A system and method for random phase shift holographic object three-dimensional measurement demodulation

Abstract

The application discloses a system and a method for random phase shift holographic object three-dimensional measurement demodulation, the system is based on the prevention of large off-axis Fresnel digital holographic microscopic measurement optical path structure, laser light source is adopted for illumination, the light emitted by the laser is split into two beams by the first beam splitter after collimation and expansion, one beam is incident to the surface of the measured circuit board solder joint at 0 degree angle with the optical axis, the object side light field is formed, after the microscopic imaging lens, the other beam reference light field interferes to form a hologram on the CCD target surface, the optical path difference is changed by using the phase shifter, a series of phase shift holograms can be generated; the technology has the characteristics of fast speed because two frames of fringe patterns are used, and can reduce the influence of mechanical vibration, environmental interference and temperature change on the measurement. In addition, the method does not need a precise phase shifter, and the phase shifter does not need to be calibrated, and there is no local phase symbol ambiguity problem, and the method has higher precision compared with the single frame technology.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of optical precision measurement, and particularly relates to a random phase shift holographic object three-dimensional measurement device and a demodulation method thereof. BACKGROUND

[0002] Electronic component package connection is realized through a large number of solder joints. Failure of the solder joints will lead to failure of the device and even the entire system. Therefore, reliability testing of the solder joints is a very important issue. The reliability testing of the solder joints mainly studies high-precision stress and strain measurement at the solder joints. By measuring the three-dimensional topographic information at the solder joints, deformation and stress data at the solder joints can be obtained. At present, the measurement methods for complex surface micro-topography and deformation are divided into contact measurement and non-contact measurement according to the measurement principle.

[0003] Among the contact measurement methods, the most widely used in the field of surface topography measurement is the stylus profilometer and the three-coordinate measuring machine. Deformation measurement mainly relies on electrical measurement and mechanical testing methods. The electrical measurement method has the advantages of high sensitivity, high testing accuracy, good testing stability, and the ability to obtain various mechanical signals through measurement, and has become the main technical means for strain detection in industrial sites. However, this method can only measure the strain of a discrete point in a fixed direction and cannot obtain the full-field strain. Moreover, the wiring is complex and the measurement efficiency is low. Since the strain gauge is directly pasted on the surface of the strain gauge, it also causes errors in the measurement to some extent.

[0004] Compared with contact measurement, non-contact surface topography and deformation measurement methods do not contact the surface of the measured sample during measurement, do not cause permanent damage to the surface, and have the advantages of fast measurement speed, high accuracy, high resolution, and large measurement range. Specifically, it can be divided into stereo vision method, structured light method, digital image correlation measurement method, holographic interference, etc. The first three measurement methods are based on the triangulation principle, and the accuracy and resolution are limited. Digital holographic technology records and saves the phase information related to the three-dimensional topography of the object in the form of an interference hologram formed by the interference of the object light and the reference light, and uses digital diffraction reconstruction to reconstruct the object topography in three dimensions. The deformation measurement method is obtained by comparing the object topography information obtained before and after deformation. This technology has the advantages of high measurement accuracy, non-contact, full-field, and dynamic measurement, and has become one of the important technologies for high-resolution detection of object topography and deformation in various application fields.

[0005] The main problem of digital holography is to overcome the image quality degradation caused by the overlap of the conjugate image and the zero-order diffraction wave with the object image. In off-axis holography, the large included angle between the reference wave and the object wave is introduced to realize the separation of the spectrum of the zero-order diffraction wave, the conjugate image and the object image in the off-axis angle direction. However, this method has a very narrow measurement area because only a small bandwidth is used. The phase shift technique can also effectively remove the conjugate image and the zero-order diffraction wave. However, the amount of the introduced phase shift must be accurately calibrated in advance. However, due to the nonlinearity of the piezoelectric sensor and environmental interference, errors inevitably occur between the actual introduced phase shift and the designed phase shift, thereby causing the phase shift technique to be unable to obtain accurate solutions. In addition, the phase shift technique often needs to collect three or more frames of holograms, which is time-consuming and laborious, and affects the measurement speed of the system.

[0006] To this end, the present patent proposes a two-frame random on-axis digital holography. This technology has the characteristics of fast speed because two frames of fringe patterns are used, and can reduce the influence of mechanical vibration, environmental interference and temperature change on measurement. In addition, this method does not need a precise phase shifter, does not need to calibrate the phase shifter, and has no local phase symbol ambiguity problem, and has higher precision than single-frame technology. SUMMARY

[0007] The present application provides a random phase shift holographic object deformation measurement device and its demodulation method, which can effectively realize high-precision measurement of stress and strain at the packaging solder joint, break through the technical difficulties of general detection system for complex micro-scale surface topography deformation, and finally construct a fast and high-precision random phase shift holographic object deformation measurement system.

[0008] In order to achieve the purpose, the technical solution provided by the present application has the following steps:

[0009] A random holographic phase shift device is designed as shown in Figure 1 , and the measured packaging circuit board is placed in the measurement space.

[0010] Firstly, the laser light source is turned on to irradiate the surface of the measured circuit board solder joint to form an object light wave, so that two frames of random phase shift holograms can be recorded by a CCD camera;

[0011] Then, a random two-frame phase shift algorithm is applied to process the collected fringe pattern and extract the wrapped phase;

[0012] Further, the fringe pattern is normalized by using the above obtained result, and a parameter optimization method is used to obtain the unwrapped measurement phase, i.e. the phase value reflecting the topography information;

[0013] On this basis, the surface profile height and the phase difference conversion relationship can be used to obtain the measured surface topography profile information. Compared with the prior art, the present application has the following advantages:

[0014] The patent proposes a random phase shift holographic object deformation measurement device and its demodulation method, which effectively realizes the measurement of complex surface micro-scale three-dimensional topography deformation.

[0015] 1、The patent uses the principle of random phase shift, proposes to use two frames of holograms with unknown phase shift to restore the object wave front, realizes the measurement of the measured circuit board solder joint, and does not need to use expensive phase shifters, which relaxes the strict calibration requirements of the phase shifter.

[0016] 2、Unlike traditional methods, the method of the patent does not need to pre-filter the fringe pattern, can save filtering time, removes the trend of light intensity through iterative approximation, and can eliminate the influence of residual background to the greatest extent.

[0017] 3、Compared with the traditional phase shift technology which requires at least three frames of fringe pattern measurement, the method only needs two frames of holograms, which saves measurement time. Moreover, the method can distinguish the convexity and concavity of the measured surface, has no phase symbol ambiguity problem, and can overcome the influence of mechanical vibration, temperature fluctuation and air disturbance on the measurement.

[0018] 4、The two-frame digital holographic technology measures the deformation by obtaining the object topography information before and after deformation, which has the advantages of high measurement accuracy, non-contact, full-field, and can realize dynamic object or transient phenomenon three-dimensional measurement. BRIEF DESCRIPTION OF DRAWINGS

[0019] Figure 1 is a schematic diagram of the measurement device of the present application.

[0020] The system is based on the prevention of large off-axis Fresnel digital holographic microscopic measurement optical path structure, uses laser light source illumination, and the light emitted by the laser is divided into two beams by the beam splitter 1 after collimation and expansion. One beam is incident at 0 degree angle to the optical axis to the surface of the measured circuit board solder joint to form the object light field, which is imaged by the microscopic imaging lens, and interferes with the other reference light field on the CCD target surface to form a hologram. The optical path difference is changed by using a phase shifter to produce a series of phase shift holograms. By processing the intensity and phase information contained in the hologram, the topography and deformation information of the solder joint can be obtained. DETAILED DESCRIPTION

[0021] The present application will be described in detail below with reference to the drawings and examples.

[0022] When measuring, the measured circuit board is moved to the measuring space of the measuring device, four light arms in the schematic diagram are adjusted to be perpendicular or parallel respectively to reduce the measurement error caused by image distortion. Then the laser light source is turned on, the camera and the phase shifter are controlled to generate and record the phase shift hologram formed by the circuit board or other objects to be measured, and the hologram is saved in the computer hard disk. The hologram is processed by the measuring software to obtain the measurement result.

[0023] A random phase shift holographic object three-dimensional measurement demodulation method has the following specific steps:

[0024] First step: place the measured object in the measuring space, turn on the laser light source, and control the camera to record the formed hologram.

[0025] At this time, the phase shift hologram light intensity distribution observed by the camera can be expressed as:

[0026] I i (x,y)=O 2 +R 2 +2ORcos(φ-Δ i )(1);

[0027] Wherein, O represents the object light wave, R represents the reference light wave, Δ i represents the phase shift, φ represents the phase, and the equation (1) is rewritten as:

[0028] I n (x,y)=a(x,y)+b(x,y)cos(φ(x,y)+Δ n ),(n=1,2)(2);

[0029] Wherein, a(x,y) and b(x,y) are the background intensity and modulation amplitude under the condition of spatial variation, wherein φ(x,y) and Δ n are the measurement phase and unknown time domain carrier frequency phase shift between the two phase shift fringe patterns. Without loss of generality, it is assumed that Δ1=0. For the sake of simplicity, the spatial coordinates (x,y) are omitted below. If the spatial average is subtracted from the sampling fringe pattern, the following can be obtained:

[0030] I rn =I n -I n =a′+bcos(φ+Δ n ),(n=1,2)(3);

[0031] . represents the average operation of all pixels. a′ is the residual background in the filtered fringe pattern I rn .

[0032] Second step: in the case of ignoring the residual background, the fringe pattern I n is rewritten as an MN×1 vector (M, N are the size of the fringe pattern), the phase shift can be estimated as:

[0033]

[0034] Further, the phase is estimated as:

[0035]

[0036] Third step: the residual background a' can be calculated by the following equation:

[0037]

[0038] Fourth step: after obtaining the residual background, the background removed image in equation (3) is updated as:

[0039]

[0040] In this case, equation (3) to equation (8) is iterated again, and the convergence criterion can be expressed as

[0041]

[0042] Where q represents the number of iterations, and ε is the predefined accuracy requirement. When the convergence criterion is met, the phase distribution to be measured is obtained.

[0043] Fifth step: on this basis, the following optimized method is used to expand the phase. First, the background and amplitude obtained by the above method are used to normalize equation (2) to equation (11)

[0044]

[0045]

[0046]

[0047] The phase is expanded by Zernike coefficients as:

[0048]

[0049] Where j is the pixel coordinate, c j is the expansion coefficient, and Z j is the Zernike polynomial. And define the following penalty function:

[0050] f(X) =∑[I' - cos(φ)] 2 (13);

[0051] Let the solution vector be X = [c1, c2,..., c j ] T, the solution vector of the minimum punishment function is searched by differential evolution algorithm, and the solution vector is:

[0052] X = argminf(X) (14);

[0053] Further, the solution vector is substituted into equation (12) to obtain the unwrapping phase.

[0054] The algorithm steps of the patent can be summarized as:

[0055] 1) The trend item of the hologram is removed by using spatial average method.

[0056] 2) The phase shift is estimated by using equation (4), and the phase is estimated by using equation (5).

[0057] 3) The residual background is estimated in combination with the results of equation (4) and equation (5).

[0058] 4) The image after background removal is updated by using equation (7), and steps 2)-4) are iterated until convergence.

[0059] 5) The background and amplitude obtained in the iteration process are used to normalize the collected fringe pattern, and then the unwrapping phase is obtained by using step 5.

Claims

1. A method for three-dimensional measurement and demodulation of random phase-shift holographic objects, characterized in that: Step 1) Remove the trend term from the hologram using the spatial averaging method; The object to be measured is placed in the measurement space, the laser light source is turned on, and the camera is controlled to record the resulting hologram; the light intensity distribution of the phase-shifted hologram observed by the camera at this time can be described as follows: I i (x,y)=|0| 2 +|R| 2 +2|0||R|cos(φ-Δ i ) (1); Where O represents the object light wave, R represents the reference light wave, Δi represents the phase shift, and φ represents the phase. After appropriate variable substitution, equation (1) can be rewritten as: In(x,y)=a(x,y)+b(x,y)cos(φ(x,y)+Δn), (n=1,2) (2); Where a(x,y) and b(x,y) are the background intensity and modulation amplitude under spatially varying conditions, and φ(x,y) and Δn are the measured phase and the phase shift of the unknown time-domain carrier frequency between the two phase-shifted fringe patterns; without loss of generality, it is assumed that Δ1 = 0; for simplicity, the spatial coordinates (x,y) are omitted below; if the spatial average is subtracted from the sampled fringe patterns, we can obtain: I rn =I n -<I n >=a′+bcos(φ+Δ n ),(n=1,2) (3); <.> represents the mean operation for all pixels; a′ is the filtered fringe pattern I. rn The residual background in; Step 2) Estimate the phase shift and phase; ignoring the residual background, rewrite the fringe pattern In as an MN×1 vector (M and N are the sizes of the fringe pattern), and the phase shift can be estimated as: The phase is then estimated as follows: Step 3) Combining the results of estimating the phase shift and the phase, estimate the residual background; the residual background a′ can be calculated using the following formula: Step 4) Update the image after background removal, and iterate through steps 2)-4) until convergence; After obtaining the residual background, the background removal image in equation (3) is updated as follows: In this case, by iterating from equation (3) to equation (8) again, the convergence criterion can be expressed as: Where q represents the number of iterations and ε is a predefined accuracy requirement; when the convergence criterion is met, the phase distribution to be measured is obtained; Step 5) Use the background and amplitude obtained during the iteration process to normalize the acquired fringe pattern, and then use the parameter optimization method to expand the phase.

2. The method as described in claim 1, characterized by the following: Step 5): Based on this, the phase is expanded using the following optimized method; first, using the background and amplitude obtained by the above method, equation (2) is normalized to equation (11). The phase is expanded using the Zenic coefficients as follows: in, Let j be the pixel coordinate, cj be the expansion coefficients, and Zj be the Zenik polynomial; and define the following penalty function: f(X)=∑[I′1-cos(φ)] 2 (13) Let the solution vector be X = [c1, x2, ..., c j ] T By using the differential evolution algorithm to search for the solution vector that minimizes the above penalty function, the solution vector can be obtained as follows: X = arg minf(X) (14); Substituting the solution vector into equation (12) yields the expanded phase.

3. The method based on claim 1, comprising a system for three-dimensional measurement and demodulation of random phase-shift holographic objects, characterized in that: The system is based on the optical path structure of the Fresnel digital holographic microscopy measurement for preventing large off-axis movements. It uses a laser light source for illumination. The light emitted by the laser is collimated and expanded, and then split into two beams by the first beam splitter. One beam is incident on the surface of the solder joint of the circuit board under test at a 0-degree angle with the optical axis. The object-space light field formed is then interfered with the other reference light field on the CCD target surface by the microscope imaging lens to form a hologram. By using a phase shifter to change the optical path difference, a series of phase-shifted holograms can be generated. By processing the intensity and phase information contained in the holograms, the morphology and deformation information of the solder joint can be obtained. During measurement, the circuit board under test is moved to the measurement space of the measuring device, and the four optical arms in the schematic diagram are adjusted to be perpendicular or parallel to reduce measurement errors caused by image distortion. Then, the laser light source is turned on, the camera and phase shifter are controlled to generate and record the phase-shifted hologram, and it is saved to the computer hard drive. The measurement software processes the hologram to obtain the measurement result.

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