit and an analogous amplifier connected to a control output for controlling said pulsed light source.

14. An interferometer according to any of claims 3 to 13, wherein said holocamera comprises a high-pass light filter and movable lamellae for stopping the passage of light.

1 5. An interferometer according to claim 14, wherein the control unit is connected with a detector sensing light pulses of said pulsed light source via a control input.

1 6. An interferometer according to any of claims 3 to 15, wherein said pulsed light source, said continuous light source, said sensing means, and said optical system are arranged on a common frame.

17. An interferometer according to claim 16, wherein said frame consists of elements joined rigidly with one another.

18. An interferometer according to claim 16 or 17, wherein said frame is arranged on a carriage capable for damping ground vibration and fixable in a determined position.

1 9. An interferometer according to claim 18, wherein said carriage comprises a hydraulic cylinder movable in a vertical direction and connected to a panoramic head for holding said frame and rendering possible the movement of said frame around a horizontal axle and in a horizontal plane.

20. An interferometer according to any of claims 3 to 19, wherein said optical system comprises positioning means comprising a clamping plate bearing up against a moving part equipped with adjusting screws and against a ground plate by means of plane shaped resilient return members arranged in perpendicular planes, the ground plate is connected in its plane by a plane shaped resilient return member to a lever plate connected by a plane shaped resilient return member to said moving part and abutting against the moving part by one of said adjusting screws, and the ground plate is connected to said moving part by a spring and by plane shaped resilient return members arranged in perpendicular planes crossing one another.

21. An interferometer according to claim 20, wherein said resilient return member is a flat spring.

22. Measuring means including an interferometer with at least four holocameras.

23. An interferometer substantially as herein described with reference to the accompanying drawings.

24. A method of holographic measurement of components of a displacement substantially as herein described.