Underwater laser radar distance measuring and calculating method and distance resolution measuring and calculating method based on Sanxi imaging

Abstract

The invention relates to underwater optical imaging, in particular to an underwater laser radar distance measuring and calculating method and a distance resolution measuring and calculating method based on Sanxi imaging, and aims to solve the problems that the underwater Sanxi imaging mainly depends on later-stage calibration to improve the detection precision, and the detection precision is poor due to the fact that an existing underwater laser radar based on Sanxi imaging does not establish a related imaging model. In order to overcome the defects that in the prior art, in the prior art, the underwater laser radar distance measurement and calculation method based on Saxuov imaging is large in measurement range and large in underwater operation amount of later calibration, an underwater Saxuov imaging system is arranged, laser is used for vertically irradiating an underwater target, a camera is used for imaging diffuse reflection laser rays on the surface of the target, and then the object space distance of the target can be directly calculated. Therefore, the distance of the target relative to the horizontal plane where the main point of the object space of the camera is located is obtained, and the distance resolution is obtained through further calculation according to the object space distance.

Description

Technical Field

[0001] The present invention relates to underwater optical imaging, and in particular to an underwater laser radar distance calculation method and a distance resolution calculation method based on Schaffer imaging. Background Art

[0002] Underwater optical imaging technology has been widely used in many aspects such as seabed target monitoring, marine resource exploration, seabed topography measurement, underwater rescue, marine biology, etc. However, traditional underwater optical detection technology uses underwater dedicated imaging objective lenses, which have limited depth of field. In order to increase the depth of field of underwater optical detection, the Sha's imaging principle in the air is introduced. The Sha's imaging principle shows that if the object plane, image plane and lens plane intersect on a straight line, all points on the object plane can be clearly imaged on the image plane. The Sha's imaging lidar uses point laser or line laser as the illumination light source, and uses the Sha's imaging principle to achieve clear imaging of the position along the laser beam, and measures the distance of the target through the pixel position of the laser point or laser line reflected from the target surface on the camera image plane. The relationship between the image plane pixel position and the target distance of the Sha's lidar in the air has been derived in many literatures.

[0003] The principle of Sabat imaging in air is as follows Figure 1 As shown, the plane where the laser beam is emitted from the laser, the main object plane of the camera, and the extended plane of the camera image plane intersect on a line. The horizontal plane perpendicular to the object plane at the intersection of the camera 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, and the distances of targets A and B can be obtained according to the pixel positions of A' and B'. The dotted line AA' represents the optical axis of the lens, BB' represents the edge light, AA" is the reverse extension line of AA', BA" is the vertical line of AA', and B'C is the vertical line of 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, represented by y. A'B' is the image height of B' relative to A', represented by x. By using ΔOA"B and ΔOCB' as similar triangles, the depth of target B relative to target A is represented by y, and the Schaber imaging formula in the air can be obtained as follows:

[0004]

[0005] In formula (1), is the angle between the camera optical axis and the camera image plane, that is, the Schall angle, which satisfies the Schall imaging principle; θ is the angle between the camera optical axis and the laser beam.

[0006] The existing laser radar model based on Schachter imaging is for air medium, and its image and object medium are both air. However, due to the different mediums and optical refractive indexes of the underwater Schachter imaging laser radar in the object space and image space, the relationship between the image pixel position and the target distance of the Schachter laser radar in the air is no longer applicable. It is necessary to establish an underwater Schachter imaging model to represent the relationship between the image pixel position and the target distance during the underwater Schachter imaging process. In the prior art, the underwater laser radar based on Schachter imaging has not established a relevant imaging model, and the relationship between the pixel position and the target distance in the underwater Schachter imaging has not been studied and derived accordingly, resulting in underwater Schachter imaging mainly relying on post-calibration to improve detection accuracy, and the amount of underwater work for post-calibration is huge. Summary of the invention

[0007] The purpose of the present invention is to solve the problem that the existing underwater laser radar based on Schabütz imaging has not established a relevant imaging model, resulting in the underwater Schabütz imaging mainly relying on post-calibration to improve the detection accuracy, and the amount of underwater operations for post-calibration is huge, and to provide an underwater laser radar distance measurement method and a distance resolution measurement method based on Schabütz imaging.

[0008] The concept of the present invention:

[0009] The distance to the target in the air is calculated as follows: Figure 1 The pinhole imaging principle shown can be directly deduced, but the underwater target is distorted due to the refraction of the echo light through three different media: water, window, and air. The distance calculation formula in the air cannot accurately calculate the target distance. When the laser beam irradiates the underwater target at different distances vertically downward, a laser line is generated on the target surface through diffuse reflection. The present invention sets a camera at the same height as the laser, and controls the camera optical axis to have a certain angle with the laser output laser. The camera is used to image the laser line generated by the diffuse reflection of the target, and then the target distance is calculated based on the pixel position of the laser line on the image plane.

[0010] To achieve the above objectives, the technical solutions provided by the present invention are as follows:

[0011] A method for underwater laser radar distance measurement based on Shashi imaging is special in that it includes the following steps:

[0012] 1) An underwater Shabbat imaging system is set up, wherein the underwater Shabbat imaging system includes a laser and a camera, and the laser emitted by the laser is controlled to illuminate the underwater target perpendicularly to the horizontal plane. It is defined that the plane where the laser fan is located is the object plane, and the position of the target surface on the object plane diffusely reflects the laser to generate 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 Among them, 0°<θ<90°, Adjust the camera's object principal plane, the extended plane of the camera's image plane, and the object plane to intersect on a line;

[0013] 2) Using a camera to image the diffusely reflected laser line on the target surface;

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

[0015]

[0016] Wherein, y is the object-side distance of the target, specifically the distance between the target and the reference plane on the object plane, the reference plane being a horizontal plane perpendicular to the object plane at the intersection of the camera optical axis and the object plane; x is the image-side height, specifically the height of the target imaging position relative to the image-side reference image height, the image-side reference image height being the intersection of the imaging lens 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; f' is the focal length of the imaging lens;

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

[0018] N=H+y

[0019] Where H is the distance between the horizontal plane where the camera's object principal point is located and the reference plane.

[0020] Further, in step 1), the adjusting of the object principal plane of the camera, the extended plane of the camera image plane and the object plane to intersect on a line is specifically as follows: adjusting the camera angle so that the angle θ between the camera optical axis and the object plane is equal to the angle θ between the camera optical axis and the camera image plane. Satisfy the following formula:

[0021]

[0022] At the same time, the present invention also provides a method for calculating the distance resolution of an underwater laser radar based on Shashi imaging, which is special in that it includes the following steps:

[0023] S1. Using the underwater laser radar distance measurement method based on Schaffner imaging, the object distance of the target in each pixel is calculated;

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

[0025] Δy=y′Δx;

[0026] Among them, y′ is the derivative of the object distance y of the target, and Δx is the width of a pixel.

[0027] Beneficial effects of the present invention:

[0028] 1. The present invention takes into account the different media of the object side and the image side, and first calculates the refraction angle of the outgoing light using the law of refraction, thereby making the Schaber laser radar model suitable for the calculation of underwater Schaber imaging distance and distance resolution.

[0029] 2. The present invention establishes an underwater Sha'abella lidar imaging model, obtains the relationship between pixel position and target distance in underwater Sha'abella imaging, and then obtains the distance resolution of underwater Sha'abella imaging lidar at different depths, providing theoretical support for the optical design of underwater Sha'abella lidar. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Figure 1 It is a schematic diagram of the principle of Sha Shi imaging in air;

[0031] Figure 2 Schematic diagram of the principle of underwater Schaffner imaging in an embodiment of the present invention;

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

[0033] Figure 4 is a diagram showing the relationship between distance resolution and target distance in an embodiment of the present invention. DETAILED DESCRIPTION

[0034] The principle of underwater Sha's imaging is as follows Figure 2 As shown in the figure, the underwater Schaber imaging is simplified to the ideal pinhole imaging. It can be seen that the media of the object and image sides are 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 and image sides, the marginal light is refracted after passing through the pinhole, and the refracted light deviates from the optical axis. Since the marginal light is offset in the image space relative to the object space, ΔOA”B and ΔOCB' are no longer similar. Therefore, the Schaber imaging formula in the air will not be applicable to underwater Schaber imaging.

[0035] In underwater Schaffhausen imaging, the edge light will be deflected according to Snell's law. The incident angle ∠BOA and the refraction angle ∠B'OC of the edge light 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 Shabbat imaging system is set up on the water surface. The underwater Shabbat imaging system includes a laser and a camera. The laser is controlled to emit laser light, which is perpendicular to the horizontal plane and irradiates the underwater target downward. The plane where the fan of the laser is located is defined as the object plane. The horizontal plane perpendicular to the object plane at the intersection of the camera optical axis and the object plane is defined as the reference plane. The position of the target surface on the object plane diffusely reflects the laser light to generate laser lines. The horizontal plane where the target is located is defined as the measurement plane. Figure 2 As shown, B is the position of the target, and B' corresponds to the imaging position.

[0039] When the measurement plane is below the reference plane, Figure 2 In the right triangle △OA”B and the right triangle △OB'C, sin i and sin i′ can be expressed as follows:

[0040]

[0041] Where y is the object distance of the target, specifically the distance between the target and the reference plane on the object plane; x is the image height, specifically the distance between the imaging position of the target on the camera image plane and the imaging position of the reference plane on the camera image plane; l is the object distance of the target, which is Figure 2 OA” in the figure is the angle between the camera optical axis and the object plane, which is Figure 2 ∠LAO shown, f′ is the focal length of the imaging lens, is the angle between the camera optical axis and the camera image plane, that is, Figure 2 The ∠B'A'O shown is also called the Schall angle, which satisfies the Schall imaging principle shown in the following formula:

[0042]

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

[0044]

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

[0046]

[0047] Since θ is less than 90° in the present invention, Take a positive value, so the positive and negative signs in the above equation are positive, organize and deform the above equation, and express the image side edge ray length OB' as l', and the expression of y is as follows:

[0048]

[0049] The above equation (7) is an expression of the relationship between the object-side distance of the target and its image-side height x in underwater Sha's imaging. When n=n'=1 in equation (7), the underwater sand imaging formula (7) will be transformed into the air Sha's imaging formula (1), so the air Sha's imaging can be regarded as a special case of underwater Sha's imaging. Because when n=n'=1, the following relationship exists:

[0050]

[0051] The distance 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] Definition: When the laser line reflected by the object plane, image plane, and target rotates clockwise to the optical axis at an acute angle, θ, When the value of i is positive, the laser line reflected by the object plane, image plane, and target rotates counterclockwise to the optical axis at an acute angle, θ, and i is negative. When the target is located above the horizontal plane where the intersection of the optical axis and the object plane is located, the values ​​of y and x are positive. When the target is located below the horizontal plane where the intersection of the optical axis and the object plane is located, the values ​​of 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. The y, The value of is negative, and 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 measuring plane is on the reference plane. In this case, The value of x is negative, and the values ​​of θ and y are positive.

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

[0055] Step 1. Set up the underwater Sha's imaging system, control the laser to emit a laser perpendicular to the horizontal plane and illuminate the underwater target downward, and 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 relations are satisfied:

[0056] 0°<θ<90°,

[0057]

[0058] Make the camera's object principal plane, the extended plane of the camera's image plane and the object plane intersect on a line.

[0059] Step 2. Using a camera to image the diffusely reflected laser line on the target surface;

[0060] Step 3. When the focal length f′ of the imaging lens, the object distance l of the target, the angle θ and the angle When all are known, the object distance y of each target is calculated according to the pixel position x corresponding to the laser line in the captured laser line image by formula (7);

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

[0062] N=H+y.

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

[0065] Δy=y′Δx,

[0066] Wherein, y′ is the derivative of the object-space distance y of the target, Δx is the width of a pixel, and Δx=p.

Claims

1. A method for underwater laser radar distance measurement based on Shashi imaging, characterized in that: The following steps are involved: 1) An underwater Shabbat imaging system is set up, wherein the underwater Shabbat imaging system includes a laser and a camera, and the laser emitted by the laser is controlled to illuminate the underwater target perpendicularly to the horizontal plane. It is defined that the plane where the laser fan is located is the object plane, and the position of the target surface on the object plane diffusely reflects the laser to generate 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 in, 90°, Adjust the camera's object principal plane, the extended plane of the camera's image plane, and the object plane to intersect on a line; 2) Using a camera to image the diffusely reflected laser line on the target surface; 3) Calculate the object distance of the target according to the image using the following formula: Wherein, y is the object-side distance of the target, specifically the distance between the target and the reference plane on the object plane, the reference plane being the horizontal plane perpendicular to the object plane at the intersection of the camera optical axis and the object plane; x is the image-side height, specifically the height of the target imaging position relative to the image-side reference image height, the image-side reference image height being the intersection of the imaging lens optical axis and the image plane; n is the refractive index of water; n' is the refractive index of air; l ′ is the length of the edge ray of the image side; l is the object distance of the target; f ′ is the focal length of the imaging lens; 4) The distance N of the target relative to the horizontal plane where the camera's object space principal point is located is calculated using the object space distance of the target according to the following formula: N=H+y Where H is the distance between the horizontal plane where the camera's object principal point is located and the reference plane.

2. The underwater laser radar distance measurement method based on Schaber imaging according to claim 1 is characterized in that: In step 1), the adjustment of the object principal plane of the camera, the extended plane of the camera image plane and the object plane to intersect on a line is specifically as follows: adjusting the camera angle so that the angle θ between the camera optical axis and the object plane is equal to the angle θ between the camera optical axis and the camera image plane. Satisfy the following formula:

3. A method for calculating the distance resolution of underwater laser radar based on Shashi imaging, characterized in that: The following steps are involved: S1. Using the underwater laser radar distance measurement method based on Schaffner imaging as described in claim 1 or 2, the object distance of the target in each pixel is calculated; S2. Using the pixel size of the camera image plane of the underwater Schaffner imaging system, the distance resolution Δy at different distances is calculated according to the following formula: Δy=y ′ Δx; Among them, y ′ is the derivative of the object-space distance y of the target, and Δx is the width of one pixel.

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