Underwater laser radar distance measurement method and distance resolution measurement method based on schlieren imaging

Abstract

The present application relates to underwater optical imaging, in particular to a method for calculating the distance of underwater laser radar based on Shash imaging and a method for calculating the distance resolution, in order to solve the problem that the existing underwater laser radar based on Shash imaging does not establish a related imaging model, resulting in that the underwater Shash imaging mainly relies on post calibration to improve the detection accuracy, and the underwater operation amount of post calibration is huge, the method for calculating the distance of underwater laser radar based on Shash imaging is provided with an underwater Shash imaging system, the laser is vertically irradiated to the target under water, the camera images the diffuse reflection laser line on the surface of the target, then the object distance of the target can be directly calculated, so as to obtain the distance of the object main point of the target relative to the horizontal plane of the camera, and the distance resolution is further calculated by using the above object distance.

Description

Technical Field

[0001] This invention relates to underwater optical imaging, specifically to a method for calculating the distance and resolution of an underwater lidar based on Saxony imaging. Background Technology

[0002] Underwater optical imaging technology has been widely applied in many fields such as seabed target monitoring, marine resource exploration, seabed topography surveying, underwater rescue, and marine biology. However, traditional underwater optical detection technologies use specialized underwater imaging lenses, which limit the depth of field. To increase the depth of field in underwater optical detection, the Safran imaging principle in air has been introduced. The Safran imaging principle states that if the object plane, image plane, and lens plane intersect on a straight line, then all points on the object plane can be clearly imaged on the image plane. Safran imaging lidar uses point lasers or line lasers as illumination sources, and achieves clear imaging of the laser beam along its line using the Safran imaging principle. The distance to the target is measured by the pixel position of the laser point or line reflected from the target surface on the camera's image plane. The relationship between the pixel position on the image plane of Safran lidar in air and the target distance has been derived in several publications.

[0003] The principle of Saxophone imaging in air is as follows: Figure 1 As shown, the plane where the laser beam emitted by the laser, the object plane of the camera, and the extension plane of the camera's image plane intersect on a single line. The horizontal plane perpendicular to the object plane at the intersection of the camera's optical axis and the object plane is defined as the reference plane. Targets A and B at different depths are imaged on A' and B' on the image plane, respectively. The distances to targets A and B can then be obtained based on the pixel positions of A' and B'. Here, the dashed line AA' represents the lens optical axis, BB' represents the edge ray, AA" is the backward extension of AA', BA" is the perpendicular line to AA', and B'C is the perpendicular line to AA'. AO is the object distance l of target A along the optical axis, and OA' represents the focal length f' of the imaging lens. AB is the detection depth of the object relative to the reference plane, denoted by y. A'B' is the image height of B' relative to A', denoted by x. By using ΔOA"B and ΔOCB' as similar triangles, and denoting the depth of target B relative to target A by y, the Saxony imaging formula in air can be obtained as follows:

[0004]

[0005] In formula (1), θ is the angle between the camera's optical axis and the camera's image plane, also known as the Saxophone angle, which satisfies the Saxophone imaging principle; θ is the angle between the camera's optical axis and the laser beam.

[0006] Existing lidar models based on Safran imaging are designed for air media, where both the image and object planes are air. However, underwater Safran imaging lidar operates on different media (object and image planes) with varying optical refractive indices, rendering the relationship between pixel position and target distance in air-based Safran lidar models inapplicable. Therefore, an underwater Safran imaging model is needed to represent the relationship between pixel position and target distance during underwater Safran imaging. Current underwater lidar technologies based on Safran imaging lack relevant imaging models, and the relationship between pixel position and target distance in underwater Safran imaging has not been researched or derived. This leads to underwater Safran imaging relying heavily on post-calibration to improve detection accuracy, which involves a significant amount of underwater work. Summary of the Invention

[0007] The purpose of this invention is to address the shortcomings of existing underwater lidar based on Saxony imaging, which lacks a relevant imaging model, leading to reliance on post-calibration to improve detection accuracy and resulting in a large amount of underwater calibration work. This invention provides a method for calculating the distance and resolution of underwater lidar based on Saxony imaging.

[0008] The concept of this invention:

[0009] Target distance calculation in the air is based on the following Figure 1 The pinhole imaging principle shown can be directly deduced. However, underwater targets suffer from distortion due to the refraction of echo light through three different media—water, window, and air—making it impossible to obtain an accurate target distance using distance calculation formulas for air. When a laser beam shines vertically downwards onto underwater targets at different distances, it generates laser lines through diffuse reflection on the target surface. This invention places a camera at the same height as the laser and controls the camera's optical axis to have a certain angle with the laser beam emitted from the laser. The camera images the laser lines generated by the diffuse reflection of the target, and then calculates the target distance based on the pixel positions of the laser lines on the image plane.

[0010] To achieve the above objectives, the technical solution provided by this invention is as follows:

[0011] A method for calculating the range of an underwater lidar based on Saxony imaging, characterized by the following steps:

[0012] 1) An underwater Shapiro imaging system is set up, comprising a laser and a camera. The laser emitted by the laser is controlled to illuminate the underwater target perpendicularly to the horizontal plane. The plane containing the laser fan is defined as the object plane. The target surface diffusely reflects the laser beam at a position on the object plane, generating a laser line. The angle between the lens optical axis and the object plane is θ, and the angle between the camera optical axis and the camera image plane is θ. Where 0° < θ < 90°, Adjust the camera's object plane, the extension plane of the camera's image plane, and the object plane so that they intersect on a single line;

[0013] 2) Image the diffuse reflection laser lines on the target surface using a camera;

[0014] 3) Calculate the object distance of the target based on the image using the following formula:

[0015]

[0016] Where y is the object-side distance of the target, specifically the distance between the target on the object plane and the reference plane, where the reference plane is the horizontal plane perpendicular to the object plane at the intersection of the camera's optical axis and the object plane; x is the image-side height, specifically the height of the target's imaging position relative to the image-side reference image height, where the image-side reference image height is the intersection of the imaging lens's optical axis and the image plane; n is the refractive index of water; n' is the refractive index of air; l' is the image-side edge ray length; l is the object distance of the target; and f' is the focal length of the imaging lens.

[0017] 4) Using the target's object-space distance, calculate the distance N of the target relative to the horizontal plane where the camera's object-space principal point is located using the following formula:

[0018] N = H + y

[0019] Where H is the distance between the horizontal plane containing the object principal point of the camera and the reference plane.

[0020] Further, in step 1), adjusting the intersection of the camera's object principal plane, the extension plane of the camera's image plane, and the object plane onto a single line specifically involves adjusting the camera angle so that the angle θ between the camera's optical axis and the object plane is equal to the angle between the camera's optical axis and the camera's image plane. Satisfy the following formula:

[0021]

[0022] Meanwhile, this invention also provides a method for calculating the range resolution of underwater lidar based on Saxony imaging, which is characterized by including the following steps:

[0023] S1. Using the above-mentioned underwater lidar distance calculation method based on Saxony imaging, the object-space distance of the target in each pixel is calculated;

[0024] S2. Using the pixel size of the camera image plane of the underwater Shadwell imaging system, calculate the distance resolution Δy at different distances according to the following formula:

[0025] Δy = y′Δx;

[0026] Where y′ is the derivative of the object distance y of the target, and Δx is the width of one pixel.

[0027] The beneficial effects of this invention are:

[0028] 1. This invention takes into account the different media of the object side and the image side, and uses the law of refraction to first calculate the refraction angle of the outgoing light, so that the Shapiro lidar model is suitable for calculating the underwater Shapiro imaging distance and range resolution.

[0029] 2. This invention establishes an underwater Shapiro lidar imaging model, obtains the relationship between pixel position and target distance in underwater Shapiro imaging, and then obtains the distance resolution of underwater Shapiro imaging lidar at different depths, providing theoretical support for the optical design of underwater Shapiro lidar. Attached Figure Description

[0030] Figure 1 This is a schematic diagram illustrating the principle of Saxophone imaging in air;

[0031] Figure 2 This is a schematic diagram illustrating the principle of underwater Saxony imaging in an embodiment of the present invention;

[0032] Figure 3 This is a graph showing the relationship between pixels and target distance in an embodiment of the present invention;

[0033] Figure 4 This is a graph showing the relationship between distance resolution and target distance in an embodiment of the present invention. Detailed Implementation

[0034] The principle of underwater Saxony imaging is as follows: Figure 2 As shown, underwater Saxony imaging can be simplified to ideal pinhole imaging. The object-side and image-side media can be considered as water and air, respectively, and the optical axis of the camera passes through the pinhole in the pinhole imaging. Due to the different refractive indices of the media in the object-side and image-side, the edge rays are refracted after passing through the pinhole. The refracted rays deviate from the optical axis. Since the edge rays are offset in the image-side space relative to the object-side space, ΔOA”B and ΔOCB' are no longer similar. Therefore, the Saxony imaging formula in air will not be applicable to underwater Saxony imaging.

[0035] In underwater Schaperone imaging, the edge rays will be deflected according to Snell's law. The incident angle ∠BOA and the refraction angle ∠B'OC of the edge rays satisfy the law of refraction and have the following relationship.

[0036] n sin i=n′ sini′ (2)

[0037] Where n is the refractive index of water, i is the angle of ∠BOA, n' is the refractive index of air, and i' is the angle of ∠B'OC;

[0038] An underwater Shapiro imaging system is set up on the water surface. This system includes a laser and a camera. The laser emits light perpendicularly downwards to illuminate the underwater target. The plane containing the laser beam is defined as the object plane. The point where the camera's optical axis intersects the object plane, and the horizontal plane perpendicular to the object plane, is defined as the reference plane. The target surface diffusely reflects the laser beam, generating a laser line. The horizontal plane containing the target is defined as the measurement plane. Figure 2 As shown, B is the position of the target, and B' is the corresponding imaging position.

[0039] When the measuring plane is below the reference plane, according to Figure 2 In right triangles △OA”B and △OB'C, sin i and sin i′ can be represented as follows:

[0040]

[0041] Where y is the object-side distance of the target, specifically the distance between the target and the reference plane on the object plane; x is the image-side height, specifically the distance between the image position of the target on the camera's image plane and the image position of the reference plane on the camera's image plane; l is the object distance of the target, i.e. Figure 2 In this context, OA and θ represent the angle between the camera's optical axis and the object plane, i.e., as shown in the figure. Figure 2 As shown, ∠LAO, f′ is the focal length of the imaging lens. The angle between the camera's optical axis and the camera's image plane is, as shown below. Figure 2 The ∠B'A'O shown is also called the Saxophone angle. The Saxophone angle satisfies the Saxophone imaging principle shown in the following formula:

[0042]

[0043] Substituting equations (3) and (4) into equation (2) respectively, we obtain the following expression, where OB′ is the length of the image-side edge ray:

[0044]

[0045] By solving the above quadratic equation (5), we obtain The expression is as follows:

[0046]

[0047] Since θ is less than 90° in this invention, therefore Taking a positive value, the sign in the above equation is positive. Organizing and transforming the equation, and representing the image-side edge ray length OB′ as l′, we obtain the expression for y as shown below:

[0048]

[0049] Equation (7) above is an expression for the relationship between the object-side distance of a target and its image-side height x in underwater Saxon imaging. When n = n' = 1 in equation (7), the underwater sand imaging formula (7) will transform into the air-based Saxon imaging formula (1). Therefore, air-based Saxon imaging can be considered a special case of underwater Saxon imaging. This is because when n = n' = 1, the following relationship exists:

[0050]

[0051] The range resolution Δy of the underwater lidar can be expressed as the distance value in the object space corresponding to each pixel on the camera, which is obtained by taking the derivative of equation (7) as shown below.

[0052] Δy=y′Δx (9)

[0053] Defined as follows: when the laser ray reflected from the object plane, image plane, and target rotates clockwise around an acute angle to the optical axis, θ, And when i is positive, and the laser lines reflected from the object plane, image plane, and target rotate counterclockwise to the optical axis at an acute angle, θ, And i is negative. When the target is above the horizontal plane where the intersection of the optical axis and the object plane is located, y and x are positive. When the target is below the horizontal plane where the intersection of the optical axis and the object plane is located, y and x are negative. The object distance l of the target and the focal length f′ of the imaging lens are always negative and positive, respectively. In this embodiment, the measurement plane is below the reference plane. In formula (7), y, The values ​​are negative, while the values ​​of x and θ are positive. In other embodiments of the present invention, the above method is also applicable to the case where the measurement plane is on the reference plane. x takes negative values, while θ and y take positive values.

[0054] Using the above principles and formulas, the underwater lidar range resolution measurement based on Saxony imaging includes the following steps:

[0055] Step 1. Set up the underwater Szadbury imaging system, controlling the laser emitted by the laser to illuminate the underwater target perpendicularly downwards from the horizontal plane. The angle θ between the lens optical axis and the object plane, and the angle between the camera optical axis and the camera image plane are... The following relationship must be satisfied:

[0056] 0° < θ < 90°

[0057]

[0058] This causes the object principal plane of the camera, the extension plane of the camera image plane, and the object plane to intersect at a single line.

[0059] Step 2. Image the diffuse reflection laser lines on the target surface using a camera;

[0060] Step 3. The focal length f′ of the imaging lens, the object distance l of the target, the included angle θ, and the included angle... Given that all the information is known, the object distance y of each target is calculated using formula (7) based on the pixel position x corresponding to the laser line in the captured laser line image.

[0061] Step 4. Based on the distance H between the horizontal plane where the camera's object principal point is located and the reference plane, calculate the distance N of each target relative to the horizontal plane where the camera's principal point is located using the following formula:

[0062] N = H + y.

[0063] Where H is the distance between the horizontal plane where the camera's principal point is located and the reference plane, and θ is an inherent parameter of the underwater Szadwell imaging system. In this embodiment, f′=22.87mm, l=-6424.7mm, and θ=48.57° are used. The underwater Szadrift imaging system has a reference distance H = -4251.26 mm. The camera image plane of the underwater Szadrift imaging system uses a sensor with a pixel size p = 11 μm and a resolution of 2048 × 2048. Based on steps 4 and 5, the distance of the target relative to the horizontal plane of the object-side principal point of the camera for each pixel is calculated. The relationship between the pixel position and the distance of the target relative to the horizontal plane of the object-side principal point of the camera is as follows: Figure 3 As shown.

[0064] Step 5. Using the pixel size of the sensor chip, calculate the distance resolution Δy at different distances according to the following formula, and the result is as follows: Figure 4 As shown;

[0065] Δy=y′Δx,

[0066] Where y′ is the derivative of the object distance y of the target, Δx is the width of one pixel, and Δx=p.

Claims

1. A method for underwater laser radar distance measurement based on Schlieren imaging, characterized in that, The method comprises the following steps: 1) set underwater schlieren imaging system, the underwater schlieren imaging system includes laser, camera, control laser emitted by laser irradiates underwater target perpendicular to horizontal plane, define, the plane where laser fan is located is object plane, the position of target surface on object plane carries out diffuse reflection to laser and generates laser line;The angle between lens optical axis and object plane is θ, the angle between camera optical axis and camera image plane is Wherein, 90°, adjust the extension of object side main plane of camera, camera image plane and object plane to intersect in a line; 2) imaging the diffuse reflection laser line of the target surface by using the camera; 3) calculating the object distance of the target according to the imaging by the following formula: Wherein y is the object distance of the target, specifically the distance between the target and the reference plane on the object surface, the reference plane is a horizontal plane at the intersection of the camera optical axis and the object surface, perpendicular to the object surface; x is the image height, specifically the height of the target imaging position relative to the reference image height, the reference image height is the intersection of the imaging lens optical axis and the image surface; n is the refractive index of water; n' is the refractive index of air; l ′ is the image edge light length; l is the object distance of the target; f ′ is the focal length of the imaging lens; 4) calculating the distance N of the target relative to the horizontal plane where the object principal point of the camera is located according to the object distance of the target by the following formula: N = H + y wherein H is the distance between the horizontal plane where the object principal point of the camera is located and the reference plane.

2. The underwater laser radar distance measurement method based on the Schlieren imaging according to claim 1, characterized in that: In the step 1), the adjustment of the intersection of the object-side principal plane of the camera, the extension plane of the camera image plane and the object plane is specifically: adjusting the camera angle, so that the included angle θ between the camera optical axis and the object plane and the included angle between the camera optical axis and the camera image plane satisfies the following formula:

3. A method for calculating the range resolution of underwater laser radar based on the Shack-Hartmann imaging, characterized in that, The method comprises the following steps: S1. calculating the object distance of the target in each pixel point by using the underwater laser radar distance measurement method based on the Schlieren imaging according to claim 1 or 2; S2. calculating the distance resolution Δy at different distances according to the following formula by using the camera image plane pixel size of the underwater Schlieren imaging system: Δy = y ′ Δx; where y ′ is the derivative of the object distance y, and Δx is the width of one pixel.

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