Device and method for contactless measurement of a change in strain on a test object

Abstract

The invention relates to a device (10) for contactless measurement of a change in strain (Δε) on a test object (20), comprising: an emission module (30) for emitting two coherent partial beams (34, 36) such that interference fringes are formed within an interference region (40). Three photodetectors (51, 52, 53) each detect light scattered from a first sector (41), second sector (42), or third sector (43) of the interference region (40). From the data measured by the photodetectors, an evaluation unit (60) calculates a two-dimensional change in strain (Δε) in the interference region (40).

Description

[0001]

[0002] Clausthal University of Technology, legal file:

[0003] Adolph-Roemer-Straße 2A 3017-0061 PCT-1

[0004] 38678 Clausthal-Zellerfeld Date:

[0005] Germany, October 27, 2025

[0006] Device and method for non-contact measurement of a strain change in a measuring object

[0007] The invention relates to a device for the non-contact measurement of a strain change in a test object, comprising an emission module with at least one light source and configured to emit a first partial beam and a second partial beam such that the first partial beam and the second partial beam overlap on the test object within an interference region, creating interference fringes within the interference region, wherein the first partial beam is coherent with the second partial beam. The device also includes a first photodetector configured to detect light scattered and / or reflected from a first section of the interference region and to provide first measurement data, encoding an intensity or change in intensity of the detected light, to an evaluation unit.The device further comprises a second photodetector configured to detect light scattered and / or reflected from a second section of the interference area and to provide second measurement data encoding an intensity or change in intensity of the detected light to the evaluation unit.

[0008] According to a second aspect, the invention relates to a method for non-contact measurement of a strain change in a test object. The method comprises the step of providing a first partial beam and a second partial beam by means of an emission module, wherein the partial beams are coherent to each other so that they interfere with each other within an interference range.

[0009] Strain measurements are primarily used to investigate the load-bearing capacity of materials and components, for example, through tensile tests in which the object is subjected to a force from one or more sides and its response to this load is recorded. Strain can have a positive sign, meaning that the distance between two points on the object increases, or a negative sign, meaning that the distance between two points decreases, indicating contraction or compression.

[0010] Conventional strain measurements are performed using mechanical strain gauges, which require a mechanical connection to the object being measured. Attaching the strain gauge to the object introduces additional influences and measurement uncertainties. This is disadvantageous, and therefore non-contact strain measuring devices are desirable.

[0011] An alternative method for strain measurement is non-contact strain measurement using optical sensors such as photodetectors. This method requires no contact or mechanical coupling with the object being measured. As a result, the measurement uncertainty is lower than with strain gauges. The measurement principle utilizes the change in interference fringes caused by strain on the object. These fringes are generated on the object by two coherent partial beams to determine the change in strain. Two photodetectors are positioned to detect the scattered and / or reflected light from the interference pattern on the object. An evaluation unit then calculates the change in strain along the direction defined by a straight line connecting the two photodetectors.

[0012] However, a disadvantage of such a strain gauge is that it can only detect strain in one direction. This makes it difficult to investigate the material behavior and load-bearing capacity of components.

[0013] Furthermore, it is desirable that a strain gauge device is portable so that it can be used at different measuring locations, ideally without requiring time-consuming readjustment of the device's components.

[0014] From DE102018111921 A1 a contactless optical strain sensor is known with two frequency-shifted laser beams which are brought to interference at an angle to each other on a surface of a measuring object, thereby generating an interference fringe pattern on the surface of the measuring object and wherein an intensity signal from two different areas of the interference fringe pattern is evaluated differentially with respect to frequency and / or phase.

[0015] A heterodyne laser Doppler vibrometer is known from DE102014216278A1.

[0016] From DE102012211549B3 a vibrometer is known which has two detectors whose signals are evaluated using signal diversity.

[0017] From DE 102011085599B3 a device for interferometric measurement of an object is known.

[0018] From W02003021237A2 a device for detecting two-dimensional translational movements of a surface is known.

[0019] Therefore, the object of the present invention is to provide a device for non-contact measurement of a change in strain of a measuring object, which reduces one or more disadvantages of the prior art.

[0020] The invention solves the problem by a generic device for contactless measurement of a strain change of a measuring object, which has a third photodetector configured to detect light that is scattered and / or reflected from a third section of the interference area and to provide first measurement data encoding an intensity or intensity change of the detected light to the evaluation unit, wherein the evaluation unit is configured to calculate a two-dimensional strain change of the interference area from the first, second and third measurement data.

[0021] The task is also solved by a generic method that includes the step

[0022] The invention comprises: detecting light by means of a first photodetector, a second photodetector, and a third photodetector, wherein the light is scattered and / or reflected from a first, second, or third section of the interference region, respectively; providing first, second, and third measurement data by the first, second, and third photodetectors, respectively, wherein the measurement data encode an intensity or change in intensity of the detected light; and calculating a two-dimensional strain change of the interference region from the first, second, and third measurement data by means of an evaluation unit. An advantage of the invention is that it enables contactless two-dimensional strain measurement of the object being measured.This allows for the precise investigation of stresses on components under a wide variety of conditions, including realistic tests involving forces acting at multiple points on the component and materials exhibiting complex strain behavior. Furthermore, no mounting or mechanical contact with the object being measured is required, resulting in lower measurement uncertainty compared to strain gauges. Surprisingly, only three photodetectors are sufficient to calculate two-dimensional strain, whereas two photodetectors are required to calculate one-dimensional strain.

[0023] The first and second partial beams are directed onto the object being measured by means of an emission module. For the purposes of this description, an emission module is understood to be a component or assembly that emits light. The emission module comprises at least one light source. The at least one light source comprises, for example, a laser, such as a laser diode, for example, operating in continuous wave mode and / or preferably with a pulse duration of at least 10 milliseconds, preferably at least 100 milliseconds. The luminous power of the at least one light source is preferably at least 10 pW, preferably at least 100 pW, preferably at least 400 pW, preferably at most 20 mW, preferably at most 12 mW, preferably at most 10 mW, preferably at most 1 mW.

[0024] The emission module comprises, for example, exactly one light source and a beam splitter configured to divide the total beam emitted by the light source into a first partial beam and a second partial beam. Alternatively, the emission module comprises, for example, a first light source configured to emit a first partial beam and a second light source configured to emit a second partial beam. In this case, the two light sources may be grouped together as an assembly and / or mechanically connected, for example, within a common housing. Alternatively, the two light sources may be spaced apart and not mechanically connected. In this case, the two light sources form the emission module as a conceptual unit due to the function of the two light sources.

[0025] It is possible, but not necessary, for the emission module to include further optical elements, such as those for deflecting and / or splitting the first and / or second partial beam. For example, the emission module may have deflection optics configured to deflect the first and / or second partial beam such that the first and second partial beams overlap on the object being measured within an interference region, resulting in interference fringes within that region. The deflection optics may, for example, include at least one mirror or other optical element with a reflectance of at least 60%, preferably at least 75%, at the wavelength of the first and / or second partial beam.

[0026] The emission module emits light in the form of a first and second partial beam. The first and second partial beams preferably have a wavelength bandwidth, defined as the difference between the smallest and largest emitted wavelengths at a given time, of a maximum of 100 pm, preferably a maximum of 10 pm, preferably a maximum of 1 pm, and preferably a maximum of 0.1 pm. A central wavelength at which the luminous intensity of the first and second partial beams is maximum is preferably at least 300 nm and preferably a maximum of 2100 nm. Preferably, the central wavelength of the first and second partial beams is at least 600 nm, and preferably at least 750 nm. For example, the central wavelength of the first and second partial beams is between 1200 nm and 1700 nm.

[0027] Preferably, the frequency difference between the central frequencies of the first and second partial beams is a maximum of 1 GHz, preferably a maximum of 100 MHz, and preferably a maximum of 50 MHz. It is possible, but not necessary, for the central wavelength of the first and / or the second partial beam to be time-varying, for example, by means of a frequency-shifting optic.

[0028] The first and second partial beams are preferably coherent to each other. For the purposes of this description, coherence means that the coherence length is at least as large as the optical path length difference in the interference region between the first and second partial beams. For example, the coherence length is at least 1 m. Preferably, the first and second partial beams are also coherent to themselves. The first and second partial beams are preferably directed by the emission module, for example, a deflection optic belonging to the emission module which may include mirrors, onto the object being measured in such a way that interference fringes are formed on the object within the interference region. The interference fringes exhibit interference maxima and interference minima.The distance between adjacent interference maxima depends, among other things, on the wavelength of the light in the partial beams and on the angle at which the first and second partial beams overlap on the object being measured. The distance between adjacent interference maxima Ad is calculated from the angle 0 at which the first and second partial beams, or rather their wavefronts, meet on the surface of the object being measured, and the wavelength A using the formula.

[0029] For a fixed wavelength, the choice of angle affects both the spacing of adjacent interference maxima and the light power reflected and / or scattered from the object onto the photodetector. The larger the angle, the smaller the spacing of adjacent interference maxima and the lower the scattered light power. With a fixed photodetector area, a smaller fringe spacing results in a higher number of fringes imaged onto the photodetector, and thus higher contrast.

[0030] Furthermore, the detected strain change is proportional to the distance between adjacent interference maxima, so a smaller distance between adjacent interference maxima is advantageous for higher lateral resolution. This is achieved by appropriately selecting the angle between the first and second partial beams and / or a shorter wavelength. Thus, the choice of the distance between adjacent interference maxima is important for the signal quality at the photodetector.

[0031] The distance between adjacent interference maxima within the interference region is preferably at least 1 pm, preferably at least 50 pm. The distance between adjacent interference maxima within the interference region is preferably a maximum of 1 mm, preferably a maximum of 200 pm. This ensures both sufficiently high light intensity and sufficiently high contrast on the photodetector. The interference fringes generated on the object by the first and second partial beams are at least partially scattered and / or reflected onto the photodetectors. The photodetectors are preferably arranged such that light from the interference fringes, which has been reflected and / or scattered by the object within the respective section, can strike the respective photodetector directly. "Directly" here means that no deflection optics are required.In contrast, at least one lens and / or objective for each photodetector is preferably located between the interference region and the photodetectors along the optical path of the reflected and / or scattered light. This improves the imaging and signal strength. Alternatively, it is possible to arrange further optical elements that direct the scattered and / or reflected light from the respective section onto the respective photodetector.

[0032] Preferably, the photodetectors are arranged such that the first photodetector can mainly detect light from the first section, and preferably a maximum of 1% of the total intensity on the photodetector originates from sections other than the first section, more preferably a maximum of 0.1%. The same preferably applies accordingly to the other photodetectors.

[0033] Preferably, all sections of the interference region are disjoint from each other, meaning that there is no overlap between the sections. Thus, the sections are spaced apart. This allows the photodetectors to be arranged so that they each detect only light from their respective section and not from other sections. This reduces the measurement uncertainty.

[0034] For example, additional imaging optics, such as lenses, are arranged between the object being measured and the photodetectors. This serves to improve the imaging of the interference fringes onto the photodetectors.

[0035] The photodetectors detect the scattered and / or reflected light of the object being measured and preferably generate measurement data from it that encodes the intensity or change in intensity, and pass this measurement data on to the evaluation unit.

[0036] The evaluation unit determines the strain change from the photodetector measurement data, preferably using the following calculation method. From the measurement data of the individual photodetectors, the evaluation unit determines, for example, a velocity vector and / or displacement vector that describes the movement of the interference fringes at a measurement point between two time points.

[0037] The evaluation unit preferably assigns a position vector to each measurement point. Strain values ​​in three directions are calculated from the three velocity vectors along these three position vectors. For this purpose, velocity vectors are calculated along three additional virtual measurement directions, each located, for example, midway between two adjacent sections or photodetectors. This can be achieved, for instance, by vector addition of two of the velocity vectors along each measurement direction. From the resulting six velocity vectors, the evaluation unit preferably calculates three strain values, each along one of these directions. The three directions are distinct from one another, meaning that their vectors form an angle with each other that is not 0°, 180°, or 360°.

[0038] Preferably, the three directions each form an angle of 120° to each other. While this is not required for the calculation, it simplifies the process. An example of such a calculation is described in detail in the explanation of Figures 2a and 2b. Alternatively, other calculation methods are also possible that calculate a two-dimensional strain change from the measurement data. In this description, a two-dimensional strain change is understood to be an absolute strain change along an x-axis and a y-axis orthogonal to it, which define a plane between them that is essentially parallel to the measured surface of the object.

[0039] The detector area of ​​the first, second and third photodetector is preferably at least 4 pm each. 2 , preferably at least 20 pm 2 The detector area is preferably a maximum of 500 mm². 2 preferably a maximum of 100 mm2 .

[0040] Preferably, the device is designed such that the first and second sections are not opposite each other; preferably, the same applies to the second and third sections and / or to the first and third sections. "Not opposite each other" here means, in particular, that the respective two sections are not opposite each other with respect to a geometric center of gravity that exists for the three sections. Specifically, the geometric center of gravity of the sections is understood here to be the center of gravity with respect to the three points that each form the geometric center of gravity of an individual section. In other words, the sections have a direction with respect to a geometric center of gravity that forms an angle other than 180°. Preferably, the angle is also other than 0° and other than 360°.

[0041] This allows three photodetectors to be sufficient for the evaluation unit to calculate the two-dimensional strain from the measurement data of the three photodetectors. This is because two opposing sections would only be able to have their corresponding photodetectors record measurement data along the same direction, meaning that with three photodetectors only a maximum of two directions could be recorded.

[0042] Preferably, the device is designed such that at least two of the three sections have a direction with respect to a geometric center of gravity of the three sections, between which an angle of at least 30° and at most 150° is, preferably at least 60°, preferably at most 130°, preferably at least 80°, preferably at least 110°, preferably at least 115°, preferably at most 125°.

[0043] Preferably, the device is designed such that the three sections are arranged in such a way that the angle of the directions between any two of the three sections is preferably 120° ± 3°.

[0044] This makes it particularly easy to calculate the two-dimensional strain from the measurement data of the three photodetectors, as described in the explanation of Figures 2a and 2b.

[0045] To achieve an arrangement of the sections as described in the previous variants, the emission module is configured, for example, to split and / or deflect the first and second partial beams. Alternatively, the device preferably includes beam-shaping optics configured to shape, split, and / or deflect the first and second partial beams such that the sections are arranged on a measuring surface of the object being measured, as described above. Beam-shaping optics are understood to mean, in particular, an optical element or a group of optical elements, whether connected or not.

[0046] Preferably, if the device has exactly three photodetectors, the beam shaping optics include at least one deflection optic configured to split the first and / or second partial beam into three section beams each, and preferably to deflect the section beams such that preferably one section beam of the first partial beam forms an interference pattern with one section beam of the second partial beam in each of the three sections. This makes it possible, in a cost-effective and space-saving manner, to ensure that hardly any light power reaches a part of the interference area that does not belong to the three sections. Thus, the energy efficiency of the device is increased.

[0047] Alternatively or additionally, the beam shaping optics may, for example, feature a computer-generated hologram. This is particularly advantageous if the device has more than three photodetectors, because splitting and deflecting more than three section beams using deflection optics is generally more expensive and space-consuming than a computer-generated hologram.

[0048] The total area of ​​the interference region is preferably at least 3 pm. 2 , preferably at least 20 pm 2 , preferably at least 1 cm 2 The total area of ​​the interference region is preferably a maximum of 20 cm². 2 , preferably a maximum of 10 cm 2 .

[0049] The device preferably includes a focusing optic. Preferably, the focusing optic for focusing the first partial beam is designed such that interference fringes are formed, for which there exist straight lines perpendicular to the interference fringes of the first, second, and third sections, all of which essentially intersect at a common point. "Essentially" here means that the deviation of each individual line from the common point of intersection is a maximum of 5% of the distance between the respective interference fring and the ideal point of intersection. For example, if the distance between an interference fring and the ideal point of intersection is 2 cm, the straight line perpendicular to this interference fring may miss the point of intersection by a maximum of 1 mm.Deviations of the interference fringes and the perpendicular lines are possible, for example, due to unevenness and / or curvature of the surface of the object being measured and / or due to optical distortions of the partial beams. In the device described here, unevenness has essentially no effect on the measurement data or on the two-dimensional strain change.

[0050] This geometric arrangement makes calculating the two-dimensional strain change of the measured object particularly simple and precise. The calculation is explained in detail in the description of Figures 2a and 2b.

[0051] For example, the focusing optics are designed to focus the first and second partial beams in two different focal planes. Focusing optics refers to a component or assembly that refracts the light of the first and / or second partial beam in such a way that it assumes a minimum beam diameter in the first and second focal planes, respectively. The first focal plane is the plane in which the first partial beam has a minimum beam diameter. The second focal plane is the plane in which the second partial beam has a minimum beam diameter.

[0052] Preferably, the focusing optics are configured to focus the first and / or second partial beam such that the wavefronts of the first and / or second partial beam are curved on a surface of the object being measured. Curved here means that the curvature of the wavefronts is non-zero. Preferably, the focusing optics are configured to focus the first and / or second partial beam such that the wavefronts of the first and / or second partial beam exhibit a curvature of at least 0.1 on a surface of the object being measured. at least 2 in sections — , preferably at least 10 — . It is possible that the mm

[0053] Focusing optics designed to focus such that only the wavefronts of the first partial beam are curved and that the wavefronts of the second partial beam are straight or have a lesser curvature than the wavefronts of the first partial beam.

[0054] The distance between the first focal plane and the second focal plane is preferably at least 1 pm, preferably at least 2 pm, preferably at least 5 pm. The distance between the first and second focal planes is preferably at most 120 cm, preferably at most 50 cm, preferably at most 10 cm, preferably at most 1 cm.

[0055] Preferably, the first and / or second focal plane are designed approximately parallel to a surface plane of the object being measured, on which the interference fringes originate. Approximately parallel here means an angular deviation of no more than 5°, preferably no more than 2°. The surface plane of the object being measured is defined as the plane resulting from averaging all surface points of the object on the surface on which the interference fringes originate and from which scattered and / or reflected light from the interference fringes is detected by the photodetectors.

[0056] It is possible, but not required, for one of the two focal planes to have a distance to the surface plane of the object being measured of a maximum of 2 mm, preferably a maximum of 1 mm.

[0057] To achieve focusing in two different focal planes, the focusing optics comprise, for example, at least one first partial optic, and preferably also a second partial optic. Each partial optic comprises, for example, at least one, and preferably at least two, lenses. The first partial optic is configured, for example, to focus the first partial beam. The second partial optic is configured, for example, to focus the second partial beam or the first and second partial beams together. The partial optics and any other optical elements of the focusing optics can be mechanically connected to one another or spaced apart without any mechanical connection.

[0058] Preferably, the emission module or focusing optics are configured such that the first and second partial beams strike a surface of the object being measured at an angle of at least 30°, preferably at least 60°, preferably at least 70°, preferably at least 80°, preferably 90° ± 1°. The angle is understood to be the smallest possible angle to the surface of the object being measured onto which the partial beams strike.

[0059] Preferably, the first, second, and third sections are each substantially the same distance from one another. "Substantially" here means that the distance measured between a geometric center of mass of the sections deviates by a maximum of 5% from the mean of all distances between the geometric centers of mass of the sections. This simplifies the calculation of the two-dimensional strain change.

[0060] Preferably, the first, second, and third sections each have substantially the same distance to a centroid of the closed line of interference fringes. "Substantially" here means that the distance measured between the geometric centroid of the sections and the geometric centroid of the closed line to which the interference fringes can be connected deviates from the mean of the distances by a maximum of 5%. This further simplifies the calculation of the two-dimensional strain change.

[0061] Preferably, the photodetectors are designed for time-dependent detection of light and provision of time-dependent measurement data to the evaluation unit, preferably with continuous detection of light or detection at discrete measurement times with an average frequency that is preferably at least 200 kilohertz, preferably at least 2 megahertz, preferably at least 20 megahertz, preferably at least 100 megahertz, preferably at least 200 megahertz, preferably at least 1 gigahertz, preferably at least 10 gigahertz.

[0062] Preferably, the evaluation unit is designed to calculate a time-dependent change in strain, preferably with a time resolution of at least 1 kilohertz, preferably at least 100 kilohertz, preferably at least 1 megahertz, preferably at least 10 megahertz, preferably at least 100 megahertz. This allows the time-varying behavior of the object being measured during a measurement period, for example, during a load such as a tensile test or other force application, or environmental changes such as a temperature change.

[0063] The device preferably has a housing in which at least the emission module, the focusing optics, and the photodetectors are mounted. This allows the device to be easily transported between different measurement locations and reused with little or no readjustment.

[0064] The evaluation unit can be located separately from the housing or inside the housing. The evaluation unit could, for example, be a computer that can be connected to the photodetectors via cable or other data connection.

[0065] The device preferably has a total weight of no more than 50 kg, preferably no more than 20 kg, preferably no more than 10 kg, preferably no more than 5 kg, preferably no more than 1 kg. This makes the device portable and easy to transport.

[0066] The device preferably has a total volume of no more than 0.5 m³ 3 , preferably a maximum of 0.1 m 3 , preferably a maximum of 0.04 m 3 , preferably a maximum of 0.005 m 3 This makes the device particularly easy to transport.

[0067] Preferably, the device includes a direction detection unit configured to detect the direction of the strain change. Preferably, the direction detection unit is configured to detect the direction of the strain change. Preferably, the direction is detected without contact, preferably optically, preferably by means of the first and second partial beams.

[0068] Preferably, the device includes a frequency shifting device configured to shift the frequency of the first partial beam and / or the second partial beam such that the difference frequency between the frequency of the first partial beam and the frequency of the second partial beam is non-zero, preferably at least 1 kilohertz, preferably at least 100 kilohertz, preferably at least 1 megahertz, and preferably at most 10 gigahertz. The difference frequency is also referred to as the carrier frequency.

[0069] The carrier frequency is preferably variably adjustable, for example by means of a phase-locked loop. This makes it possible to adapt the carrier frequency to the strain change frequencies of the object being measured.

[0070] The evaluation unit is preferably designed to calculate a change in strain from the measurement data of the photodetectors and from the carrier frequency.

[0071] This makes the device suitable for determining the sign and / or direction of the strain change. The exact calculation is explained by way of example in the description of Figures 2a and 2b. The evaluation unit is preferably configured to determine the frequency of a temporal change in the intensity of the light detected by the photodetectors from the measurement data. For example, the evaluation unit is configured to calculate a Doppler shift, for example by calculating the difference frequency between the frequency of the intensity change of the detected light and the frequency of the frequency shift device.

[0072] The frequency-shifting device includes, for example, an acousto-optic modulator, also known as a Bragg cell. This shifts the frequency of the incident light and simultaneously changes its polarization. It is possible to shift the frequency of either the first or the second partial beam. Alternatively, it is possible to shift the frequencies of the first and second partial beams such that the difference frequency between them changes over time, preferably periodically, preferably with a frequency of at least 1 kilohertz, and preferably a maximum of 950 megahertz.

[0073] Alternatively or additionally to the frequency shift device, the device may include a phase shift unit configured to shift the phase of the first and / or second partial beam such that the phase difference between the phases of the first and second partial beams is non-zero. In this case, the evaluation unit is configured to calculate a Doppler shift, for example, by determining the difference frequency between the frequency of the intensity change of the detected light and the frequency of the frequency shift device. The magnitude of the strain change of the measured object can be calculated from the magnitude of the difference frequency. The direction of the strain change can be determined from the sign of the difference frequency.

[0074] Each photodetector preferably comprises at least one first sub-detector and one second sub-detector, configured such that the first sub-detector detects a first polarization component and / or a first angular component of the light that differs from a second polarization component and / or a second angular component of the light detected by the second sub-detector. Preferably, the evaluation unit is configured to average the measurement data of the first and second sub-detectors for each photodetector. Preferably, in the case of different angular components, the angular components detected by the sub-detectors do not overlap. Preferably, in the case of different polarization components, the polarization components detected by the first and second sub-detectors are orthogonal to each other.

[0075] This reduces the noise from the intensity changes of the light, caused by speckle patterns scattered and / or reflected by the interference fringes, through a combination of the individual signals by the evaluation unit. Such a combination includes, for example, calculating a weighted average or root mean square of the measurement data from the sub-detectors. Speckle patterns are interference patterns that arise from the coherent illumination of uneven or rough surfaces. The speckle patterns detected by two detectors at non-overlapping angles or with mutually orthogonal polarization are statistically independent of each other for all surfaces if, as in this case, only scattered light without reflected components is collected. Thus, combining the measurement data from the two sub-detectors reduces the noise due to possible speckle patterns on the surface of the object being measured. This reduces the measurement uncertainty.

[0076] For example, the emission module emits a linearly polarized laser beam, which is split by the beam splitter unit, for example, a polarization beam splitter, into an s-polarized first partial beam and a p-polarized second partial beam. The polarization of the p-polarized partial beam is changed by 90° using, for example, a delay plate, such as by rotation and / or circular polarization. The photodetectors then each have, for example, a first and second partial detector, each of which detects only s- or p-polarized light. This achieves polarization diversity. Preferably, the evaluation unit is designed to average the measurement data from each pair of partial detectors, thus reducing potential speckle noise.

[0077] Alternatively or additionally, for example, a further splitter unit, such as a non-polarizing beam splitter, divides the total beam or partial beams into two sub-beams. These sub-beams are then directed onto the measurement surface, for example, using mirrors, so that they strike the surface at different angles. The light scattered and / or reflected by the interference pattern on the measurement object preferably passes through another splitter unit, such as a mirrored knife-edge prism, which allows for splitting in two directions without significant wavefront distortion. This ensures that both sub-detectors of each photodetector receive approximately the same signal. As a result, the two sub-detectors can be arranged separately, and their distance is not limited by the housing dimensions of the sub-detectors.This simplifies the design of the device. Furthermore, the angular diversity enables a reduction in speckle noise.

[0078] In cases where angular diversity and polarization diversity are used, each photodetector preferably has four subdetectors, of which two subdetectors differ with respect to the polarizations or angles they detect. Details can be found in the description of Figures 3a to 3c.

[0079] Preferably, in the case of polarization diversity, the emission module and / or focusing optics are configured such that the angle of incidence of the first and / or second partial beam on the surface of the object being measured is not equal to 90°, preferably a minimum of 60° and a maximum of 120°. This essentially depolarizes the light scattered by the sample, enabling the detection of two different polarization components by the two partial detectors of the photodetectors, for example, the perpendicular and parallel polarization components. Preferably, the partial detectors are configured to detect different signal powers. Preferably, the evaluation unit is configured to calculate a weighted combination that preferably takes into account power differences between the signal powers of the partial detectors, for example, by means of maximum ratio combining, in particular as described in F. Wang, S. Krause, and C.Rembe “Signal diversity for the reduction of signal dropouts and speckle noise in a laser-Doppler extensometer”, Measurement: Sensors, 2022. DOI: 10.1016 / j.measen.2022.100377, the disclosure of which is hereby incorporated.

[0080] In one embodiment, the device comprises a fourth, a fifth, and a sixth photodetector, configured to detect light scattered and / or reflected from the fourth, fifth, or sixth section of the interference region, respectively, and to provide fourth, fifth, or sixth measurement data encoded as the intensity or change in intensity of the detected light. The evaluation unit is preferably configured to calculate the strain change from the first through sixth measurement data. This allows for averaging the measurement data from any two photodetectors, thus achieving a lower measurement uncertainty.

[0081] Preferably, the first and second sections are opposite each other. Preferably, the third and fourth sections are opposite each other. Preferably, the fifth and sixth sections are opposite each other. "Opposite" here refers preferably to a geometric center of gravity of the closed line to which the interference fringes can be connected. In this arrangement, a particularly simple calculation of the strain change and a simple noise averaging of the measurement data from opposing photodetectors are possible.

[0082] Preferably, the first and second sections are substantially the same distance from each other and / or from a centroid of the closed line to which the interference fringes can be connected. Preferably, the third and fourth sections are substantially the same distance from each other and / or from a centroid of the closed line to which the interference fringes can be connected. Preferably, the fifth and sixth sections are substantially the same distance from each other and / or from a centroid of the closed line to which the interference fringes can be connected. The distance is measured from the geometric centroid of each section. "Substantially" is understood to mean a maximum deviation of 5% of the distance.This makes it particularly easy to combine the noise of the measurement data from the first and second photodetectors, the third and fourth photodetectors, and the fifth and sixth photodetectors, for example by averaging or using maximum ratio combining, and thus reduce the measurement uncertainty.

[0083] Preferably, the closed line to which the interference fringes can be connected forms essentially a circle or an ellipse. This allows for a simple calculation of the two-dimensional strain change from the measurement data. "Essentially" here means that the sum of all deviations from a circle or an ellipse is at most 10% of the circle's radius in the case of a circle, or the average of the two semi-axes in the case of an ellipse. Preferably, the radius or major semi-axis of the closed line to which an outermost interference fring can be connected is at most 30 cm, preferably at most 20 cm, preferably at most 10 cm, preferably at least 1 pm, preferably at least 1 mm, and preferably at least 5 mm.

[0084] Preferably, the emission module and / or the focusing optics are designed such that the geometric centers of the sections can be connected to form a closed circle, the center of which preferably coincides substantially with an intersection point of the lines that can be constructed perpendicular to the interference fringes of the sections. "Substantially" here means a deviation of no more than 5% of the circle's radius.

[0085] Such an arrangement makes calculating the two-dimensional strain change from the measurement data particularly easy. Furthermore, the measurement uncertainty is especially low, since the noise from each pair of opposing photodetectors—in this case, the photodetectors for the first and second sections, the third and fourth sections, and the fifth and sixth sections—can be averaged.

[0086] Preferably, all sections are disjoint from each other. Disjoint here means that there is no overlap between the sections. Thus, the sections are spaced apart. This allows the photodetectors to be arranged so that only the scattered and / or reflected light from exactly one section reaches each photodetector. This reduces the measurement uncertainty.

[0087] Preferably, if there are a first to sixth section, the connecting lines of the geometric centroids of adjacent sections to the center of the circle each form an angle between 40° and 80°, preferably between 50° and 70°, preferably between 56° and 64°, preferably between 59° and 61°. This allows for a particularly simple calculation of the change in strain.

[0088] Preferably, the device includes a beam-shaping optic configured to shape the first and second partial beams such that a maximum of 10% of the light power is directed into parts of the interference region that do not belong to either of the sections, preferably a maximum of 5%, and more preferably a maximum of 2%. This increases the signal strength at the photodetectors for a given light power of the emission module. The beam-shaping optic preferably incorporates a computer-generated hologram. Alternatively, the beam-shaping optic may comprise, for example, individual or combined, preferably integrated, optics, preferably micro-optics.

[0089] It is possible for the device to have a different number of sections and photodetectors than three or six. For example, for specific material properties such as symmetries in the elasticity of the object being measured, it is advantageous for the device to have four, eight, or nine sections and photodetectors. The sections are preferably arranged symmetrically around a geometric center of gravity of all sections. Preferably, all sections are equidistant from each other and from the center of gravity.

[0090] Another aspect of the invention relates to a method for non-contact measurement of the strain of a test object. The method comprises the step of providing a first partial beam and a second partial beam, which are coherent to each other so that they interfere with each other within an interference range, by means of an emission module.

[0091] Furthermore, the method assigns the following steps to the evaluation unit: detecting light scattered and / or reflected from a second section of the interference region; providing second measurement data encoding the intensity or intensity change of the detected light; and calculating a two-dimensional strain change of the interference region from the first, second, and third measurement data using an evaluation unit.

[0092] The same possibilities and advantages apply to the design of the individual steps as explained above for the device.

[0093] For example, the method may additionally acquire fourth, fifth, and sixth measurement data from the fourth, fifth, and sixth photodetectors, wherein the fourth, fifth, and sixth measurement data comprise an intensity or intensity change of light scattered and / or reflected from, respectively, a fourth, fifth, or sixth section of the interference region. In this case, the calculation is preferably performed from the first through sixth measurement data and provided directional information. Furthermore, the method may include a frequency shifting step using a frequency shifting device such that a difference frequency between a frequency of the first and second partial beam, i.e., a carrier frequency, changes over time, preferably periodically, preferably with a carrier frequency of at least 1 kilohertz, preferably at least 100 kilohertz, and preferably a maximum of 10 gigahertz.In this case, the calculation preferably includes determining a Doppler shift or a difference frequency between a frequency of the detected intensity change and the carrier frequency.

[0094] Preferably, the directional information or the direction of the strain change is determined from the sign of the Doppler shift or the calculated difference frequency. Preferably, the magnitude of a strain change along at least one direction, preferably along three different directions, is determined from the magnitude of the Doppler shift or the calculated difference frequency.

[0095] The invention will now be explained in more detail with reference to the accompanying drawings. These drawings show

[0096] Figure 1a shows a schematic sketch of a device according to the invention for contactless measurement of a change in strain of a measuring object in a side view.

[0097] Figure 1b shows a top view of the interference region and the first to third sections from which scattered and / or reflected light is detected by the first to third photodetectors.

[0098] Figure 1c shows a top view of the interference region and the first to sixth sections from which scattered and / or reflected light is detected by the first to sixth photodetectors.

[0099] Figures 2a and 2b each show a sketch for calculating the two-dimensional strain change from the measurement data of the three and six sections respectively.

[0100] Figures 3a, 3b, and 3c each show a section of a device according to the invention with two partial detectors 51a and 51b for the first photodetector 51 for detecting polarization diversity (3a) and angular diversity (3b), as well as with four partial detectors 51a to 51d for detecting polarization diversity and angular diversity (3c).

[0101] Figure 4 shows a device with an acousto-optic modulator as a frequency shifting device for determining the direction information and

[0102] Figure 5 shows a device with two phase-coupled lasers as a frequency shift device for determining the direction information,

[0103] Figure 6 shows a device with detailed focusing optics and beam shaping optics in a three-dimensional oblique view.

[0104] Figures 7a, b and 8 show a device with detailed focusing optics and beam shaping optics in three different sectional planes.

[0105] Figure 9 shows a device with detailed focusing optics and beam shaping optics in the first section plane E1 with a Bragg cell as a frequency shift device.

[0106] Figure 1a shows a device 10 according to the invention for contactless measurement of a strain change in a test object 20. The device 10 has an emission module 30 which includes a light source 32. The light source 32 is a laser with a wavelength of, for example, 1550 nm. The light source 32 is configured to emit a first, preferably coherent, partial beam 34 and a second, preferably coherent, partial beam 36 such that the first partial beam 34 and the second partial beam 36 overlap on the test object 20 within an interference range 40, so that interference fringes are formed within the interference range 40. The first partial beam 34 is coherent with the second partial beam 36.

[0107] Here, for example, the device is configured such that the first section 41 and the second section 42 are not opposite each other, and the same applies, for example, to all sections 41, 42, and 43. With respect to a geometric center of gravity, which here is an intersection point 75 of the lines 74 that exist perpendicular to the interference fringes, sections 41, 42, and 43 each have a direction that forms an angle of 120° with the directions of the other two sections. Furthermore, the device preferably has a focusing optic 38 (shown in Figures 4 and 5) for focusing the first partial beam 34 and the second partial beam 36.

[0108] The device also includes a first photodetector 51 (shown in Figure 1b) configured to detect light scattered and / or reflected from a first section 41 (shown in Figure 1b) of the interference region 40 and to provide initial measurement data, encoding an intensity or change in intensity of the detected light, to the evaluation unit. The device also includes a second photodetector 52 (shown in Figure 1b) configured to detect light scattered and / or reflected from a second section 42 (shown in Figure 1b) of the interference region 40 and to provide secondary measurement data, encoding an intensity or change in intensity of the detected light, to the evaluation unit.The device also includes a third photodetector 53 (shown in Figure 1b) configured to detect light scattered and / or reflected from a third section 43 (shown in Figure 1b) of the interference region 40, and to provide third measurement data encoding an intensity or change in intensity of the detected light to the evaluation unit.

[0109] The device also has an evaluation unit 60, which is designed to calculate a two-dimensional strain change of the object being measured within the interference range 40 from the first, second and third measurement data.

[0110] Figure 1b shows a top view of the interference region 40 with the first section 41, the second section 42 and the third section 43, from which scattered and / or reflected light passes through the first photodetector 51, second photodetector

[0111] Figure 1c shows a top view of the interference region and the first to sixth sections 41, 42, 43, 44, 45, 46, from which scattered and / or reflected light is detected by the first to sixth photodetectors 51, 52, 53, 54, 55, 56. The arrangements of the sections and photodetectors shown in Figures 1b and 1c represent two different embodiments of the invention.

[0112] The distance between adjacent interference maxima Ad is shown here such that almost three interference maxima are located within the area defined by the photodetectors 51 to

[0113] The interference maxima are located in the portion of sections 41 to 43 and 41 to 46, respectively, as shown in sections 53 and 51 to 56. This is for simplified illustration purposes only. In reality, there are preferably at least five interference maxima within the portion detected by the photodetectors, preferably at least ten, preferably a maximum of 500, and preferably a maximum of 200.

[0114] The interference fringes of the same order, as shown in Figures 1b and 1c, form, for example, a closed line 70, which in each case forms a circle. Furthermore, straight lines 74 exist for each of the interference fringes of sections 41 to 43 and 41 to 46, respectively, which are perpendicular to the interference fringes and all intersect at a point 75. This point 75 is identical here to the center of the circle to which the interference fringes can be connected. However, this is not necessary; for example, it is possible for the interference fringes to have no curvature, as shown in Figures 2a and 2b.

[0115] With the geometry shown in Figures 1b and 1c, it is easily possible to determine the strain along the three directions from the measurement data of the photodetectors. Calculate e2 and E3ZU.

[0116] Figures 2a and 2b each show a sketch for calculating the two-dimensional strain change from the measurement data of the three sections 41, 42, 43 and six sections 41 to 46, respectively. For this purpose, as indicated by the straight interference fringes in Figures 2a and 2b, it is assumed that the interference fringes within the part of the first to third sections 41, 42, 43 and the first to sixth sections 41 to 46 detected by the respective photodetectors are approximately curvature-free, i.e., they are approximated with straight lines.

[0117] Alternatively, it is possible to generate the curvature-free interference fringes shown in Figures 2a and 2b on the surface of the object being measured using a corresponding focusing optic 38 and optionally further optical elements such as deflection optics (see also Figures 6 to 9).

[0118] Starting from the intersection point 75 of the lines 74, which are perpendicular to the interference fringes, the evaluation unit preferably calculates three position vectors r lt r2 and r3 are calculated for the three sections 41, 42, 43. For example, the geometric centroid of each section is used to calculate the position vector. For each of the position vectors r t A speed v will be achieved t The evaluation unit determines the position vectors, with the positive sign pointing in the direction of the position vectors. Each of the position and velocity vectors has an angle. <p l t <p2und <p3zur x-Achse, die beispielsweise wie in Figur 2a und 2b definiert sein kann. Andere Koordinatensysteme sind alternativ möglich.

[0119] The intensity of the scattered and / or reflected light from the sections of the interference region is composed, for example, of a DC component I. Q i, which is constant over time and is preferably suppressed by means of a high-pass filter of the photodetectors, a time-dependent amplitude I of a carrier frequency f c , which denotes the difference between the frequencies of the first and second partial beams, generated, for example, by a frequency-shifting device such as a Bragg cell or by a phase-locking unit. Furthermore, the intensity exhibits a phase shift. f (t) is caused by surface movement. The overall intensity can then be expressed as:

[0120] The photodetectors transmit measurement data to the evaluation unit, which encodes the intensity or change in intensity of the detected light. If the photodetectors transmit time-dependent measurement data to the evaluation unit, and the measurement data represents the intensity in the form of a time-dependent electrical voltage... To encode the measurement data, for example, it can be formulated using the time-dependent voltage amplitude üt, the sensitivity K, and the active detector area A of the photodetector as:

[0121] Using the coordinate system shown in Figures 2a and 2b, the evaluation unit preferably calculates the position and velocity vectors as follows: The evaluation unit then preferably calculates an intermediate virtual additional vector from each pair of vectors, each for the velocity-

[0122] — » digkeit and the place, for example, as v'i = Vj- + V[ +1 for the speed at — »

[0123] Location r'i = r i-1 + r i+1 , where (1,2,3) and 3+1 =1 and 1 -1 =3 hold true, resulting in a modulo addition.

[0124] The evaluation unit preferably calculates the interference maximum distance Ad along three directions. with i = 1, 2, 3, which can be arbitrarily oriented as long as there is no angle of 180° between the directions, wherein the directions preferably each have an angle of 120° to each other

[0125] Preferably, the photodetectors are designed for time-dependent acquisition of the measurement data and the evaluation unit is preferably designed for calculating the time-dependent strain change, such that three time-dependent strain changes £ f (t) with i = 1 , 2, 3 are obtained, where b L = \r t + r' t \ applies:

[0126] The calculation is performed analogously if the device has six sections 41 to 46 and six photodetectors 51 to 56, whereby the first calculation steps involving the formation of the virtual additional vectors are omitted.

[0127] Figures 3a, 3b and 3c each show a section of a device according to the invention for detecting polarization and / or angular diversity.

[0128] Figure 3a shows a section of a device in which polarization diversity is used to reduce speckle noise. Preferably, the first and second partial beams 34, 36 (see Figures 1 and 2) have circular polarization, for example, due to one or each a quarter-wavelength delay plate 160, 162 (see also Figures 4 and 5). Preferably, the device is also configured to detect the different polarization components for speckle noise reduction. For this purpose, each of the photodetectors has two partial detectors. This is shown in Figure 3a using the example of the first photodetector 51, which has two partial detectors 51a, 51b, onto which the light scattered and / or reflected by the first section 41 is split by means of a beam splitter 80 and imaged by means of a lens 81a, 81b, respectively.

[0129] Alternatively, imaging without a lens or with multiple optical elements is also possible. In this example, the beam splitter 80 is a polarizing beam splitter. For improved imaging, an objective 90, here consisting of two lenses, is positioned between the object 20 being measured and the beam splitter 80.

[0130] Alternatively, it is possible that the device does not have a lens or other optical elements to improve the image on the photodetectors.

[0131] Figure 3b shows a section of a device in which angular diversity is used to reduce speckle noise. This requires the detection of the different angular components. For this purpose, each of the photodetectors has two sub-detectors. This is shown in Figure 3b using the example of the first photodetector 51, which has two sub-detectors 51a and 51b. A portion of the light scattered and / or reflected by the first section 41 is imaged onto each sub-detector by means of a lens 90a and 90b, respectively, and a lens 81a and 81b. Alternatively, imaging without a lens and / or lenses or with other optical elements is also possible.

[0132] Figure 3c shows a section of a device in which polarization and angular diversity are used to reduce speckle noise. Each of the photodetectors has four subdetectors. This is shown in Figure 3c using the example of the first photodetector 51, which has four subdetectors 51a, 51b, 51c, and 51d. Two subdetectors 51a, 51c and 51b, 51d each detect the same proportion of the light scattered and / or reflected by the first section 41 at one of two different solid angles using a lens 90a, 90b. The two angular components are each split by a beam splitter 80a, 80b. The beam splitters 80a, 80b are, for example, polarizing beam splitters. The light is focused onto each of the sub-detectors 51a, 51b, 51c, 51d by means of a lens 81a, 81b, 81c, 81d. Alternatively, focusing without a lens and / or lenses or with other optical elements is also possible.

[0133] From the measurement data generated by the sub-detectors, the evaluation unit 60 calculates the two-dimensional strain change with reduced speckle noise influence due to polarization or angle diversity.

[0134] Figure 4 shows a device 10 with an acousto-optic modulator as a frequency shift device 110 for determining the directional information. The frequency of the coherent light emitted from the emission module 30 is shifted by means of the frequency shift device 110, in this case by a frequency of, for example, 40 megahertz. The light is directed via two mirrors 120a, 120b onto a beam splitter 130, which splits the light into a first partial beam 34 and a second partial beam 36. The beam splitter 130 is designed here, for example, as a beam splitter, such as a polarizing or a non-polarizing beam splitter.

[0135] The first partial beam 34 is again passed through the frequency shifting device 110, for example, by means of a non-polarizing beam splitter 140. This shifts the frequency of the first partial beam 34 relative to the frequency of the second partial beam 36, so that the difference frequency between the frequencies of the first and second partial beams 34, 36 changes. During the second passage of the first partial beam through the frequency shifting device 110, the polarization of the first partial beam 34 is rotated by 90°. If the frequency shifting device 110 is non-polarizing, a delay plate is preferably arranged so that it is in the beam path of the first or second partial beam 34, 36, so that the polarization of the first and second partial beams 34, 36 is different when they strike the object 20. The first partial beam 34 then passes through the mirrors 120a, 120b again to the splitter unit 130.From there, the first and second partial beams 34, 36 are directed together onto the measuring object 20.

[0136] Furthermore, in this example, the device 10 includes a beam-shaping optic 150 configured to shape the first and second partial beams 34, 36 such that essentially no light power reaches parts of the interference region 40 that do not belong to one of the sections 41 to 43 or 41 to 46. The beam-shaping optic 150 features a computer-generated hologram configured such that light from the emission module reaches only sections 41 to 43 or 41 to 46, respectively. This maximizes the light power. The device 10 also includes a lens 165 for improving the beam shape and the imaging onto the photodetectors.

[0137] The device 10 also includes a delay plate 160, which rotates the polarization of the first and second partial beams 34, 36. In this case, the delay plate 160 is a quarter-wavelength plate, so that the polarization of the partial beams 34, 36 is circular thereafter.

[0138] In total, the acousto-optic modulator 110, the beam splitter 130 and the delay plate 160 form the diversity unit 165.

[0139] The light scattered and / or reflected by the object 20 is detected by means of the photodetectors, of which the first photodetector 51 is shown here. Here, the photodetector 51 has two sub-detectors 51a, 51b which, as explained in Figure 3a, measure under two different polarizations to reduce speckle noise.

[0140] Figure 5 shows a device 10 with two phase-coupled and frequency-modulated lasers as light sources 32a, 32b of the emission module 30, such that the frequency-shifting device 110 forms part of the emission module 30. The first light source 32a emits the first partial beam 34, the second light source 32b emits the second partial beam 36.

[0141] In this example, the partial beams 34 and 36 are coherent with each other. This is achieved by means of the phase-locking unit 170. The phase-locking unit 170 also includes a frequency-shifting device 110, which here is configured as a phase-locked loop, for example, as a combination of a local oscillator, a mixer, a filter, and a voltage-controlled current source. As a result, the frequency of the first partial beam 34 is time-varying, here periodically at a frequency of 40 megahertz. The second partial beam 36 has a fixed frequency. Therefore, the difference frequency between the first partial beam 34 and the second partial beam 36 is time-varying, here periodically at a frequency of 40 megahertz. Other frequencies are possible. A difference frequency that satisfies the Carson bandwidth formula is advantageous. The Carson bandwidth formula can be found, for example, in J.R. Carson, "Notes on the theory of modulation," Proc.This is described in IRE, Vol. 10, No. 1 (Feb. 1922), pages 57-64. In other words, the difference frequency is preferably at least half the sum of the maximum expected strain frequency of the object being measured and the frequency deviation. The frequency deviation is the difference between the highest and lowest instantaneous frequencies in the light and depends on the selected carrier frequency and the strain frequency of the object being measured.

[0142] Furthermore, the device in this example includes an additional delay plate 162, which rotates the polarization of the second partial beam 36 by 90°. The second partial beam 36 passes through the beam splitter 140c and the delay plate 162 to the splitting unit 130, which in this case is a beam splitter. The portion of the first partial beam 34 that does not reach the beam splitter 140b and the phase coupling unit 170 strikes the splitting unit 130. From the splitting unit 130, the first partial beam 34 and the second partial beam 36 are guided together to the object being measured 20.

[0143] In this example, the beam splitter 130 and the delay plate 162 form a diversity unit 135, which is configured to shift a polarization of the first and / or second partial beam 34, 36, such that there is a polarization difference between the first and second partial beam 34, 36.

[0144] The imaging onto the object being measured and the recording using the photodetectors is carried out as explained in Figure 4.

[0145] Figure 6 shows a device 10 with detailed focusing optics 38 and beam shaping optics 150 in a three-dimensional oblique view. Only those components not already described in detail in the previous figures are discussed here.

[0146] The first partial beam 34 is split into section beams 34a, 34b, and 34c by means of deflection optics, which may, for example, include beam splitters and prism mirrors. The second partial beam 36 is split into section beams 36a, 36b, and 36c in a similar manner. Here, the deflection optics form, for example, a beam shaping optic 150. Furthermore, the device here includes, for example, a focusing optic 38, which may have one or more lenses for each section beam. The focusing optic 38 is configured, for example, to focus the section beams 34a and 36a such that they interfere with each other in the first section 41 and form interference fringes. Similarly, the section beams 34b and 36b form the interference fringes of the second section 42, and the section beams 34c and 36c form the interference fringes of the third section 43.

[0147] The light 180 scattered and / or reflected by the measuring object 20, shown here as a dashed line, is guided via the lens 38b of the focusing optic 38 and the lens 81 to a splitting optic 82, which splits the light and directs it to the three photodetectors 51, 52, 53. Each of the photodetectors 51, 52, 53 has, for example, two sub-detectors and one lens per photodetector, or one lens per sub-detector.

[0148] Figures 7, 8a, and 8b show a device 10 with detailed focusing optics 38 and beam shaping optics 150 in three different section planes E1 (Figure 7), E2 (Figure 8a), and E3 (Figure 8b). The arrangements of the three section planes E1, E2, and E3 are shown in Figure 8c. Compared to plane E1, planes E2 and E3 are rotated by +60° and -60°, respectively, about the z-axis.

[0149] Figures 7 and 9 show the plane E1 in which the section rays 34a and 36a are located, forming the interference fringes on the object being measured, which belong to the first section 41.

[0150] Figure 8a shows plane E2, in which section rays 34b and 36b are located, forming the interference fringes on the object being measured, which belong to the second section 42.

[0151] Figure 9 shows a device 10 similar to that shown along the first section plane E1 from Figure 7, wherein a Bragg cell is used as the frequency shifting device 110 instead of a phase coupling unit.

[0152] Each of the figures shown is schematic and not to scale. Reference list

[0153] 10 Device

[0154] 20 measuring objects

[0155] 30 Emission module

[0156] 32 Light source

[0157] 34 first partial beam

[0158] 36 second partial beam

[0159] 38 Focusing optics

[0160] 40 Interference area

[0161] 41 first section

[0162] 42 second section

[0163] 43 third section

[0164] 44 fourth section

[0165] 45 fifth section

[0166] 46 sixth section

[0167] 51 first photodetector

[0168] 52 second photodetector

[0169] 53 third photodetector

[0170] 54 fourth photodetector

[0171] 55 fifth photodetector

[0172] 56 sixth photodetector

[0173] 60 Direction detection unit

[0174] 70 closed line

[0175] 74 straight lines perpendicular to interference fringes

[0176] 75 Intersection of the lines

[0177] 80 beam splitters

[0178] 81 lens

[0179] 82 Split optics for photodetectors

[0180] 90 lens

[0181] 110 Frequency shift device

[0182] 120 mirrors

[0183] 130 partition unit

[0184] 135 Diversity Unit

[0185] 140 beam splitters

[0186] 150 Beam shaping optics 160, 162 Delay plate

[0187] 165 lens

[0188] 170 phase coupling unit

[0189] 180 scattered / reflected light

[0190] Ad distance between adjacent interference maxima

[0191] As strain change

[0192] I Intensity

[0193] AI intensity change

[0194] A wavelength

[0195] 6 Angles between the first and second partial beam

Claims

in te ll ec tualproper ty Clausthal University of Technology, Attorney's File: Adolph-Roemer-Straße 2A 3017-0061 PCT-1 38678 Clausthal-Zellerfeld Germany Date: October 27, 2025 Patent claims 1. Device (10) for non-contact measurement of a strain change (As) of a measuring object (20), comprising: (a) an emission module (30) (i) has at least one light source (32), (ii) is designed to emit a first partial beam (34) and a second partial beam (36) such that the first partial beam (34) and the second partial beam (36) overlap on the object (20) within an interference area (40), so that interference fringes are formed within the interference area (40), (iii) wherein the first partial beam (34) is coherent to the second partial beam (36), (b) a first photodetector (51) which is configured to (i) Detecting light that is scattered and / or reflected from a first section (41) of the interference region (40), and (ii) Providing initial measurement data encoding an intensity (I) or intensity change (AI) of the detected light to an evaluation unit (60), (c) a second photodetector (52) which is configured to (i) Detecting light that is scattered and / or reflected from a second section (42) of the interference region (40), and (ii) Providing second measurement data encoding an intensity (I) or intensity change (AI) of the detected light to the evaluation unit (60), characterized by (d) a third photodetector (53) which is configured to (i) Detecting light that is scattered and / or reflected from a third section (43) of the interference region (40) third measurement data, and (ii) Providing third-party measurement data encoding an intensity (I) or intensity change (AI) of the detected light to the evaluation unit (60), (e) wherein the evaluation unit (60) is configured to calculate a two-dimensional strain change (As) of the interference area (40) from the first, second and third measurement data.

2. Device (10) according to claim 1, characterized in that the device (10) is designed such that (a) the first section (41) and the second section (42) are not opposite each other, (b) preferably the second section (42) and the third section (43) are not opposite each other (c) and preferably the first section (41) and the third section (43) are not opposite each other.

3. Device (10) according to claim 1 or 2, characterized in that at least two of the three sections (41 , 42, 43) have a direction with respect to a geometric center of gravity of the three sections (41 , 42, 43) between which an angle is at least 30° and at most 150°, preferably at least 60°, preferably at most 140°, preferably at least 80°, preferably at least 110°, preferably at least 115°, preferably at most 125°.

4. Device (10) according to one of the preceding claims, characterized by (a) a focusing optic (38) for focusing the first partial beam (34) and / or the second partial beam (36), (b) wherein the focusing optics are designed such that interference fringes are formed to which there exist lines (74) perpendicular to the interference fringes of the first, second and third sections (41 , 42, 43) and which all meet substantially at a common intersection point (75).

5. Device (10) according to one of the preceding claims, characterized in that (a) the photodetectors (51 , 52, 53) are designed to detect light in a time-dependent manner and to provide time-dependent measurement data to the evaluation unit (60), (b) the evaluation unit (60) is designed to calculate a time-dependent strain change (As(t)).

6. Device (10) according to claim 5, characterized by (a) a frequency shift device (110) configured to (i) Shifting the frequency of the first partial beam (34) and / or the second partial beam (36) such that the frequency of the first partial beam (34) differs from the frequency of the second partial beam (36), (ii) wherein the difference between the frequency of the first and second partial beam is preferably at least 100 kilohertz, preferably at least 1 megahertz, preferably at least 10 megahertz, (b) and wherein the evaluation unit (60) is configured to calculate a strain change (As), preferably comprising an amount and a direction of the strain change (As), from the measurement data of the photodetectors (51 , 52, 53).

7. Device (10) according to claim 6, characterized in that the frequency shift device (110) comprises an acousto-optic modulator or a Bragg cell.

8. Device (10) according to one of the preceding claims, characterized by a phase coupling unit (170), (i) which is set up to couple phases of a first light source (32a) of the emission module (30) and a second light source (32b) of the emission module (30), (ii) wherein the phase coupling unit (170) preferably comprises an optical phase control loop.

9. Device (10) according to one of the preceding claims, characterized in that (a) each of the photodetectors (51 , 52, 53) has a first sub-detector (51a, 52a, 53a) and a second sub-detector (51b, 52b, 53b) configured such that (b) the first subdetector (51a, 52a, 53a) in each case (i) a first polarization component and / or (ii) captures a first angular component of the light, (c) that is different from a (i) second polarization component or (ii) a second angular component of the light detected by the second subdetector (51b, 52b, 53b).

10. Method for non-contact measurement of the strain of a measuring object (20) using a device (10) according to one of the preceding claims, wherein the method comprises the step (a) providing a first partial beam (34) and a second partial beam (36) which are coherent to each other so that they interfere with each other within an interference range, by means of an emission module (30), characterized by the steps (b) Detecting light by means of a first photodetector (51), a second photodetector (52) and a third photodetector (53) which is scattered and / or reflected from a first, second or third section (41, 42, 43) of the interference region (40) respectively, (c) Providing first, second and third measurement data by the first, second and third photodetectors (51, 52, 53) respectively, wherein the measurement data encode an intensity (I) or intensity change (AI) of the detected light, and (d) Calculating a two-dimensional strain change (As) of the interference area, using an evaluation unit (60), from the first, second and third measurement data.

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