Non-contact optical lens center thickness measuring method

Abstract

The invention provides a non-contact optical lens center thickness measuring method, which comprises the steps of S1, acquiring image data of a lens, and shooting a surface image of the lens through a high-resolution camera, S2, preprocessing the image data, including denoising and contrast enhancement, so as to improve the image quality, S3, identifying the position of the lens in the image, and S4, determining the center thickness of the lens. S4, determining the contour of the lens by using an edge detection algorithm, and determining an optimal fitting ellipse of the lens by using an ellipse fitting algorithm according to the contour; according to the method, a high-precision image processing algorithm and an advanced optical interference principle are fused, so that rapid, accurate and completely lossless measurement of the center thickness of the optical lens is realized, the measurement efficiency is greatly improved, the operation complexity is reduced, and the measurement precision is improved. And any physical damage possibly caused by a traditional contact type measurement method to the lens is fundamentally avoided, and the optical performance of the lens is ensured not to be influenced.

Description

Technical Field

[0001] The invention belongs to the technical field of optical lens measurement, and specifically refers to a non-contact optical lens center thickness measurement method. Background Art

[0002] In the production and testing process of optical lenses, accurate measurement of the center thickness of the lens is a crucial step. Traditional contact measurement methods, such as using tools such as vernier calipers or micrometers, have many inconveniences and limitations. These methods are not only cumbersome and inefficient, but also easily damage the lens due to contact pressure, affecting its optical performance.

[0003] With the advancement of science and technology, non-contact measurement methods have gradually emerged, but the existing non-contact optical lens center thickness measurement methods still have some problems. They rely on complex mechanical equipment and cumbersome operating procedures, resulting in high measurement costs and difficulty in popularization. At the same time, during measurement, they may be affected by factors such as impurities and scratches on the lens surface, resulting in low measurement accuracy and unable to meet the production and testing needs of high-precision lenses. Summary of the invention

[0004] The technical problem to be solved by the present invention is to provide a non-contact optical lens center thickness measurement method with higher accuracy.

[0005] The present invention is achieved in that: A method for measuring the center thickness of a non-contact optical lens comprises the following steps: S1: Acquire image data of the lens: Take a surface image of the lens by using a high-resolution camera; S2: Preprocessing the image data, including: denoising and contrast enhancement; S3: Identify the position of the lens in the image and determine the outline of the lens using an edge detection algorithm; S4: Determine the best fitting ellipse of the lens using an ellipse fitting algorithm according to the contour; S5: Calculate the coordinates of the center point of the best fitting ellipse as the center point of the lens; S6: Using the principle of optical interference, the optical path difference at the center point of the optical lens is measured by a non-contact interferometer, and the thickness of the lens at that point is calculated based on the optical path difference; S7: Correcting the measured thickness: Correcting the measured thickness according to the refractive index of the lens material to obtain the final center thickness of the lens.

[0006] Furthermore, the denoising in step S2 adopts a median filtering algorithm, and the formula of the median filtering algorithm is: ; Where f(x, y) is the grayscale value of the original image at the coordinate (x, y), g(x, y) is the grayscale value of the denoised image at the coordinate (x, y), W is the filtering window, and med{} means taking the median of all grayscale values ​​in the window.

[0007] Furthermore, the edge detection algorithm in step S3 is a Canny edge detection algorithm, and the Canny edge detection algorithm includes gradient calculation, and its formula is: ; ; in and are the gradient components of the image in the x and y directions respectively, G is the gradient amplitude, and θ is the gradient direction.

[0008] Furthermore, the ellipse fitting algorithm formula in step S4 is: ; in is a point on the contour, is the center of the ellipse, c is the semi-major axis, θ is the rotation angle of the ellipse, a, b, c, d are the parameter vectors to be optimized, and N is the number of contour points; It also includes iterative optimization of the fitting results until a preset convergence condition is reached. The iterative optimization adopts the Levenberg-Marquardt algorithm, and its iterative formula is: ; in is the parameter vector at the kth iteration, J is the Jacobian matrix, r is the residual vector, μ is the damping factor, and I is the unit matrix.

[0009] Furthermore, the optical interferometer in step S6 is a Michelson interferometer, and the formula for measuring the optical path difference is: ; Where L is the optical path difference, N is the order of interference fringes, and λ is the wavelength of light used in the interferometer.

[0010] Furthermore, the correction formula in step S7 is: ; in is the corrected lens center thickness, is the measured center thickness of the lens, is the wavelength of light used by the optical interferometer and is a known constant, n is the refractive index of the lens material and is an inherent property of the lens material, and L is the optical path difference measured by the optical interferometer.

[0011] Furthermore, the method further comprises step S8: repeating steps S1 to S7 for multiple times, taking the average value of the multiple measurement results as the final lens center thickness, and the average value calculation formula is: ; in is the average value of multiple measurement results, M is the number of measurements, is the center thickness of the lens obtained by the i-th measurement.

[0012] Furthermore, after the step S4, the symmetry of the best fitting ellipse is verified to ensure that the lens is not deformed. The symmetry verification formula is: ; Where a and b are the major and minor axes of the fitted ellipse respectively. If the asymmetry value Asymmetry exceeds a preset threshold, it is determined that the lens is deformed.

[0013] Furthermore, the preset threshold is determined according to the design requirements and manufacturing standards of the lens, and the calculation formula of the preset threshold is: ; Among them, Threshold is the preset threshold, k is the safety factor, and Tolerance is the design tolerance of the lens.

[0014] Furthermore, the method further includes: after step S7, comparing the final center thickness of the lens with a preset standard thickness to evaluate the manufacturing quality of the lens, and the comparison formula is: ; Among them, Deviation is the thickness deviation, is the corrected lens center thickness, The preset standard thickness.

[0015] The advantages of the present invention are: by integrating high-precision image processing algorithms with advanced optical interference principles, the present invention achieves rapid, accurate and completely non-destructive measurement of the center thickness of an optical lens. This method not only greatly improves measurement efficiency and reduces operational complexity, but also fundamentally avoids any physical damage to the lens that may be caused by traditional contact measurement methods, thereby ensuring that the optical performance of the lens is not affected. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] The present invention will be further described below in conjunction with embodiments with reference to the accompanying drawings.

[0017] Figure 1 1 is a simplified flowchart of the method of the first and second embodiments of the present invention.

[0018] Figure 2 4 is a simplified flowchart of the method of the third embodiment of the present invention. DETAILED DESCRIPTION Embodiment 1

[0019] like Figure 1 and Figure 2As shown, a non-contact optical lens center thickness measurement method comprises the following steps: S1: Obtain image data of the lens. Use a high-resolution camera to capture the surface image of the lens, avoiding problems such as scratches and contamination that may be caused by traditional contact measurement, while improving measurement accuracy and efficiency; S2: Preprocess the image data, including denoising and contrast enhancement, to improve image quality. By optimizing image quality, measurement errors are reduced and measurement accuracy is improved. S3: Identify the position of the lens in the image and use the edge detection algorithm to determine the outline of the lens. It can automatically and accurately locate the lens, reduce manual intervention, and improve the degree of automation of measurement. S4: Based on the contour, an ellipse fitting algorithm is used to determine the best fitting ellipse of the lens. By accurately fitting the ellipse, the center point of the lens is accurately determined, providing accurate position information for subsequent thickness measurement; S5: Calculate the coordinates of the center point of the best fitting ellipse as the center point of the lens, and use the high precision of interferometric measurement to achieve accurate measurement of the lens thickness; S6: Using the principle of optical interference, the optical path difference at the center point of the lens is measured by a non-contact interferometer, and the thickness of the lens at that point is calculated based on the optical path difference; S7: Correct the measured thickness. Correct the measured thickness according to the refractive index of the lens material to obtain the final center thickness of the lens. By considering the influence of the refractive index, the measurement accuracy is improved, making the measurement result more consistent with the actual situation.

[0020] As can be seen from the above, the present invention first uses a high-resolution camera to capture a high-definition image of the lens under appropriate lighting conditions, and then removes noise and enhances contrast through an image preprocessing step to more accurately identify the position of the lens; then, an advanced ellipse fitting algorithm is used to fit the edge of the lens to determine the precise coordinates of the center point of the lens; on this basis, a non-contact optical interferometer is used to measure the optical path difference at the center point of the lens, and correction is performed in combination with the refractive index of the lens material, thereby achieving non-destructive and high-precision measurement of the center thickness of the lens. Embodiment 2

[0021] This embodiment is basically the same as the previous embodiment, except that: The denoising in step S2 adopts the median filtering algorithm, and the formula of the median filtering algorithm is: ; Where f(x, y) is the gray value of the original image at the coordinate (x, y), g(x, y) is the gray value of the denoised image at the coordinate (x, y), W is the filtering window, med{} represents the median of all gray values ​​in the window, and the median filtering algorithm can more effectively remove salt and pepper noise and speckle noise in the image while retaining the edge details of the image, which is crucial for subsequent edge detection and ellipse fitting. By using the median filtering algorithm for denoising, the image quality can be significantly improved, providing clear image input for subsequent steps, thereby improving the accuracy and stability of the entire measurement process, ensuring the best effect of image preprocessing, and laying a solid foundation for subsequent steps.

[0022] The edge detection algorithm in step S3 is the Canny edge detection algorithm, which includes gradient calculation, and its formula is: ; ; in and are the gradient components of the image in the x and y directions respectively, G is the gradient amplitude, and θ is the gradient direction. The Canny edge detection algorithm can accurately identify the edge contour of the lens and provide accurate input data for the subsequent ellipse fitting algorithm, which helps to improve the positioning accuracy of the lens center point and the overall measurement accuracy.

[0023] The ellipse fitting algorithm formula in step S4 is: ; in is a point on the contour, is the center of the ellipse, c is the semi-major axis, θ is the rotation angle of the ellipse, a, b, c, d are the parameter vectors to be optimized, and N is the number of contour points; The ellipse fitting algorithm in step S4 also includes iterative optimization of the fitting results until the preset convergence condition is reached. The iterative optimization adopts the Levenberg-Marquardt algorithm, and its iterative formula is: ; in is the parameter vector at the kth iteration, J is the Jacobian matrix, r is the residual vector, μ is the damping factor, and I is the unit matrix. The ellipse fitting algorithm adopted in the present invention has higher fitting accuracy and stronger robustness. By adopting this high-precision ellipse fitting algorithm, the best fitting ellipse of the lens can be determined more accurately, thereby obtaining more accurate center point coordinates, which is helpful to improve the accuracy of the overall measurement.

[0024] The optical interferometer in step S6 is a Michelson interferometer, and the formula for measuring the optical path difference is: ; Where L is the optical path difference, N is the order of interference fringes, and λ is the wavelength of light used by the interferometer. Using a Michelson interferometer for measurement can ensure accurate optical path difference data, providing a reliable basis for subsequent thickness correction steps. This helps to improve the accuracy and stability of the overall measurement.

[0025] The present invention improves image quality by using a median filter algorithm to perform image denoising; uses a Canny edge detection algorithm to accurately identify the lens contour, providing a reliable basis for subsequent ellipse fitting; determines the best fitting ellipse of the lens through an ellipse fitting algorithm, and uses a Levenberg-Marquardt algorithm for iterative optimization to ensure the accuracy and stability of ellipse fitting; and provides strong support for the accurate calculation of the center thickness of the lens by using a Michelson interferometer to measure the optical path difference. The optimization and clarification of these technical details jointly improve the measurement accuracy and reliability of the method of the present invention. Embodiment 3

[0026] like Figure 2 As shown, based on the first embodiment, the correction formula in step S7 is: Where T corrected is the corrected lens center thickness, T measured is the measured center thickness of the lens, λ is the wavelength of light used by the optical interferometer and is a known constant, n is the refractive index of the lens material, which is the inherent property of the lens material, and L is the optical path difference measured by the optical interferometer. A specific formula for thickness correction is given, taking into account the influence of the refractive index of the lens material on the measurement result. By adopting this correction method, the influence of the refractive index of the lens material on the measurement result can be eliminated, and a more accurate value of the center thickness of the lens can be obtained. This helps to improve the accuracy and reliability of the overall measurement. This step is the key to ensure that the measurement result is not affected by the refractive index of the lens material, and helps to improve the measurement accuracy of the overall solution.

[0027] The method further includes step S8: repeating steps S1 to S7 multiple times, taking the average value of the multiple measurement results as the final lens center thickness, and the average value calculation formula is: ; in is the average value of multiple measurement results, M is the number of measurements, For the center thickness of the lens obtained by the i-th measurement, a method of taking the average value of multiple measurements is proposed to reduce the error and uncertainty caused by a single measurement. By taking the average value of multiple measurements, the discreteness and error of the measurement results can be significantly reduced, and the stability and reliability of the overall measurement can be improved.

[0028] It also includes: after step S4, the best fitting ellipse is symmetric verified to ensure that the lens is not deformed, and the symmetry verification formula is: ; Where a and b are the major and minor semi-axis of the fitted ellipse, respectively. If the asymmetry value Asymmetry exceeds the preset threshold, the lens is determined to be deformed. This step proposes a step for symmetry verification of the best fitting ellipse to ensure that the lens is not deformed. By verifying the symmetry of the fitted ellipse, possible deformation problems of the lens can be discovered in time, thereby avoiding misjudging an unqualified lens as qualified, which helps to improve the reliability of the overall measurement process and product quality.

[0029] Specifically, the preset threshold is determined according to the design requirements and manufacturing standards of the lens, and the calculation formula of the preset threshold is: , where Threshold is the preset threshold, k is the safety factor (usually 1.0~1.5), and Tolerance is the design tolerance of the lens. By adopting this method to determine the preset threshold, the threshold value can be flexibly adjusted according to the design requirements and manufacturing standards of the lens, thereby ensuring that the results of symmetry verification are more accurate and reliable. This helps to improve the flexibility and adaptability of the overall measurement process. This step is the key to ensuring the accuracy and reliability of symmetry verification, and helps to improve the practicality and measurement accuracy of the overall solution.

[0030] The method further includes: after step S7, comparing the final center thickness of the lens with a preset standard thickness to evaluate the manufacturing quality of the lens, and the comparison formula is: ; Among them, Deviation is the thickness deviation, is the corrected lens center thickness, By comparing the final lens center thickness with the preset standard thickness, it is possible to intuitively evaluate whether the manufacturing quality of the lens meets the requirements. This helps to promptly discover possible problems in the manufacturing process and take corresponding measures to improve and optimize. At the same time, this step also provides strong support for product quality traceability and evaluation.

[0031] The present invention uses a Michelson interferometer and a specific formula to accurately measure the optical path difference and then calculate the lens thickness; uses the refractive index to correct the measured thickness to improve the accuracy of the final result, averages multiple measurements to reduce errors and ensure the stability of the measurement results; verifies the symmetry to ensure that the lens is not deformed and improves the measurement reliability; and compares the measurement results with the standard thickness to quickly evaluate the lens manufacturing quality. These steps together promote the realization of an efficient, accurate and comprehensive non-contact optical lens center thickness measurement solution.

[0032] The above embodiments and drawings do not limit the product form and style of the present invention. Any appropriate changes or modifications made by ordinary technicians in the relevant technical field should be deemed to be within the patent scope of the present invention.

Claims

1. A non-contact optical lens center thickness measurement method, characterized in that: The steps include: S1: Acquire image data of the lens: Take a surface image of the lens by using a high-resolution camera; S2: Preprocessing the image data, including: denoising and contrast enhancement; S3: Identify the position of the lens in the image and determine the outline of the lens using an edge detection algorithm; S4: Determine the best fitting ellipse of the lens using an ellipse fitting algorithm according to the contour; S5: Calculate the coordinates of the center point of the best fitting ellipse as the center point of the lens; S6: Using the principle of optical interference, the optical path difference at the center point of the optical lens is measured by a non-contact interferometer, and the thickness of the lens at that point is calculated based on the optical path difference; S7: Correcting the measured thickness: Correcting the measured thickness according to the refractive index of the lens material to obtain the final center thickness of the lens.

2. A non-contact optical lens center thickness measurement method as claimed in claim 1, characterized in that: The denoising in step S2 adopts a median filtering algorithm, and the formula of the median filtering algorithm is: ; Where f(x, y) is the grayscale value of the original image at the coordinate (x, y), g(x, y) is the grayscale value of the denoised image at the coordinate (x, y), W is the filtering window, and med{} means taking the median of all grayscale values ​​in the window.

3. A non-contact optical lens center thickness measurement method as claimed in claim 2, characterized in that: The edge detection algorithm in step S3 is the Canny edge detection algorithm, which includes gradient calculation, and its formula is: ; ; in and are the gradient components of the image in the x and y directions respectively, G is the gradient amplitude, and θ is the gradient direction.

4. A non-contact optical lens center thickness measurement method as claimed in claim 3, characterized in that: The ellipse fitting algorithm formula in step S4 is: ； in is a point on the contour, is the center of the ellipse, c is the semi-major axis, θ is the rotation angle of the ellipse, a, b, c, d are the parameter vectors to be optimized, and N is the number of contour points; It also includes iterative optimization of the fitting results until a preset convergence condition is reached. The iterative optimization adopts the Levenberg-Marquardt algorithm, and its iterative formula is: ; in is the parameter vector at the kth iteration, J is the Jacobian matrix, r is the residual vector, μ is the damping factor, and I is the unit matrix.

5. A non-contact optical lens center thickness measurement method as claimed in claim 4, characterized in that: The optical interferometer in step S6 is a Michelson interferometer, and the formula for measuring the optical path difference is: ; Where L is the optical path difference, N is the order of interference fringes, and λ is the wavelength of light used in the interferometer.

6. A non-contact optical lens center thickness measurement method as claimed in claim 5, characterized in that: The correction formula in step S7 is: ; in is the corrected lens center thickness, is the measured center thickness of the lens, is the wavelength of light used by the optical interferometer and is a known constant, n is the refractive index of the lens material and is an inherent property of the lens material, and L is the optical path difference measured by the optical interferometer.

7. A non-contact optical lens center thickness measurement method as claimed in claim 1, characterized in that: The method further comprises step S8: repeating steps S1 to S7 for multiple times, taking the average value of the multiple measurement results as the final lens center thickness, and the average value calculation formula is: ; in is the average value of multiple measurement results, M is the number of measurements, is the center thickness of the lens obtained by the i-th measurement.

8. A non-contact optical lens center thickness measurement method as claimed in claim 1, characterized in that: After the step S4, the symmetry of the best fitting ellipse is verified to ensure that the lens is not deformed. The symmetry verification formula is: ; Where a and b are the major and minor axes of the fitted ellipse, respectively. If the asymmetry value Asymmetry exceeds a preset threshold, it is determined that the lens is deformed.

9. A non-contact optical lens center thickness measurement method as claimed in claim 8, characterized in that: The preset threshold is determined according to the design requirements and manufacturing standards of the lens, and the calculation formula of the preset threshold is: ; Among them, Threshold is the preset threshold, k is the safety factor, and Tolerance is the design tolerance of the lens.

10. A non-contact optical lens center thickness measurement method as claimed in claim 1, characterized in that: Also includes: After step S7, the final center thickness of the lens is compared with a preset standard thickness to evaluate the manufacturing quality of the lens. The comparison formula is: ; Among them, Deviation is the thickness deviation, is the corrected lens center thickness, The preset standard thickness.

Images