A method for compensating unevenness in laser range-gated three-dimensional imaging

Abstract

The present invention provides a method for compensating non-uniformity of laser distance-gated three-dimensional imaging. The method obtains a detector pixel distance error matrix through a three-dimensional depth image at different distances and a geometric correspondence between a detection field of view and image pixels. The distance error matrix is ​​used to compensate an original distance matrix, and the distance matrix obtained after compensation is converted into a three-dimensional depth image, thereby realizing correction of the original three-dimensional depth image. The non-uniformity of distance-gated three-dimensional imaging caused by device technology can be effectively reduced, and the measurement accuracy of laser distance-gated three-dimensional imaging can be improved. The method has good engineering application value.

Description

Technical Field

[0001] The invention relates to the technical field of laser range-gated imaging, and in particular to a laser range-gated three-dimensional imaging non-uniformity compensation method. Background Art

[0002] Laser range-gated 3D imaging is a non-scanning active 3D imaging technology. It uses the principle of range gating to effectively shield invalid backscattered signals between the detector and the target. It has the advantages of high signal-to-noise ratio, long detection distance, and high resolution. It is widely used in long-distance mapping, underwater 3D imaging, target identification in complex media, and other fields. Laser range-gated 3D imaging generally uses nanosecond pulsed lasers and nanosecond shutter detectors to achieve ultrafast imaging at a fixed distance slice in a single shot. Through the accumulation and inversion of multiple range slice images, a 3D image of the entire scene can be further obtained. As the core imaging device of the above imaging system, the ICCD (Enhanced Charge Coupled Device, Intensified CCD) detector has a nanosecond opening time and high gain characteristics. Its switching time characteristics directly affect the accuracy of range-gated 3D imaging. In the same frame image, if different pixels cannot be exposed at the same time, then the measured distance value must have a large error.

[0003] The switching time characteristics of the ICCD detector are determined by the photocathode target surface of the image intensifier at the front end of the detector. Common image intensifier manufacturers at home and abroad include Photek in the UK, Teem Photonics in France, and Northern Night Vision Company in my country. Due to the differences in the process levels of each company, the image intensifiers produced are different, especially in terms of shutter time characteristics. Foreign devices have better shutter time characteristics, but they are expensive, and there are export restrictions on devices above the third generation. Domestic device prices are relatively low, but key indicators such as resolution and switching time characteristics are significantly different from those of foreign countries.

[0004] Therefore, how to eliminate the unevenness of laser range-gated three-dimensional imaging caused by the defects of ICCD detectors is a difficulty in the current laser range-gated three-dimensional imaging technology. Summary of the invention

[0005] In order to overcome the shortcomings and deficiencies of the prior art, the present invention provides a method for compensating for unevenness of laser distance-gated three-dimensional imaging. Through the geometric correspondence between three-dimensional depth images at different distances and the detection field of view and image pixels, a detector pixel distance error matrix is ​​obtained, and the original three-dimensional depth image is corrected. This method can effectively reduce the unevenness of distance-gated three-dimensional imaging caused by device technology, improve the measurement accuracy of laser distance-gated three-dimensional imaging, and has good engineering application value.

[0006] The present invention provides a method for compensating for unevenness of laser distance-gated three-dimensional imaging, comprising the following steps:

[0007] Step 1: Place the target object in front of the ICCD detector, adjust the ICCD detector parameters so that the target object covers the entire field of view of the ICCD detector, and use the range-gated 3D imaging algorithm to calculate the 3D depth image I (x, y) of the target object;

[0008] Step 2: Repeat step 1 to obtain N grayscale values ​​of the target 3D depth images at the same distance and field of view. 1 (x,y)……I N (x, y), and calculate the average gray value of the target object's three-dimensional depth image

[0009]

[0010] Step 3: Calculate the original distance matrix from the target to the ICCD detector based on the correspondence between the 3D imaging depth of field and the image grayscale value. Among them, Z 1 , Z 2 is the distance from the front and rear boundaries of the 3D imaging depth of field to the ICCD detector;

[0011] Step 4: Calculate the theoretical distance matrix from the target to the ICCD detector based on the laser range gating imaging principle Where d is the horizontal projection distance from any point in the field of view to the ICCD detector, x i ,y i is the coordinate of any point in the field of view, w is the pixel width of the circular field of view, θ r is the detector field of view;

[0012] Step 5: According to the original distance matrix D(x, y) and the theoretical distance matrix D p (x, y) is calculated to obtain the distance error matrix ΔD(x, y) = D(x, y) - D p (x,y);

[0013] Step 6: Adjust the distance between the target and the ICCD detector, repeat steps 1 to 5, and obtain a series of distance error matrices ΔD 1 (x,y)……ΔD N (x,y), calculate the average value to get the final distance error matrix

[0014]

[0015] Step 7: Use the final distance error matrix to compensate the original distance matrix to obtain a compensated distance matrix D r(x, y) = D(x, y) - ΔD'(x, y) and converted into a three-dimensional depth image of the target object

[0016]

[0017] Furthermore, the range-gated three-dimensional imaging algorithm includes a step-by-step time-delay scanning method, a centroid method, a triangle range-energy correlation algorithm, and a trapezoidal range-energy correlation algorithm.

[0018] After adopting the above technical solution, the present invention has at least the following beneficial effects:

[0019] By compensating and correcting the three-dimensional depth image, the three-dimensional imaging non-uniformity caused by the inherent characteristics of the detector can be greatly reduced, the three-dimensional measurement accuracy and distance resolution can be improved, and to a certain extent, the technical barriers of the core imaging devices of the laser range-gated imaging system can be eliminated. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative labor.

[0021] Figure 1 A flow chart of a laser distance-gated three-dimensional imaging non-uniformity compensation method provided in this embodiment;

[0022] Figure 2 The schematic diagram of the theoretical distance matrix calculation from the target to the ICCD detector;

[0023] Figure 3 This is the principle diagram of triangle distance energy correlation algorithm imaging;

[0024] Figure 4 This is a three-dimensional depth image of a pure gray flat target plate located 2m in front of the ICCD detector;

[0025] Figure 5 The figure shows the distance error comparison before (left) and after (right) compensation of a pure gray plane target plate located 5m in front of the ICCD detector. DETAILED DESCRIPTION

[0026] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0027] refer to Figure 1-3 This embodiment takes the triangle distance energy correlation algorithm as an example to illustrate its imaging principle. Figure 1 As shown in the figure, taking a pure gray plane target plate as the target object, two consecutive frames of distance slice images are needed to invert the three-dimensional depth image. The back depth of the previous frame slice image overlaps with the foreground depth of the next frame slice image. According to the grayscale relationship between the two frames of slice images, the distance information of the target within the minimum delay unit is obtained to achieve super-resolution three-dimensional imaging. According to the mapping relationship between distance and energy in the triangle distance energy correlation algorithm, the specific formula is as follows:

[0028]

[0029] Among them, Z r is the actual distance to the target, τ 1 and Z 1 is the gating delay of the previous slice image and the corresponding imaging distance, τ laser is the laser pulse width, τ gate is the gate width, c is the propagation speed of light in the medium, n is the refractive index, I 1 and I 2 It is the gray value of the target in two consecutive slice images.

[0030] Then proceed as follows:

[0031] Step 1: Place a pure gray plane target plate 2 meters in front of the ICCD detector, adjust the ICCD detector parameters so that the target plate covers the entire field of view of the ICCD detector, and use the above triangle distance energy correlation algorithm to obtain the three-dimensional depth image I (x, y) of the target plate;

[0032] Step 2: To eliminate the influence of pixel distortion caused by noise signals on the measurement, repeat step 1 to obtain the grayscale value I of 10 target plate 3D depth images at 2 m. 1 (x,y)……I 10 (x, y), and calculate the average gray value I'(x, y) of the target plate's three-dimensional depth image, such as Figure 4 As shown;

[0033] Step 3: Calculate the original distance matrix D(x,y) from the target plate to the ICCD detector according to the correspondence between the 3D imaging depth of field and the image grayscale value;

[0034] Among them, the image grayscale value range is 0 to 255, and the three-dimensional imaging depth of field is Z1 to Z2, and the two change linearly in a one-to-one correspondence.

[0035] Step 4: Calculate the theoretical distance matrix D from the target to the ICCD detector based on the laser range gating imaging principle p (x,y);

[0036] Step 5: According to D(x,y) and D in steps 3 and 4 p (x, y) calculates the distance error matrix ΔD(x, y);

[0037] Step 6: To improve the accuracy of the calculation results, change the position of the target plate and place it at 4m, 6m, ..., 10m away from the ICCD detector. Repeat steps 1 to 5 to obtain a series of distance error matrices. Calculate the average value to obtain the final distance error matrix ΔD'(x, y);

[0038] In order to verify the compensation effect, the target plate is placed 5m away from the ICCD detector to verify the distance error matrix, as shown in Figure 5 As shown, Figure 5 The distance errors before and after compensation are shown. The distance error before compensation reaches 0.5m, and the errors after compensation are less than 0.05m, indicating that the compensation method used significantly reduces the unevenness of laser distance-gated three-dimensional imaging.

[0039] Step 7: Use the final distance error matrix ΔD'(x, y) to compensate the original distance matrix D(x, y) to obtain the compensated distance matrix D r (x,y)=D(x,y)-ΔD'(x,y), and converted into a three-dimensional depth image of the target plate

[0040] Through the above-mentioned laser range-gated three-dimensional imaging unevenness compensation method, the three-dimensional depth image obtained by laser range-gated three-dimensional imaging of the target to be measured is corrected, which greatly reduces the three-dimensional imaging unevenness caused by the inherent characteristics of the device and improves the three-dimensional measurement accuracy and distance resolution.

[0041] Although the present invention has been disclosed as above by way of embodiments, it is not intended to limit the present invention. Any person skilled in the art may make some changes and modifications without departing from the spirit and scope of the present invention. Therefore, the protection scope of the present invention shall be determined by the claims.

Claims

1. A method for compensating for unevenness in laser range-gated three-dimensional imaging. It is characterized in that include: Step 1: Place the target object in front of the ICCD detector, adjust the ICCD detector parameters so that the target object covers the entire field of view of the ICCD detector, and use the range-gated 3D imaging algorithm to calculate the 3D depth image I (x, y) of the target object; Step 2: Repeat step 1 to obtain N grayscale values ​​of the target 3D depth images at the same distance and field of view. 1 (x,y)……I N (x, y), and calculate the average gray value of the target object's three-dimensional depth image Step 3: Calculate the original distance matrix from the target to the ICCD detector based on the correspondence between the 3D imaging depth of field and the image grayscale value. Among them, Z 1 , Z 2 is the distance from the front and rear boundaries of the 3D imaging depth of field to the ICCD detector; Step 4: Calculate the theoretical distance matrix from the target to the ICCD detector based on the laser range gating imaging principle Where d is the horizontal projection distance from any point in the field of view to the ICCD detector, x i ,y i is the coordinate of any point in the field of view, w is the pixel width of the circular field of view, θ r is the detector field of view; Step 5: According to the original distance matrix D(x, y) and the theoretical distance matrix D p (x, y) is calculated to obtain the distance error matrix ΔD(x, y) = D(x, y) - D p (x,y); Step 6: Adjust the distance between the target and the ICCD detector, repeat steps 1 to 5, and obtain a series of distance error matrices ΔD 1 (x,y)……ΔD N (x,y), calculate the average value to get the final distance error matrix Step 7: Use the final distance error matrix to compensate the original distance matrix to obtain a compensated distance matrix D r (x, y) = D(x, y) - ΔD'(x, y) and converted into a three-dimensional depth image of the target object 2. The laser distance gating three-dimensional imaging non-uniformity compensation method according to claim 1, It is characterized in that The distance-gated three-dimensional imaging algorithm includes a step-by-step time-delay scanning method, a centroid method, a triangle distance-energy correlation algorithm, and a trapezoidal distance-energy correlation algorithm.

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