CN116735167B - Lens multiplying power and distortion detection method - Google Patents
Lens multiplying power and distortion detection method Download PDFInfo
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- CN116735167B CN116735167B CN202311029037.2A CN202311029037A CN116735167B CN 116735167 B CN116735167 B CN 116735167B CN 202311029037 A CN202311029037 A CN 202311029037A CN 116735167 B CN116735167 B CN 116735167B
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01M—TESTING STATIC OR DYNAMIC BALANCE OF MACHINES OR STRUCTURES; TESTING OF STRUCTURES OR APPARATUS, NOT OTHERWISE PROVIDED FOR
- G01M11/00—Testing of optical apparatus; Testing structures by optical methods not otherwise provided for
- G01M11/02—Testing optical properties
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01M—TESTING STATIC OR DYNAMIC BALANCE OF MACHINES OR STRUCTURES; TESTING OF STRUCTURES OR APPARATUS, NOT OTHERWISE PROVIDED FOR
- G01M11/00—Testing of optical apparatus; Testing structures by optical methods not otherwise provided for
- G01M11/02—Testing optical properties
- G01M11/0242—Testing optical properties by measuring geometrical properties or aberrations
- G01M11/0257—Testing optical properties by measuring geometrical properties or aberrations by analyzing the image formed by the object to be tested
- G01M11/0264—Testing optical properties by measuring geometrical properties or aberrations by analyzing the image formed by the object to be tested by using targets or reference patterns
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Abstract
The invention discloses a lens multiplying power and distortion detection method, which relates to the field of lens detection and comprises the steps of installing a lens to be detected on a line scanning camera, aiming at a black-and-white stripe grating plate for shooting, and collecting image data; performing frequency domain conversion and noise reduction treatment on the acquired image data, and calculating to obtain actual frequency; based on the actual frequency, further calculating the grating width, and determining the multiplying power of the lens by comparing the preset width with the grating width; selecting a plurality of sampling points in a visual field range, calculating the optimal focal plane position, adjusting the relative position of the lens and the grating plate, eliminating distortion caused by an inclination angle, and calculating distortion parameters of the lens. The invention can accurately calculate the multiplying power and the inclination angle of the lens, effectively eliminate the distortion of the lens and optimize the testing flow and the precision of the lens.
Description
Technical Field
The invention relates to the field of lens detection, in particular to a lens multiplying power and distortion detection method.
Background
In the intelligent rapid development of modern optical instruments, the application range of the lens in the technical field of optics is wider and wider, and the specifications are diversified. The magnification and distortion are important parameters in the lens detection process, and as one of the basic parameters of the lens, the distortion directly affects the imaging quality of the lens. Limited by the photosensitive elements, there may be insufficient data to accurately calculate distortion, which makes detecting more and more complex lens magnification and distortion an important challenge.
In the development and popularization process of industrial automation, requirements on a machine vision system are continuously increased, and requirements on lens performance are also more and more strict. This makes the specifications of the lens more and more biased to customization, which further increases the difficulty of detecting the magnification and distortion of the lens. The conventional detection method generally uses a camera to observe a calibration plate, and then compares an image acquired by the camera with the actual size of the calibration plate, so as to determine the magnification and distortion of the lens. However, due to the influence of the lens specification, when attempting to calculate the size of the calibration plate in the image, the size of the calibration plate may not be directly and accurately calculated due to insufficient information contained in the image, and thus the expected measurement accuracy may not be achieved.
Therefore, a new lens magnification and distortion calculation method is urgently needed in the market. This new approach needs to be able to solve two major challenges: firstly, the image acquired by the camera contains less information, and the size of the calibration plate cannot be directly acquired from the image; secondly, the lens itself is inclined, which may cause left-right asymmetry of multiplying power in the field of view, thereby affecting accuracy of distortion result of the lens.
Disclosure of Invention
The invention aims to provide a lens multiplying power and distortion detection method for solving the problems in the background technology.
In order to achieve the above purpose, the invention adopts the following technical scheme:
a lens multiplying power and distortion detection method comprises the following steps:
s1: mounting a lens to be tested on a line scanning camera, and shooting the line scanning camera aiming at a black-and-white stripe grating plate with a preset width so as to acquire black-and-white stripe images;
s2: converting the black-and-white stripe image data acquired in the step S1 into a frequency domain, carrying out noise reduction treatment in the frequency domain, determining the position of each harmonic component, calculating the actual frequency corresponding to each harmonic component, and combining the frequencies to obtain the actual frequency of the black-and-white stripe image;
s3: calculating the grating width in the black-and-white stripe image according to the actual frequency calculated in the step S2, wherein the grating width is equal to the preset width divided by the actual frequency; comparing the grating width with a preset width on a grating plate, and determining the multiplying power of the lens by using the ratio of the two widths;
s4: selecting a plurality of sampling points in the visual field range of the lens, focusing each sampling point to obtain an optimal focal plane position corresponding to each sampling point, fitting a discrete focal plane topographic map formed by the optimal focal plane positions corresponding to all the sampling points to obtain a fitting plane, determining a relative inclination angle between the fitting plane and the grating plate plane, and adjusting the relative positions of the lens and the grating plate according to the angle to level the lens so as to eliminate distortion caused by the inclination angle;
and comparing the leveled black-and-white stripe image with an ideal grating image, respectively quantifying the distortion degree at each sampling point by using an aberration analysis method, and further averaging the distortion degrees of all sampling points, thereby calculating the distortion parameters of the leveled lens.
In some embodiments, the noise reduction process includes removing non-harmonic components in the frequency domain image.
In some embodiments, the number of sampling points is selected to be between 5 and 10.
In some embodiments, the lens leveling method in step S4 is a numerical optimization method.
In some embodiments, during the focusing process in step S4, for each sampling point, a lens position with the highest definition of the black and white stripe image at the corresponding sampling point is found by fine-tuning the distance between the lens and the grating plate, and the sampling point corresponds to the best focal plane position of the sampling point under the lens position.
In some embodiments, the plurality of sampling points are evenly distributed.
In some embodiments, the fitting of the focal plane topography in step S4 employs a least squares method.
In some embodiments, the aberration analysis method includes quantifying the degree of distortion using a Zernike polynomial.
In some embodiments, in the step S4, the method further includes calculating a real physical coordinate of the best focal plane position corresponding to each sampling point, including the following steps:
acquiring pixel coordinates obtained when each sampling point is focused;
according to the resolution ratio of the black-and-white stripe image and the lens multiplying power obtained in the step S3, converting the pixel coordinates into physical coordinates of the optimal focal plane position of the sampling point relative to a lens coordinate system;
and combining the lens position of each sampling point during focusing to obtain the real physical coordinates of the optimal focal plane position of each sampling point relative to the grid plate coordinate system.
In some embodiments, the converting the pixel coordinates to physical coordinates of the sample point best focus plane position relative to the lens coordinate system includes:
determining a physical size of each pixel on the image sensor;
determining a proportional relationship between an actual physical size of the object on the image sensor and a size in the field of view of the lens using the lens magnification calculated in step S3;
and calculating the physical coordinates of the optimal focal plane position of the sampling point in a lens coordinate system through a proportional relation by utilizing the pixel coordinates of the sampling point.
Compared with the prior art, the lens testing technology has the advantages that the lens to be tested is subjected to deep testing and analysis through the fine steps and calculation. In the S1 stage, the lens to be tested is mounted on the line scanning camera and is aligned to the black-and-white stripe grating plate for shooting, and the operation mode can accurately acquire image data, so that a reliable basis is provided for the subsequent steps. In the S2 stage, the actual frequency can be accurately obtained through frequency domain conversion and noise reduction processing of the image data, and the image quality is improved and the reliability of the data is enhanced. The S3 stage determines the magnification of the lens by calculating the actual frequency, which enables us to accurately measure the optical performance of the lens. Finally, in the S4 stage, distortion caused by the inclination angle can be effectively eliminated by focusing the sampling point and adjusting the relative position of the lens and the grating plate, and the image quality is optimized. Through the series of steps, the invention can accurately calculate the multiplying power and distortion of the lens under the condition that the acquired image contains limited information.
Drawings
Fig. 1 is a flow chart of the method of the present invention.
Detailed Description
The following describes specific embodiments of the present invention with reference to the drawings.
A flow chart of the method of the invention is shown in figure 1.
The invention provides a method for detecting lens multiplying power and distortion, which comprises the following steps:
step S1: and mounting the lens to be tested on the line scanning camera, and then shooting the line scanning camera aiming at the black and white stripe grating plate with the preset width. The distance between the line scanning camera and the grating plate can be set according to actual needs. In the shooting process, the line scanning camera can acquire black and white stripe images on the grating plate.
Step S2: and (3) converting the black and white stripe image data acquired in the step S1 into a frequency domain. In the frequency domain, the noise reduction processing may be performed by using a frequency domain filtering method or the like, and mainly includes removing non-harmonic components. Then, the position of each harmonic component is determined, and the actual frequency corresponding to each harmonic component is calculated according to the position of the harmonic component. These actual frequencies are then combined to obtain the actual frequency of the image.
Step S3: and (3) according to the actual frequency calculated in the step S2, the grating width in the image is calculated. Specifically, the grating width is equal to the preset width divided by the actual frequency. And then comparing the calculated grating width with a preset width on the grating plate, wherein the ratio of the two widths is the multiplying power of the lens to be tested.
Step S4: and selecting a plurality of (e.g. 5 to 10) sampling points in the visual field of the lens, and focusing each sampling point to calculate the optimal focal plane position. Then, the relative tilt angle between the lens and the grating plate is determined using the magnification obtained in step S3. According to the angle, the relative positions of the lens and the grating plate are adjusted to level the lens, and the step can be performed by adopting a numerical optimization method. After leveling, the lens faces the grating plate, in such a way that distortions due to the angle of inclination are eliminated. Next, by comparing the leveled image with an ideal grating image, the distortion degree is quantified at each sampling point by aberration analysis or the like, respectively. And finally, averaging the distortion degrees of all the sampling points, and calculating to obtain distortion parameters of the lens after leveling.
More specifically, step S4 may include the following sub-steps:
s41: selecting sampling points: a plurality of (e.g., 5 to 10) sampling points are selected within the field of view of the lens. The sampling points are typically chosen to be evenly distributed so as to cover the entire field of view as much as possible.
S42: calculating the best focal plane position: for each sample point, focus is performed. In practical operation, focusing can be achieved by fine-tuning the distance between the lens and the grating plate, or by changing the focal length of the lens. For each sample point, a lens position is found that maximizes the image sharpness (e.g., contrast) at the position corresponding to the sample point, and the focal point produced by the lens position is the best focal plane position for that sample point.
S43: determining the inclination angle: a discrete focal surface topography can be constructed using the magnification obtained in step S3 and the optimal focal surface position for each sample point calculated in the above steps. The surface is then fitted to the topography to obtain a surface. The angle between this plane and the ideal focal plane (i.e. the plane parallel to the grating plate) is the relative tilt angle between the lens and the grating plate.
S44: leveling lens: and adjusting the relative positions of the lens and the grating plate according to the calculated inclination angle so as to level the lens. In a specific operation, a numerical optimization method can be adopted, and the average error of focal plane positions of all sampling points is minimized by fine adjustment of lens positions.
S45: quantization distortion degree: after leveling, the lens faces the grating plate, and distortion due to the tilt angle is eliminated. Next, by comparing the leveled image with an ideal grating image, the distortion degree is quantified at each sampling point by aberration analysis or the like, respectively. Specifically, the degree of distortion can be quantified by comparing the difference between the actual image and the ideal image. This difference may be a simple pixel difference or a more complex image feature difference.
S46: calculating distortion parameters: and finally, averaging the distortion degrees of all the sampling points, and calculating to obtain distortion parameters of the lens after leveling. This parameter is a statistic used to quantify the degree of distortion throughout the field of view, and can be used to evaluate the performance of the lens, as well as to guide the design and manufacture of the lens.
In the following examples, the present invention is described by taking specific parameters as examples, and the actual parameters may vary from case to case:
step S1: firstly, a lens to be tested is mounted on a line scanning camera, and then the line scanning camera is aligned to a black-and-white stripe grating plate with a preset width of 10mm for shooting. The distance between the line scanning camera and the grating plate was set to 1000mm.
Step S2: the line scan camera captures and converts image data to the frequency domain. Assuming that the location of the main harmonic component obtained in the frequency domain is at 10Hz, its corresponding actual frequency is 1/10=0.1 Hz.
Step S3: the grating width was found to be 10mm/0.1 hz=100 mm using the preset width divided by the actual frequency. The calculated grating width is then compared with a preset width on the grating plate, i.e. 100mm/10 mm=10, which is the magnification of the lens to be measured.
Step S4: 9 sampling points are uniformly selected in the visual field range of the lens, and the optimal focal plane position of each point can be obtained by adjusting the distance between the lens and the grating plate.
Substeps S41-S42: for example, the lens positions corresponding to the 9 best focal planes obtained are: 995mm, 996mm, 997mm, 998mm, 999mm, 1000mm, 1001mm, 1002mm and 1003mm. The lens position and the lens multiplying power can be combined to calculate the real physical coordinates of the corresponding optimal focal plane position.
Using these data, a discrete focal surface topography was obtained, and by plane fitting, the angle between the fitted plane and the ideal focal surface (i.e., the plane parallel to the grating plate) was found to be 0.9 degrees.
Then, the distance between the lens and the grating plate is finely adjusted, so that the included angle is reduced to 0, and the leveling is finished. The lens can be directly rotated by 0.9 degrees in the opposite direction, then the process is repeated to obtain the optimal focal plane position of the sampling point again, a fitting plane is obtained, whether the included angle between the fitting plane and the grating plate is 0 is verified again, and if the included angle is not 0, the lens can be rotated according to the included angle, and the process is repeated until the included angle is 0.
After leveling, the distortion level was quantified by aberration analysis at each sampling point. For example, if it is found that the distortion degrees are 0.01mm, 0.02mm, 0.01mm, 0.02mm, 0.015mm, 0.01mm, 0.02mm, 0.01mm and 0.02mm, respectively, at 9 sampling points.
Then the average value, i.e. (0.01+0.02+0.01+0.02+0.015+0.01+0.02+0.01+0.02)/9=0.015 mm, is calculated, which is the distortion parameter of the lens after we level.
The lens multiplying power and distortion detection method can effectively acquire multiplying power and distortion parameters of the lens, improves measurement accuracy, and is suitable for testing and calibrating various lenses.
The foregoing is only a preferred embodiment of the present invention, but the scope of the present invention is not limited thereto, and any person skilled in the art, who is within the scope of the present invention, should be covered by the protection scope of the present invention by making equivalents and modifications to the technical solution and the inventive concept thereof.
Claims (10)
1. The lens multiplying power and distortion detection method is characterized by comprising the following steps:
s1: mounting a lens to be tested on a line scanning camera, and shooting the line scanning camera aiming at a black-and-white stripe grating plate with a preset width so as to acquire black-and-white stripe images;
s2: converting the black-and-white stripe image data acquired in the step S1 into a frequency domain, carrying out noise reduction treatment in the frequency domain, determining the position of each harmonic component, calculating the actual frequency corresponding to each harmonic component, and combining the frequencies to obtain the actual frequency of the black-and-white stripe image;
s3: calculating the grating width in the black-and-white stripe image according to the actual frequency calculated in the step S2, wherein the grating width is equal to the preset width divided by the actual frequency; comparing the grating width with a preset width on a grating plate, and determining the multiplying power of the lens by using the ratio of the two widths;
s4: selecting a plurality of sampling points in the visual field range of the lens, focusing each sampling point to obtain an optimal focal plane position corresponding to each sampling point, fitting a discrete focal plane topographic map formed by the optimal focal plane positions corresponding to all the sampling points to obtain a fitting plane, determining a relative inclination angle between the fitting plane and the grating plate plane, and adjusting the relative positions of the lens and the grating plate according to the angle to level the lens so as to eliminate distortion caused by the inclination angle;
and comparing the leveled black-and-white stripe image with an ideal grating image, respectively quantifying the distortion degree at each sampling point by using an aberration analysis method, and further averaging the distortion degrees of all sampling points, thereby calculating the distortion parameters of the leveled lens.
2. The lens magnification and distortion detection method according to claim 1, wherein the noise reduction processing includes removing non-harmonic components in the frequency domain image.
3. The lens magnification and distortion detection method according to claim 1, wherein the number of sampling points is selected to be between 5 and 10.
4. The method for detecting lens magnification and distortion according to claim 1, wherein the lens leveling method in step S4 is a numerical optimization method.
5. The method according to claim 1, wherein during the focusing in step S4, for each sampling point, a lens position is found by fine-tuning the distance between the lens and the grating plate so that the sharpness of the black-and-white streak image at the corresponding sampling point is highest, and the focusing point generated correspondingly by the sampling point at the lens position corresponds to the best focal plane position of the sampling point.
6. The lens magnification and distortion detection method according to claim 1, wherein the plurality of sampling points are uniformly distributed.
7. The lens magnification and distortion detection method according to claim 1, wherein the fitting of the focal plane topography in step S4 employs a least square method.
8. The lens magnification and distortion detection method according to claim 1, wherein the aberration analysis method includes quantifying the degree of distortion using a Zernike polynomial.
9. The method for detecting lens magnification and distortion as defined in claim 5, wherein in step S4, further comprising calculating a true physical coordinate of a best focal plane position corresponding to each sampling point, comprising the steps of:
acquiring pixel coordinates obtained when each sampling point is focused;
according to the resolution ratio of the black-and-white stripe image and the lens multiplying power obtained in the step S3, converting the pixel coordinates into physical coordinates of the optimal focal plane position of the sampling point relative to a lens coordinate system;
and combining the lens position of each sampling point during focusing to obtain the real physical coordinates of the optimal focal plane position of each sampling point relative to the grid plate coordinate system.
10. The lens magnification and distortion detection method according to claim 9, wherein the process of converting the pixel coordinates into physical coordinates of the sampling point best focal plane position with respect to the lens coordinate system comprises:
determining a physical size of each pixel on the image sensor;
determining a proportional relationship between an actual physical size of the object on the image sensor and a size in the field of view of the lens using the lens magnification calculated in step S3;
and calculating the physical coordinates of the optimal focal plane position of the sampling point in a lens coordinate system through a proportional relation by utilizing the pixel coordinates of the sampling point.
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