Method for detecting micro-deformation of glass by double exposure method based on cosine modulation

Abstract

The invention relates to the technical field of optical holographic detection, in particular to a method for detecting micro-deformation of glass by a double-exposure method based on cosine modulation. The method comprises the following steps: respectively recording a light field before glass deformation and a light field after glass deformation; interference is carried out on the light field before glass deformation and the light field after glass deformation through the same reference light, and a corresponding first hologram before glass deformation and a corresponding second hologram after glass deformation are obtained; overlapping and reproducing the first hologram and the second hologram to obtain a double-exposure hologram; obtaining a cosine function fringe field of the double-exposure hologram; and calculating a displacement vector by adopting a displacement sensitivity model according to the cosine function fringe field to obtain a glass deformation quantity. The double-exposure method is used for performing optical superposition deformation to generate the cosine fringe field, so that the amplitude interference of the object light field can be eliminated, the fringe contrast is improved, and the signal-to-noise ratio of glass deformation detection is improved.

Description

Technical Field

[0001] The present invention relates to the technical field of optical holographic detection, and in particular to a method for detecting glass micro-deformation by a double exposure method based on cosine modulation. Background Art

[0002] Holographic interferometry has a wide range of applications in precise micro-deformation measurement. It measures the number of shifted fringes to determine the corresponding material deformation. Currently, it is widely used in areas such as object deformation measurement, quantitative visualization of welds, and solution concentration measurement. Therefore, research on digital holographic interferometry is of great significance.

[0003] Traditional deformation detection methods (such as strain gauges) have low detection accuracy and cannot achieve full-field non-contact measurement. In existing research, common holographic interferometry methods include: single exposure method, double exposure method and continuous exposure method. The single exposure method can realize dynamic monitoring of the surface of the object, but the data volume calculation is very large in the time series fringe phase measurement; the continuous exposure method records the vibration of the object through a longer exposure time, and can only solve the distribution of the amplitude within the exposure time, and the reproduction and processing of the hologram takes a relatively long time; the double exposure method has the advantages of simple experimental setup, full utilization of light energy, and smaller speckle structure on the interference pattern. At present, the double exposure method has not been used in glass deformation detection. Summary of the Invention

[0004] In order to solve the problems existing in the prior art, the present invention provides a method for detecting micro-deformation of glass using a double-exposure method based on cosine modulation. The method records the light field before and after deformation of the glass respectively; interferes the light field before and after deformation of the glass using the same reference light to obtain a first hologram corresponding to the glass before deformation and a second hologram corresponding to the glass after deformation; superimposes and reconstructs the first and second holograms to obtain a double-exposure hologram; obtains a cosine function fringe field of the double-exposure hologram; and calculates the displacement vector using a displacement sensitivity model based on the cosine function fringe field to obtain the glass deformation variable. The present invention generates a cosine fringe field by optically superimposing and deforming the double-exposure method, which can eliminate the interference of the object light field amplitude, improve the fringe contrast, and thus improve the signal-to-noise ratio of glass deformation detection.

[0005] The present invention adopts the following technical solution, a method for detecting micro-deformation of glass based on a double exposure method of cosine modulation, comprising: Record the light field before and after the glass is deformed respectively; Interfering the light field before and after the glass is deformed with the same reference light to obtain a first hologram corresponding to the glass before deformation and a second hologram corresponding to the glass after deformation; Superimposing and reproducing the first hologram and the second hologram to obtain a double-exposure hologram; Obtain the cosine function fringe field of the double-exposure hologram; The displacement vector is calculated based on the cosine function fringe field using the displacement sensitivity model to obtain the glass deformation.

[0006] Furthermore, obtaining the cosine function fringe field of the double-exposure hologram includes: illuminating the double-exposure hologram with reference light to generate a diffraction field of the double-exposure hologram; Extract the first-order diffraction terms in the diffraction field; By calculating the intensity of the first-order diffraction term and eliminating the influence of the constant term, the cosine function fringe field of the double-exposure hologram is obtained.

[0007] Furthermore, the displacement vector is calculated based on the cosine function fringe field using the displacement sensitivity model to obtain the glass deformation, specifically: The fringe field phase difference is obtained by performing an inverse cosine operation on the cosine function fringe field of the double-exposure hologram; Establishing a light path geometric model, which includes: an illumination vector and an observation vector; Calculate the displacement sensitivity vector based on the optical path geometry model; Substitute the fringe field phase difference and the displacement sensitivity vector into the displacement sensitivity model and solve to obtain the displacement vector.

[0008] Furthermore, after the displacement vector is obtained, the following steps are also included: Perform a preliminary correction on the displacement vector according to the glass refractive index; The displacement vector after preliminary correction is corrected twice using the glass Poisson's ratio.

[0009] Furthermore, the method for calculating the displacement sensitivity vector according to the optical path geometric model is: The displacement sensitivity vector is calculated based on the difference between the illumination vector and the observation vector.

[0010] Furthermore, after performing an inverse cosine operation on the cosine function fringe field of the double-exposure hologram to obtain the fringe field phase difference, the method further includes: The path-independent algorithm is used to perform phase unwrapping calculation on the fringe field phase difference to obtain the continuous phase difference of the fringe field.

[0011] Furthermore, by calculating the intensity of the first-order diffraction term and eliminating the influence of the constant term, the cosine function fringe field of the double-exposure hologram is obtained, specifically: The expression for calculating the intensity of the first-order diffraction term is: Where: , represents the intensity of the reference light, and we can get: Where, is the amplitude-related term, where is the modulation depth factor, is the transmittance coefficient, is the reference light amplitude, is the object light wave amplitude; is the phase modulation term, where is the fringe field phase difference.

[0012] The beneficial effects of the present invention are as follows: the present invention realizes the optical superposition deformation of two holograms through the double exposure method, generates an interference fringe field characterized by the original function, can effectively eliminate the amplitude interference of the object light field, and achieve a significant improvement in the fringe contrast, thereby significantly improving the signal-to-noise ratio of glass deformation detection; further, a linear relationship between the glass deformation variable and the fringe movement number is established through a displacement sensitivity model, and the deformation variable is quickly quantitatively calculated, thereby improving the detection efficiency of the glass deformation; finally, the glass refractive index and Poisson's ratio are introduced to correct the obtained displacement vector, further improving the detection accuracy of the glass deformation and realizing micron-level deformation detection. BRIEF DESCRIPTION OF THE DRAWINGS

[0013] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments or the description of the prior art. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0014] Figure 1 Schematic diagram of a method for detecting micro-deformation of glass using a double exposure method based on cosine modulation according to an embodiment of the present invention; Figure 2 Schematic diagram of the principle of double-exposure holographic interferometry according to an embodiment of the present invention; Figure 3 A schematic diagram of the relationship between glass surface deformation and interference fringe field according to an embodiment of the present invention; Figure 4 A schematic diagram of a double-exposure holographic interferometry optical path for stress detection according to an embodiment of the present invention; Figure 5 Schematic diagram of deformation detection results using a double exposure method according to an embodiment of the present invention. DETAILED DESCRIPTION

[0015] The following will clearly and completely describe the technical solutions in the embodiments of the present invention in conjunction with the accompanying drawings. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of the present invention.

[0016] A schematic flow chart of a method for detecting micro-deformation of glass using a double exposure method based on cosine modulation according to an embodiment of the present invention is shown in FIG. Figure 1 Shown, including: Record the light field before and after the glass is deformed respectively; When glass is affected by external forces, temperature changes, and other factors, its shape will change. Before the glass deforms, the propagation of light through the glass will form a specific light field. When the glass deforms, due to the change in the shape of the glass, the propagation path and intensity distribution of the light will also change, thus forming a light field different from that before the deformation. Interfering the light field before and after the glass is deformed with the same reference light to obtain a first hologram corresponding to the glass before deformation and a second hologram corresponding to the glass after deformation; In an embodiment of the present invention, a stable light beam with known characteristics (such as wavelength, amplitude, phase, etc.) is selected as the same reference light, which remains unchanged during the entire measurement process and is used to interfere with the light reflected or transmitted by the glass.

[0017] To obtain the first hologram corresponding to the glass before deformation, a specific light source is used to illuminate the glass. The light reflected or transmitted by the glass surface forms object light. The object light is interfered with the reference light, and the interference fringes are recorded on a holographic recording medium. After processing such as development, the first hologram corresponding to the glass before deformation is obtained; the first hologram contains the amplitude and phase information of the light field in the initial state of the glass.

[0018] A second hologram corresponding to the deformed glass is obtained. Some action, such as temperature change or pressure change, is applied to the glass to cause it to deform. The same specific light source used to obtain the first hologram corresponding to the glass before deformation is then used to illuminate the deformed glass to generate new object light. Similarly, this new object light is interfered with the same reference light, and the interference fringes are recorded to obtain the second hologram corresponding to the deformed glass. The second hologram reflects the state of the light field after the glass is deformed.

[0019] In a specific embodiment of the present invention: The light field before the glass is deformed is expressed as , and the reference light After interference, the first hologram is obtained , expressed as: in, is the first hologram, is the reference light, represents the intensity of the reference light, is the light field before the glass is deformed, is the intensity of the light field before the glass is deformed, represents the complex conjugate of the reference light, represents the complex conjugate of the light field before the glass is deformed.

[0020] The light field after glass deformation is expressed as , and set the exposure to the same value, with the same reference light After interference, the second hologram is obtained , expressed as: in, is the second hologram, is the light field after the glass is deformed, is the intensity of the light field after the glass is deformed, represents the complex conjugate of the light field after the glass is deformed.

[0021] Superimposing and reproducing the first hologram and the second hologram to obtain a double-exposure hologram; In a specific embodiment of the present invention, the first hologram and the second hologram are superimposed and reproduced by superimposing the same pixel coordinates to generate a double-exposure hologram, which is specifically expressed as follows: in: For double exposure holograms, is the first hologram, is the reference light, represents the intensity of the reference light, is the light field before the glass is deformed, is the intensity of the light field before the glass is deformed, represents the complex conjugate of the reference light, represents the complex conjugate of the light field before the glass is deformed, is the second hologram, is the light field after the glass is deformed, is the intensity of the light field after the glass is deformed, represents the complex conjugate of the light field after the glass is deformed.

[0022] Obtain the cosine function fringe field of the double-exposure hologram; Double exposure holographic interferometry is the most widely used method for quantitatively analyzing the surface and deformation of objects in holographic interferometry. The schematic diagram of its principle is shown in the figure below. Figure 2As shown in the figure, the holograms of the object before and after deformation are recorded on the same hologram, and then reproduced with the original reference light in the same optical path. Through the double-exposure hologram observation, interference fringes can be seen on the surface of the object caused by the interference of the object light waves in the two states before and after deformation. The interference fringes give the equal displacement lines of the object in the direction of its detection sensitivity.

[0023] In a specific embodiment of the present invention: For the reconstruction of the double-exposure hologram, the original reference light is continued to be used in the embodiment of the present invention. To illuminate the double-exposure hologram, the resulting light field is expressed as follows: Where, , represents the DC component of the reference light, , represents the intensity term of the double-exposure hologram light field, , representing the first-order diffraction term, and , represents the conjugate term of the hologram light field; for ,because , its propagation direction deviates from the conjugate light direction of the original object light. If the virtual image is extended in the reverse direction, interference fringes can be obtained. ,because The propagation direction deviates from the conjugate light direction of the original object light, and the real image is intersected, and interference fringes can be observed.

[0024] Assume that the complex amplitude of the reference light is It means that the complex amplitudes of the object light before and after the glass deformation are and , then: in, Indicates that the object light is at point The complex amplitude at Indicates that after the glass is deformed, the object light is at point The complex amplitude at is the complex amplitude of the reference light, Indicates that the object light is at a point The amplitude at , which remains unchanged before and after deformation, Indicates that the reference light is at point The amplitude at is the imaginary unit, Indicates that the object light is at point The phase at Indicates that after the glass is deformed, the object light is at point The phase at Indicates that the object light is at a point The phase at .

[0025] When the object is deformed, the object light changes, the amplitude does not change, but the phase changes. This is a small deformation. After the first exposure, when the second exposure is made, the deformed object light interferes with the reference light. After the first exposure, the complex amplitude and intensity recorded on the double-exposure hologram are: in, represents the complex amplitude recorded on the double-exposure hologram of the first exposure, represents the intensity recorded on the first exposure double exposure hologram, yes The complex conjugate of .

[0026] After the second exposure, the complex amplitude and intensity distribution recorded on the double-exposure hologram are: in, represents the complex amplitude recorded on the double-exposure hologram after the second exposure, represents the intensity recorded on the double-exposure hologram after the second exposure, yes The complex conjugate of .

[0027] Assume that the two exposure times are equal, both , then the total energy on the double-exposure hologram is: The amplitude transmittance of the double-exposure hologram is: in, is the amplitude transmittance of the double-exposure hologram, is the total energy on the double-exposure hologram, is the background transmittance constant, i.e. the basic transmittance in the double-exposure hologram when it is not modulated by light intensity, is the modulation sensitivity coefficient.

[0028] For the double-exposure hologram reconstruction in the above equation, the complex amplitude of the diffracted light wave passing through the double-exposure hologram is: in, represents the complex amplitude of the diffracted light wave passing through the double-exposure hologram.

[0029] Comprehensive substitution gives: Where, is the amplitude of the object light, is the amplitude of the reference light, is the background transmittance constant, is the modulation sensitivity coefficient, is the reference light amplitude, is the exposure time, is the object-light phase before the glass is deformed, is the reference light phase, is the phase of the object light after the glass is deformed; the formula contains three terms: the first term is the transmitted light, which still propagates along the direction of the original reference beam; the second term is the coherent superposition of the object light waves before and after the deformation, which is called the first-order diffraction term; the third term is the conjugate object light wave.

[0030] The first-order diffraction term described by the second term in the above formula is the term of interest in holographic interferometry and is expressed as , then the expression of the first-order diffraction term is: The expression for further calculating the intensity of the first-order diffraction term is: Where: , represents the intensity of the reference light, and we can get: Where, is the amplitude-related term, where is the modulation depth factor, is the transmittance coefficient, is the reference light amplitude, is the object light wave amplitude; is the phase modulation term, where is the fringe field phase difference.

[0031] It can be seen that the intensity of the double exposure obtained is a light field modulated by a cosine function. The value of the cosine function is only related to the deformation field. This deformation field is the phase difference caused by the phase, and its fringe contrast is only 1, that is, the fringe contrast is very good and there is no noise.

[0032] The displacement vector is calculated based on the cosine function fringe field using the displacement sensitivity model to obtain the glass deformation.

[0033] In a specific embodiment of the present invention: like Figure 3 The figure shows the relationship between the deformation of the glass surface and the interference fringe field. For any point on the glass surface , if it is deformed by Point displacement to As shown in the figure below, in holographic interferometry, for For an observer at point The phase change caused by the displacement of the point before and after deformation is .

[0034] The light field before the object is deformed is used To indicate that after deformation Point light field Indicates that these two vectors are called illumination vectors. Point displacement to Point, using vector This is called the displacement vector; the observation vector is the distance from Point to Point is called , deformed to Point is , all lighting is directed towards the object, and all observation vectors are directed towards the observer.

[0035] The expression of the fringe field phase difference obtained by performing inverse cosine operation on the cosine function fringe field of the double-exposure hologram is: in, represents the phase difference, is the wavelength, is the optical path difference.

[0036] Phase difference The solution process is as follows, definition: and: , ; , ; in, is the mean of the illumination vector, is the difference of the illumination vector, is the mean of the displacement vector, is the difference of displacement vectors.

[0037] The optical path difference is calculated as: Where, is the optical path difference, is the mean of the illumination vector, is the difference of the illumination vector, is the mean of the displacement vector, is the difference of displacement vectors, represents the complex amplitude vector of the object light field before the glass is deformed, and its direction is from the light source S to the surface point P. represents the complex amplitude vector of the object light field after the glass is deformed, representing the light field scattered when point P moves to position P1, and its direction is from light source S to the new position P1. represents the vector from the undeformed point P to the observed point Q; represents the vector pointing from the deformed point P1 to the observation point Q; when When , the displacement sensitivity model is established as: The displacement sensitivity vector can be calculated based on the difference between the illumination vector and the observation vector: but: At this time, the expression of the displacement vector is: The expression for the lighting vector is: Where, 、 、 for point The three-dimensional coordinates of 、 、 is the coordinate of the light source, 、 、 Represent the components of the lighting vector on the coordinate axes respectively.

[0038] The expression of the observation vector is: Where, 、 、 for point The three-dimensional coordinates of 、 、 are the coordinates of the light source, 、 、 Represent the components of the observation vector on the coordinate axis respectively.

[0039] By combining the above expressions and substituting them into the established displacement sensitivity model for solution, we can obtain the displacement vector, which is the deformation of the glass.

[0040] In the embodiment of the present invention, after the displacement vector is obtained, the displacement vector is further preliminarily corrected according to the refractive index of the glass, and the preliminarily corrected displacement vector is further secondarily corrected using the Poisson's ratio of the glass.

[0041] In an experimental embodiment of the present invention: Build as Figure 4 The optical path diagram for double-exposure holographic interferometry for stress detection is shown. The embodiment of the present invention uses a HeNe laser with a power of approximately 40 MW and a continuous output mode. The laser's output wavelength is a critical parameter for the photosensitivity of the holographic recording medium. The CMOS (Complementary Metal Oxide Semiconductor) is model OV7670, with a maximum frame rate of 30 frames / s and a pixel clock of up to 24 MHz. The photosensitive array measures 640 x 480 pixels, and the package dimensions are 3785 μm x 4235 μm. The flat glass has a thickness of t = 4.00 mm, a radius of a = 17.50 mm, a Poisson's ratio of μ = 0.2, and a refractive index of n = 1.517.

[0042] like Figure 4 As shown, it includes a He-Ne laser, two full reflective mirrors M1 and M2, five beam expanders L1, L2, L3, L4, and L5, two beam splitters BS1 and BS2, organic glass T to be tested, CMOS (complementary metal oxide semiconductor), and a computer. , where the wavelength of the laser is The emitted laser beam is split into two beams by beam splitter BS1. One beam strikes the surface of total reflection mirror M2, then passes through a beam expansion and collimation system to become parallel light. After being split by beam splitter BS2 and reflected by beam expander L5, it reaches the CMOS image sensor, serving as the reference light wave. The other beam is reflected by total reflection mirror M1, then passes through a beam expansion and collimation system to form a plane light wave, which then strikes the organic glass to be measured (T). The light reflected by the object passes through BS2 and beam expander L5, reaching the CMOS target surface as the object light wave. The two beams superimpose and interfere, forming a Fresnel off-axis digital hologram. The hologram is recorded by the CMOS image sensor, converted to digital by an image acquisition card, and stored in a computer. The recorded digital hologram is digitally reconstructed and phase unwrapped to obtain the phase distribution of the first hologram. Then, pressure is applied to the organic glass in a direction parallel to the surface, while maintaining other conditions unchanged. A second digital hologram is recorded and digitally reconstructed and phase unwrapped to obtain the phase distribution of the second hologram.

[0043] Double exposure holographic interferometry experiment was used. Since it is digital holography, two holograms were recorded in two exposures. The pixel size is , with a resolution of Pixels, sampling frequency is 10 frames / second. The distance (object distance) between transparent organic glass T and CMOS is 0.120m. An off-axis plane reference light is constructed and the reconstructed image is moved to the center of the image. At this time, the angles between the reference light and the X-axis and Y-axis are , .

[0044] The experimental results are as follows Figure 5 As shown, Figure 5 (a) is the first digital hologram reconstruction, where the cracks on the organic glass can be clearly seen. Figure 5 (b) and 5 (c) are the reconstructed images of the double-exposure digital hologram, where interference fringes can be observed. Figure 5 (d) shows the interference fringes formed by the light field of the reproduced images of two double-exposure digital holograms. It can be observed that the holographic interference fringes move from the center to the surrounding areas. The size of the fringes movement can be used to determine the distribution of glass deformation.

[0045] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention should be included in the scope of protection of the present invention.

Claims

1. A method for detecting micro-deformation of glass using a double exposure method based on cosine modulation, characterized in that: include: Record the light field before and after the glass is deformed respectively; Interfering the light field before and after the glass is deformed with the same reference light to obtain a first hologram corresponding to the glass before deformation and a second hologram corresponding to the glass after deformation; superimposing and reproducing the first hologram and the second hologram to obtain a double-exposure hologram; Obtain the cosine function fringe field of the double-exposure hologram; The displacement vector is calculated based on the cosine function fringe field using the displacement sensitivity model to obtain the glass deformation.

2. The method for detecting glass micro-deformation using a double exposure method based on cosine modulation according to claim 1, characterized in that: Obtain the cosine function fringe field of the double-exposure hologram, including: illuminating the double-exposure hologram with reference light to generate a diffraction field of the double-exposure hologram; extracting a first-order diffraction term from the diffraction field; By calculating the intensity of the first-order diffraction term and eliminating the influence of the constant term, the cosine function fringe field of the double-exposure hologram is obtained.

3. The method for detecting glass micro-deformation using a double exposure method based on cosine modulation according to claim 1, wherein: According to the cosine function fringe field, the displacement vector is solved using the displacement sensitivity model to obtain the glass deformation variable, which is specifically: The fringe field phase difference is obtained by performing an inverse cosine operation on the cosine function fringe field of the double-exposure hologram; Establishing a light path geometric model, wherein the light path geometric model includes: an illumination vector and an observation vector; Calculating a displacement sensitivity vector according to the optical path geometric model; The fringe field phase difference and the displacement sensitivity vector are substituted into a displacement sensitivity model to obtain a displacement vector by solution.

4. The method for detecting glass micro-deformation using a double exposure method based on cosine modulation according to claim 3, wherein: After the displacement vector is obtained, it also includes: Performing a preliminary correction on the displacement vector according to the refractive index of the glass; The displacement vector after preliminary correction is corrected twice using the glass Poisson's ratio.

5. The method for detecting micro-deformation of glass using a double exposure method based on cosine modulation according to claim 3, wherein: The method for calculating the displacement sensitivity vector according to the optical path geometric model is: A displacement sensitivity vector is calculated according to the difference between the illumination vector and the observation vector.

6. The method for detecting micro-deformation of glass using a double exposure method based on cosine modulation according to claim 3, wherein: After performing an inverse cosine operation on the cosine function fringe field of the double-exposure hologram to obtain the fringe field phase difference, the method further includes: The path-independent algorithm is used to perform phase unwrapping calculation on the fringe field phase difference to obtain the continuous phase difference of the fringe field.

7. The method for detecting micro-deformation of glass using a double exposure method based on cosine modulation according to claim 2, wherein: By calculating the intensity of the first-order diffraction term and eliminating the influence of the constant term, the cosine function fringe field of the double-exposure hologram is obtained, specifically: The expression for calculating the intensity of the first-order diffraction term is: Where: , represents the intensity of the reference light, and we can get: Where, is the amplitude-related term, where is the modulation depth factor, is the transmittance coefficient, is the reference light amplitude, is the object light wave amplitude; is the phase modulation term, where is the fringe field phase difference.

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