A high-sensitivity three-dimensional topography recovery method based on double-channel differential

Abstract

This invention belongs to the field of precision measurement technology, specifically a high-sensitivity three-dimensional topography recovery method based on dual-channel differential scanning. Based on a traditional microscopic imaging system, this invention adds a beam-splitting prism between the objective lens and the tube lens. The additional optical path is equipped with the same tube lens and camera as the original optical path, but the distance from the tube lens to the camera is slightly different from the original optical path. During the axial scanning of the object under test by driving the objective lens, the focus evaluation function curve corresponding to each pixel is calculated based on the images captured by the two cameras. The difference between the focus evaluation function curves from the two cameras is used to obtain an evaluation function curve that changes more drastically near the recording depth, thereby improving the system's sensitivity. This invention improves sensitivity by fixing the difference component of the focus evaluation function in hardware, while ensuring scanning efficiency and flexibility in scanning scheme settings.

Description

Technical Field

[0001] This invention belongs to the field of precision measurement technology, specifically relating to a method for restoring three-dimensional surface morphology. Background Technology

[0002] Three-dimensional topography reconstruction based on microscopic imaging systems is a non-contact optical inspection method with a large dynamic range and high measurement accuracy. Its ability to reconstruct the topography of steep surfaces can meet the special inspection requirements of some precision parts. In recent years, it has attracted widespread attention in the fields of medicine, aviation, and aerospace [Y.Wang, X.Zhang, and H.Chen, "Depth Measurement for the Objects with a Small Height Using Depth-Focus-Based Microscopic Vision System", 2020 IEEE International Conference on Mechatronics and Automation: 653-658.].

[0003] The core of 3D topography restoration lies in finding the optimal focusing depth during scanning. Therefore, a reliable focus evaluation function is crucial for this measurement method. For a fixed lateral position, the focus evaluation function value is determined by the scanning depth, reaching its maximum value at the optimal focusing depth. Generally, an ideal focus evaluation function should simultaneously possess characteristics such as unimodality, high sensitivity, and strong robustness. Researchers have consistently proposed various focus evaluation functions suitable for different scenarios, continuously driving the development of 3D topography restoration methods [S. Pertuz, D. Puig, and M. Garcia, "Analysis of focus measure operators for shape-from-focus", Pattern Recognition 2013; 46:1415-1432]. Unfortunately, due to the imaging characteristics of optical systems, current focus evaluation function curves are generally Gaussian-like. This means that near the recording depth, i.e., the optimal focusing depth, the change in the focus evaluation function is very gradual, undoubtedly leading to a decrease in measurement sensitivity. Moreover, when noise during data acquisition is significant, a smoother focus evaluation function will be more severely affected. This problem severely limits the application of 3D topography restoration methods based on microscopic imaging systems in certain ultra-precision detection scenarios.

[0004] To address this issue, this invention solves the problem of low sensitivity of 3D topography restoration methods near the recording depth without significantly increasing hardware costs and system complexity, while ensuring high measurement efficiency and flexible scanning scheme settings. Summary of the Invention

[0005] The purpose of this invention is to provide a highly sensitive three-dimensional surface topography measurement and recovery method based on a microscopic imaging system, without significantly increasing measurement time and hardware costs.

[0006] The high-sensitivity three-dimensional surface topography measurement and reconstruction method provided by this invention is based on dual-channel differential technology, including: employing a microscopic imaging system, scanning the object under test by axially moving the objective lens or the entire microscopic imaging system; calculating the focus evaluation function curve of the relative scanning depth change for the lateral position corresponding to each pixel; ensuring a stable difference component by using two cameras with a fixed difference in the relative distance between the two optical paths; and obtaining a more drastic evaluation function curve near the recording depth by subtracting the focus evaluation function curves corresponding to the two cameras, thereby improving the sensitivity of the system; specifically as follows:

[0007] (a) Constructing a measurement system, including a microscopic measurement system and a motion control mechanism;

[0008] The microscopic measurement system includes the following components: two identical cameras, two identical tube lenses, two beam splitters, objective lens, stage, converging lens, and light source;

[0009] The first camera, first tube mirror, first beam splitter, second beam splitter, objective lens, and stage are arranged coaxially from top to bottom in the optical path to form the imaging optical path; the stage is used to place the object to be measured; the second tube mirror and second camera are sequentially arranged in the optical path of the first beam splitter; the converging lens and light source are sequentially arranged in the optical path of the second beam splitter to form the illumination optical path.

[0010] The relative distances between the two sets of tube lenses and cameras are different, and the difference in distance is fixed, which is a preset small amount, such as 0.5-1.5mm. When the first camera clearly images a certain point, the second camera captures the out-of-focus image of that point.

[0011] The motion control mechanism includes a motor and a grating ruler, which is used to control the axial movement of the objective lens or the entire microscopic imaging system.

[0012] (II) Axial scanning and focusing evaluation:

[0013] (1) Fix the object to be measured on the stage, adjust the objective lens or the entire microscopic imaging system, and determine the axial position where the highest and lowest points of the object to be measured can be clearly imaged, so as to set the scanning range and scanning interval of the measurement.

[0014] (2) Based on the scanning range and scanning interval, the two cameras will each capture a sequence of n images, denoted as I. i 1 I i 2 ,(i=1,2,…,n);

[0015] (3) Select a focus evaluation function based on the actual scenario, such as the Sobel function or the Laplace function, and calculate the pixel-level focus evaluation function value for the above image sequence. For m pixels, m focus evaluation function curves are obtained, denoted as S. i 1 S i 2 , (i=1,2,…,m), which records the focusing status of each lateral position at different scanning depths.

[0016] (III) Differential and 3D Reconstruction:

[0017] (1) Since there is only one corresponding optimal focus depth for a given horizontal position, and the focus evaluation function curve corresponding to that horizontal position reaches its maximum value at that optimal focus depth, therefore S i 1 S i 2 All of them followed a trend of first increasing and then decreasing with different scanning depths, for S i 1 S i 2 Making a difference:

[0018]

[0019] The new evaluation function curve F i In this context, the recording depth corresponds to the zero point of the evaluation function, compared to the Gaussian-like S. i 1 S i 2 Its changes are more dramatic near the recording depth, thus it has higher sensitivity to depth changes and stronger noise resistance.

[0020] (2) For F i By fitting the zero point separately and combining it with the lateral position of the corresponding pixel, the height map formed by all recorded depths is the restored three-dimensional shape.

[0021] This invention, based on a traditional microscopic imaging system, adds a beam-splitting prism between the objective lens and the tube lens. The additional optical path is equipped with the same tube lens and camera as the original optical path, but the distance from the tube lens to the camera is slightly different. During the axial scanning of the object under test using the driving objective lens, the focus evaluation function curve for each pixel is calculated based on the images captured by the two cameras. The difference between the focus evaluation function curves from the two cameras is used to obtain an evaluation function curve that changes more dramatically near the recording depth, thereby improving the system's sensitivity. This invention fixes the difference component of the focus evaluation function in hardware, ensuring both high scanning efficiency and flexibility in scanning scheme settings while improving sensitivity. Attached Figure Description

[0022] Figure 1 This is a schematic diagram of a microscopic measurement system.

[0023] Figure 2 This is a flowchart of the method of the present invention.

[0024] Figure 3 The resolution plate being measured in Example 1.

[0025] Figure 4 The curve of the traditional focus evaluation function as a function of depth in Example 1 is shown.

[0026] Figure 5 The difference between the two traditional focus evaluation function curves in Example 1 varies with depth.

[0027] Figure 1 Numbers: 1 and 9 are two cameras of the same model, 2 and 8 are two tube lenses of the same model, 3 and 4 are beam splitters, 5 is the objective lens, 6 is the object to be measured, 7 is the stage, 10 is the converging lens, 11 is the light source, the dashed line represents the imaging optical path, and the solid line represents the illumination optical path. Detailed Implementation

[0028] The present invention will be further described below with reference to specific embodiments and accompanying drawings. However, the embodiments do not constitute a limitation of the present invention. All other embodiments obtained by those skilled in the art without inventive effort are within the scope of protection of the present invention.

[0029] Example 1: The microscopic measurement system designed in this invention, see [link to example]. Figure 1As shown, the system includes two identical cameras 1 and 9, two identical tube mirrors 2 and 8, two beam splitters 3 and 4, an objective lens 5, a stage 7, a converging lens 10, and a light source 11. The first camera 1, the first tube mirror 2, the first beam splitter 3, the second beam splitter 4, the objective lens 5, and the stage 7 are arranged coaxially from top to bottom in the optical path to form the imaging optical path. The stage 7 is used to place the object to be measured 6. The second tube mirror 8 and the second camera 9 are sequentially arranged in the optical path of the beam splitter 3. The converging lens 10 and the light source 11 are sequentially arranged in the optical path of the beam splitter 4 to form the illumination optical path. Figure 1 The dashed line represents the imaging optical path, and the solid line represents the illumination optical path. The relative distances between the two sets of lenses and cameras are different, and the difference is fixed at 1 mm. When the first camera captures a clear image of a point, the second camera captures a defocused image of that point. The measurement system also includes motion mechanisms such as motors and grating rulers to achieve axial movement of the objectives or the entire microscopic imaging system. Furthermore, the microscope objective 6 has a numerical aperture of 0.25 mm and a working distance of 6.5 mm.

[0030] For a place at a fixed height, such as Figure 3 Measurements were performed on a resolution board with a binary image plotted on it. The scanning range was 20 mm, and the scanning interval was 100 μm. The Sobel function was used as the focus evaluation function. For the lateral position corresponding to pixels (150, 250), the normalized curves of the two focus evaluation functions versus depth were plotted, as shown below. Figure 4 As shown in the figure. At this point, the recording depth corresponds to the optimal focus depth during scanning, which is also the maximum value of the focus evaluation function curve. It can be seen that the changes of the two focus evaluation curves near this position are relatively slow, making the system's sensitivity to depth insufficient, and the depth recovery results are easily affected by noise.

[0031] The difference between the two focus evaluation functions is as follows: Figure 5 As shown, the recording depth at this point corresponds to the zero point of the evaluation function. For an axial deviation of 1 mm relative to the recording depth, the changes in the overall peak-to-valley values ​​of the relative curves in the original focus evaluation function curve and the difference between them are 3.97% and 18.16%, respectively. It is evident that the invented method can provide higher axial sensitivity for three-dimensional topography recovery.

Claims

1. A high-sensitivity three-dimensional topography reconstruction method based on dual-channel differential, characterized in that, include: A microscopic imaging system is used to scan the object under test by axially moving the objective lens or the entire microscopic imaging system. For the lateral position corresponding to each pixel, the focus evaluation function curve of the relative scanning depth change is calculated. A stable difference component is ensured by using two cameras with a fixed difference in the relative distance between the two optical paths and the tube mirrors. By subtracting the focus evaluation function curves corresponding to the two cameras, an evaluation function curve with more dramatic changes near the recording depth is obtained to improve the sensitivity of the system. The specific steps are as follows: (a) Constructing the measurement system; the measurement system includes a microscopic measurement system and a motion control mechanism; (1) The microscopic measurement system includes: two cameras of the same type, two tube mirrors of the same type, two beam splitters, an objective lens, a stage, a converging lens, and a light source; wherein, the first camera, the first tube mirror, the first beam splitter, the second beam splitter, the objective lens, and the stage are arranged coaxially from top to bottom in the optical path to form an imaging optical path; the stage is used to place the object to be measured; the second tube mirror and the second camera are sequentially arranged in the optical path of the beam splitter of the first beam splitter; the converging lens and the light source are sequentially arranged in the optical path of the beam splitter of the second beam splitter to form an illumination optical path; (2) The relative distances between the two sets of tubes and cameras are different, and the difference in distance is fixed and is a preset small amount. When the first camera clearly images a certain object point, the second camera captures the defocused image of that object point. (3) The motion control mechanism includes a motor and a grating ruler, which are used to control the axial movement of the objective lens or the entire microscopic imaging system; (II) Axial scanning and focusing evaluation: (1) Fix the object to be measured on the stage, adjust the objective lens or the entire microscopic imaging system, and determine the axial position where the highest and lowest points of the object to be measured can be clearly imaged, so as to set the scanning range and scanning interval of the measurement. (2) Based on the scanning range and scanning interval, the two cameras will each capture a sequence of n images, denoted as I. i 1 , i = 1, 2, ..., n; (3) Select the focus evaluation function according to the actual scene, and calculate the pixel-level focus evaluation function value for the above image sequence. For m pixels, m focus evaluation function curves are obtained, denoted as S. i 1 , i = 1, 2, ..., m, which records the focusing status of each lateral position at different scanning depths; (III) Differential and 3D Reconstruction: (1) Since there is only one corresponding optimal focus depth for a given horizontal position, and the focus evaluation function curve corresponding to that horizontal position reaches its maximum value at that optimal focus depth, therefore S i 1 , All of them followed a trend of first increasing and then decreasing with different scanning depths, for S i 1 , Making a difference: The new evaluation function curve F i In this context, the recording depth corresponds to the zero point of the evaluation function, compared to the Gaussian-like S. i 1 , Its changes are more dramatic near the recording depth, thus it has higher sensitivity to depth changes and stronger noise resistance. (2) For F i By fitting the zero point separately and combining it with the lateral position of the corresponding pixel, the height map formed by all recorded depths is the restored three-dimensional shape.

2. The high-sensitivity three-dimensional topography restoration method based on dual-channel differential as described in claim 1, characterized in that, The focus evaluation function is selected from the Sobel function and the Laplace function.

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