Laser beam divergence angle measuring method based on image fusion

Abstract

The invention discloses a laser beam divergence angle measuring method based on image fusion, and belongs to the field of photoelectric testing. A supersaturated light spot image and an unsaturated light spot image are fused to obtain an ideal fused image, the beam width of a light beam is obtained in a mode of calculating the second moment of the light spot, and then the divergence angle of the laser beam is obtained; according to the method, background noise of CCD camera imaging is removed by imaging the imaging plane in the shielding state for multiple times, and salt and pepper noise is removed by means of median filtering, so that the measurement precision is improved.

Description

Technical Field

[0001] The invention belongs to the field of photoelectric testing, and in particular relates to a laser beam divergence measurement method based on image fusion. Background Art

[0002] Laser beam divergence is a parameter that describes the degree of diffusion of a laser beam during propagation. It indicates the angle at which the laser beam diameter increases as the propagation distance increases, starting from the beam waist position. Laser beam divergence is an important parameter for evaluating laser quality. It can reflect the development characteristics of the laser during long-distance transmission and can be used to evaluate the quality of the laser. Traditional methods for measuring laser beam divergence include the knife-edge scanning method, the trepanning method, the BBO frequency doubling method, and the second-order moment method of the spot based on CCD imaging. The former methods have strict requirements on the light beam, require complex measurement equipment structures, and have high measurement costs. The CCD camera is a digital camera that uses a charge-coupled device as an image sensor. The second-order moment method of the spot based on CCD imaging only requires a CCD camera to be implemented, but its test error is large. In particular, the under-saturation or over-saturation of the image caused by the exposure during CCD imaging has a great impact on the accuracy of laser beam divergence measurement.

[0003] In response to the problems existing in the second-order moment method of the spot based on CCD imaging, the present invention proposes a laser beam divergence measurement method based on image fusion. By denoising and fusing spot images with different exposure degrees, the laser beam divergence measurement accuracy of the second-order moment method of the spot based on CCD imaging is improved.

[0004] The laser beam divergence angle measurement method of the prior art, Chinese invention patent: Laser beam divergence angle measurement method, application publication number CN118190353A, discloses: step S1: pre-blocking the image plane, and then measuring the background noise parameters of the camera under the blockage; the background noise parameters include background average noise and background noise standard deviation; step S2: removing the blockage and using the laser to be measured to project a laser beam to the image plane, and using the camera to collect a noisy laser image; step S3: filtering the noisy laser image using the background noise parameters to obtain a filtered image; step S4: using the filtered image to calculate the laser beam divergence angle.

[0005] The above file has the following problems: 1. Improving the accuracy of laser beam divergence measurement using the second-order moment calculation method by simply removing the camera background noise will affect the improvement of measurement accuracy.

[0006] 2. During the measurement process, the CCD camera exposure has a significant impact on the laser beam divergence measurement due to the spot image exposure, which affects the improvement of measurement accuracy. Summary of the Invention

[0007] The purpose of the present invention is to overcome the above-mentioned existing problems and propose a laser beam divergence angle measurement method based on image fusion to solve the problem that the accuracy of laser beam divergence measurement by the second-order moment calculation method is improved only by removing the camera background noise in the existing technology, which affects the improvement of measurement accuracy.

[0008] To achieve the above object, the technical solution adopted by the present invention is as follows: A laser beam divergence measurement method based on image fusion, characterized by the following specific steps: Step s1: Use a CCD camera to perform imaging and obtain the camera's background noise mean μ; Step s2: Use the CCD camera to image the laser beam, adjust the laser energy attenuation plate attenuation magnification and the CCD camera integration time to obtain an unsaturated spot image I 1 (x, y) With an oversaturated spot image I 2 (x, y) ; Step s3: Convert the unsaturated spot image I 1 (x, y) and oversaturated spot images I 2 (x, y) Subtract the noise mean μ respectively, use median filtering to remove imaging noise, and obtain the image I 11 (x, y) and I 21 (x, y) ; Step s4: The image will be obtained I 11 (x, y) and I 21 (x, y) Fusion to obtain image I 3 (x, y) ; Step s5: Calculate by first-order moment I 3 (x, y) The center position of the light spot ; Step s6: passing the center position of the light spot Calculation using the second-order moment of the light field I 3 (x, y) Laser spot diameter ω x With ω y ; Step s7: Based on the spot diameter ω in the x-direction and y-direction x With ω y Get the final spot diameter D ; Step s8: By the final spot diameterD The laser beam divergence angle is calculated to obtain the laser beam divergence angle.

[0009] In the step s1, the imaging plane is blocked and imaging is performed using a CCD camera.

[0010] The grayscale values ​​of the pixels in the unsaturated light spot image in step s2 conform to a two-dimensional Gaussian distribution, and the maximum grayscale value is between 240 and 255.

[0011] The maximum grayscale value of the pixels of the oversaturated light spot image in step s2 is 255.

[0012] In step s3, 3*3 median filtering is used to remove imaging noise.

[0013] In step s4 I 11 (x, y) and I 21 (x, y) Fusion I 3 (x, y) The formula is as follows:

[0014] Among them, η is the fusion threshold, m is the grayscale width of pixel fusion, m Take 5.

[0015] When the image I 21 (x, y) When the pixel value is less than η, I 3 (x, y) The corresponding pixel value is taken from I 21 (x, y) , η is taken as 230.

[0016] When the image I 21 (x, y) When the pixel value is greater than η, I 3 (x, y) The corresponding pixel value is no longer taken from I 21 (x, y) , by taking the unsaturated spot image I 11 (x, y) The pixel value at the corresponding position is multiplied by a proportional coefficient get.

[0017] The proportionality coefficient The pixel value of the unsaturated spot image is between η and η+ m Corresponding area within the range I21 (x, y) and I 11 (x, y) The ratio of the total pixel values.

[0018] The first-order moment calculation in step s5 I 3 (x, y) The center position of the light spot The formula is as follows:

[0019]

[0020] x For each pixel I 3 (x, y) The horizontal axis value of y For each pixel I 3 σ The vertical coordinate value of .

[0021] In step s6, the second-order moment of the light field is used to calculate the laser spot diameter ω. x With ω y The formula is as follows:

[0022]

[0023] σ x 2 （z） and Figure 1 y 2 （z） is the second-order moment of the spot image.

[0024] In step s7, x Direction and y The geometric mean of the directional diameters is taken to obtain the final spot diameter.

[0025] In step s7, x Direction and y The formula for taking the geometric mean of the directional diameters is as follows:

[0026] in, D is the final spot diameter.

[0027] The laser beam divergence in step s8 The calculation formula is as follows:

[0028] Wherein, L is the distance between the imaging plane and the laser light source.

[0029] The advantages of adopting the present invention are: 1. Compared with the existing technology, the present invention fuses an oversaturated spot image with an unsaturated spot image to obtain an ideal fused image, and then uses the calculation method of the second-order moment of the spot to obtain the beam width, and then calculates the laser beam divergence angle; the background noise of the CCD camera imaging is removed by imaging the imaging plane in the occluded state multiple times, and the salt and pepper noise is removed by median filtering, thereby improving the measurement accuracy.

[0030] 2. The present invention fuses the saturated spot image with the unsaturated spot image. The grayscale value of the fused image breaks through the 0-255 limit of the 8-bit camera, avoids the influence of the CCD camera exposure on the laser beam divergence measurement, and further improves the measurement accuracy.

[0031] 3. The present invention is also applicable to the measurement of laser beam divergence by fusing multiple images. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] Figure 2 This is a flow chart of the laser beam divergence measurement method integrated in the present invention; Figure 3 Schematic diagram of an unsaturated light spot of the present invention; Figure 4 Schematic diagram of an oversaturated light spot of the present invention; Figure 5 This is a schematic diagram of the fusion spot of the present invention; (x, y) This is the grayscale profile of the light spot image of the present invention. DETAILED DESCRIPTION

[0033] Example 1 A laser beam divergence measurement method based on image fusion, characterized by the following specific steps: Step s1: Use a CCD camera to perform imaging and obtain the camera's background noise mean μ; Step s2: Use the CCD camera to image the laser beam, adjust the laser energy attenuation plate attenuation magnification and the CCD camera integration time to obtain an unsaturated spot image I 1 (x, y) With an oversaturated spot image I 2 (x, y) ; Step s3: Convert the unsaturated spot image I 1 (x, y) and oversaturated spot images I 2 (x, y) Subtract the noise mean μ respectively, use median filtering to remove imaging noise, and obtain the imageI 11 (x, y) and I 21 (x, y) ; Step s4: obtaining image I 11 (x, y) and I 21 (x, y) fusion, obtaining image I 3 (x, y) ; Step s5: calculating the first moment I 3 (x, y) of the light spot center position ; Step s6: calculating the second moment of light field 3 I of the light spot center position (x, y) laser spot diameter ω x and ω y ; Step s7: based on the x direction and y direction light spot diameter ω x and ω y get the final light spot diameter D ; Step s8: calculating the laser beam divergence angle by the final light spot diameter D , get the laser beam divergence angle.

[0034] In the step s1, the imaging plane is blocked, and the CCD camera is used for imaging.

[0035] In the step s2, the pixel gray value of the unsaturated light spot image conforms to two-dimensional Gaussian distribution, and the maximum gray value is between 240-255.

[0036] In the step s2, the maximum gray value of the oversaturated light spot image pixel is 255.

[0037] In the step s3, 3*3 median filter is used to remove imaging noise.

[0038] In the step s4, I 11 (x, y) and I 21 (x, y) fusion to obtain I 3 (x, y) The formula is as follows:

[0039] Wherein, η is the fusion threshold, mis the grayscale width of pixel fusion, m Take 5.

[0040] When the image I 21 (x, y) When the pixel value is less than η, I 3 (x, y) The pixel value at the corresponding position is taken from I 21 (x, y) , η is taken as 230.

[0041] When the image I 21 (x, y) When the pixel value is greater than η, I 3 (x, y) The corresponding pixel value is no longer taken from I 21 (x, y) , by taking the unsaturated spot image I 11 (x, y) The pixel value at the corresponding position is multiplied by a proportional coefficient get.

[0042] The proportionality coefficient The pixel value of the unsaturated spot image is between η and η+ m Corresponding area within the range I 21 (x, y) and I 11 (x, y) The ratio of the total pixel values.

[0043] The first-order moment calculation in step s5 I 3 (x, y) The center position of the light spot The formula is as follows:

[0044]

[0045] x For each pixel I 3 σ The horizontal axis value of y For each pixel I 3 σ The vertical coordinate value of .

[0046] In step s6, the second-order moment of the light field is used to calculate the laser spot diameter ω. x With ω y The formula is as follows:

[0047]

[0048] Figure 1 x 2 （z） and (x, y) y 2 （z） is the second-order moment of the spot image.

[0049] In step s7, x Direction and y The geometric mean of the directional diameters is taken to obtain the final spot diameter.

[0050] In step s7, x Direction and y The formula for taking the geometric mean of the directional diameters is as follows:

[0051] in, D is the final spot diameter.

[0052] The laser beam divergence in step s8 The calculation formula is as follows:

[0053] Wherein, L is the distance between the imaging plane and the laser light source.

[0054] like (x, y) As shown, the present invention proposes a method for measuring laser beam divergence based on image fusion, comprising the following steps: Step 1: Block the imaging plane and use the CCD camera to perform multiple imaging to obtain the average value μ of the camera's background noise; Step 2: Use the CCD camera system to image the laser beam, adjust the attenuation rate of the laser energy attenuator and the CCD camera integration time, and obtain an unsaturated spot image. I 1 (x, y) With an oversaturated spot image I 2 （x， y） The standards for unsaturated and oversaturated spot images are as follows: 1. The pixel grayscale value of the unsaturated spot image conforms to the two-dimensional Gaussian distribution, and the maximum grayscale value is between 240 and 255; 2. The pixel grayscale value of the oversaturated spot image conforms to the "truncated" two-dimensional Gaussian distribution, and the maximum grayscale value is 255, appearing like a "flat-top mountain".

[0055] Step 3: I 1(x, y) and I 2 (x, y) The image is subtracted from the noise mean μ and a 3*3 median filter is performed to remove the imaging noise to obtain the image I 11 (x, y) and I 21 (x, y) ; Step 4: Use the following formula to transform the image I 11 (x, y) and I 21 (x, y) Fusion to obtain image I 3 (x, y) ; Among them, η is the fusion threshold, when the image I 21 （x， y） When the pixel value is less than η, I 3 (x, y) The pixel value at the corresponding position is directly taken from I 21 (x, y) , that is, retain I 21 (x, y) The unsaturated part of the image. η is about 230, which is a good effect. I 21 (x, y) When the pixel value is greater than η, I 3 (x, y) The corresponding pixel value is no longer taken from I 21 (x, y) , but by taking the unsaturated spot image I 11 (x, y) The corresponding position pixel value is multiplied by a proportional coefficient. The pixel value of the unsaturated spot image is between η and η+ m Corresponding area within the range I 21 (x, y) and I 11 (x, y) The ratio of the sum of the pixel values ​​of ; m Usually around 5 is used.

[0056] Step 5: Calculate by first-order moment I 3 (x, y) The center position of the light spot ;

[0057]

[0058] x For each pixel I 3 Figure 2 The horizontal axis value of y For each pixel I 3 Figure 3 The vertical coordinate value of .

[0059] Step 6: Calculate the laser spot diameter ω using the second-order moment of the light field x With ω y :

[0060] .

[0061] Spot diameter: There are many ways to define the spot diameter. Here we use the second-order moment 4σ definition method.

[0062] Step 7: x Direction and y The geometric mean of the directional diameters is taken to obtain the final spot diameter;

[0063] Step 8: Calculate the beam divergence angle :

[0064] Wherein, L is the distance between the imaging plane and the laser light source.

[0065] Example 2 Generate a simulated light spot with an intensity that conforms to the Gaussian distribution, such as Figure 4 As shown in , the maximum grayscale value of the center point is 255; Figure 5 As shown in , an oversaturated simulated light spot is also generated, and the grayscale value of its central area is 255; ​ As shown in Figure 2, the spot fusion algorithm is used to fuse them to obtain a fused spot with a maximum grayscale value of 395, breaking the 255 limit, and the shape conforms to the Gaussian distribution; the grayscale profiles of the three spot images and the fusion threshold eta are shown in Figure 2. ​ shown.

[0066] CCD camera model: industrial camera MV-CS050-10GM / NPOE.

Claims

1. A laser beam divergence measurement method based on image fusion, characterized by: The specific steps are as follows: Step s1: Use a CCD camera to perform imaging and obtain the camera's background noise mean μ; Step s2: Use the CCD camera to image the laser beam, adjust the laser energy attenuation plate attenuation magnification and the CCD camera integration time to obtain an unsaturated spot image I 1 (x, y) With an oversaturated spot image I 2 (x, y) ; Step s3: Convert the unsaturated spot image I 1 (x, y) and oversaturated spot images I 2 (x, y) Subtract the noise mean μ respectively, use median filtering to remove imaging noise, and obtain the image I 11 (x, y) and I 21 (x, y) ; Step s4: The image will be obtained I 11 (x, y) and I 21 (x, y) Fusion to obtain image I 3 (x, y) ; Step s5: Calculate by first-order moment I 3 (x, y) The center position of the light spot ; Step s6: passing the center position of the light spot Calculation using the second-order moment of the light field I 3 (x, y) Laser spot diameter ω x With ω y ; Step s7: Based on the spot diameter ω in the x-direction and y-direction x With ω y Take the geometric average to get the final spot diameter D ; Step s8: By the final spot diameter D The laser beam divergence angle is calculated to obtain the laser beam divergence angle.

2. The method for measuring laser beam divergence based on image fusion according to claim 1, characterized in that: In the step s1, the imaging plane is blocked and imaging is performed using a CCD camera.

3. The laser beam divergence measurement method based on image fusion according to claim 1 or 2, characterized in that: The grayscale values ​​of the pixels in the unsaturated light spot image in step s2 conform to a two-dimensional Gaussian distribution, and the maximum grayscale value is between 240 and 255.

4. The method for measuring laser beam divergence based on image fusion according to claim 3, characterized in that: The maximum grayscale value of the pixels of the oversaturated light spot image in step s2 is 255.

5. The method for measuring laser beam divergence based on image fusion according to claim 4, characterized in that: In step s3, 3*3 median filtering is used to remove imaging noise.

6. The method for measuring laser beam divergence based on image fusion according to claim 5, characterized in that: In step s4 I 11 (x, y) and I 21 (x, y) Fusion I 3 (x, y) The formula is as follows: Among them, η is the fusion threshold, m is the grayscale width of pixel fusion, m Take 5.

7. The method for measuring laser beam divergence based on image fusion according to claim 6, characterized in that: When the image I 21 (x, y) When the pixel value is less than η, I 3 (x, y) The pixel value at the corresponding position is taken from I 21 (x, y) , η is taken as 230.

8. The method for measuring laser beam divergence based on image fusion according to claim 7, characterized in that: When the image I 21 (x, y) When the pixel value is greater than η, I 3 (x, y) The corresponding pixel value is no longer taken from I 21 (x, y) , by taking the unsaturated spot image I 11 (x, y) The pixel value at the corresponding position is multiplied by a proportional coefficient get.

9. The method for measuring laser beam divergence based on image fusion according to claim 8, characterized in that: The proportionality coefficient The pixel value of the unsaturated spot image is between η and η+ m Corresponding area within the range I 21 (x, y) and I 11 (x, y) The ratio of the total pixel values.

10. The method for measuring laser beam divergence based on image fusion according to claim 6, characterized in that: The first-order moment calculation in step s5 I 3 (x, y) The center position of the light spot The formula is as follows: x For each pixel I 3 (x, y) The horizontal coordinate value of y For each pixel I 3 (x, y) The vertical coordinate value of .

11. The method for measuring laser beam divergence based on image fusion according to claim 10, characterized in that: In step s6, the second-order moment of the light field is used to calculate the laser spot diameter ω. x With ω y The formula is as follows: σ x 2 （z） and σ y 2 （z） is the second-order moment of the spot image.

12. The method for measuring laser beam divergence based on image fusion according to claim 10, characterized in that: In step s7, x Direction and y The formula for taking the geometric mean of the directional diameters is as follows: in, D is the final spot diameter.

13. The method for measuring laser beam divergence based on image fusion according to claim 12, characterized in that: The laser beam divergence in step s8 The calculation formula is as follows: Wherein, L is the distance between the imaging plane and the laser light source.

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