CN111862233B - Lens parameter calibration method based on equivalent coaxial spherical optical system - Google Patents
Lens parameter calibration method based on equivalent coaxial spherical optical system Download PDFInfo
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- G06T7/80—Analysis of captured images to determine intrinsic or extrinsic camera parameters, i.e. camera calibration
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Abstract
The invention discloses a lens parameter calibration method based on an equivalent coaxial spherical optical system, wherein the components of the equivalent coaxial spherical optical system comprise an object side focus, an image side focus and an image side main surface; and based on the object side and image side focuses, the object side and image side main surfaces, calibrating object side focal length, image side focal length, entrance pupil and main surface interval parameters of the lens. Forming light spots with different radiuses by using point light sources at different positions during calibration, and obtaining a functional relation between the light source position and the light spot radius through a ray tracing method of an equivalent optical system; parameters concerning magnification and entrance pupil can be obtained by imaging of the parallel light source and the calibration plate; and then combining the Newton formula and the experimentally measured distance parameter, and obtaining model parameters such as the focal length, the main surface position and the like of the lens by solving the equation set. After the model parameters are determined, the position of the conjugate point and the imaging magnification can be calculated by using a Gaussian formula or a Newton formula, and the light is tracked.
Description
Technical Field
The invention belongs to the field of light field imaging application, and particularly relates to a calibration method of a typical optical lens.
Background
The focal length of the lens is a very important internal parameter of the lens, which determines the size and accuracy of the image formed by the subject on the imaging plane. At present, a small-hole imaging model is mostly adopted in the lens imaging simulation, and the focal length in the model is the distance between the optical center of the lens and the origin of an image coordinate system, namely the distance. The image coordinates and the world coordinates have a linear relation, so that camera calibration parameters can be obtained by solving a linear equation set. In practical cases, the lens cannot satisfy the relationship of objects and images like a triangle due to the complexity of the lens structure and the errors of processing and assembly.
The Zhang Zhengyou calibration method adopts a small-hole imaging model, and obtains parameters in the camera by observing a plane to be calibrated. The calibration method obtains constraint conditions of internal parameters according to homography relation between three-dimensional points and two-dimensional projection points of the three-dimensional points on an image plane. For a given homography matrix, there are two basic constraints on its internal parameters. Since the matrix contains 8 degrees of freedom and 6 external parameters, only two internal parameter constraints can be obtained. In order to solve the camera calibration parameters, a closed solution is required to be obtained first, then a nonlinear optimal solution is obtained through maximum likelihood estimation, and finally an analytic solution and a nonlinear solution are obtained by considering the radial distortion of the lens.
In summary, the existing lens calibration model has a large gap from the actual lens, and the calibration method is complex in calculation, so that a more accurate calibration model suitable for a typical optical imaging lens needs to be established, and a set of efficient and accurate optical lens focal length calibration method is formed.
Disclosure of Invention
The invention aims to solve the technical problems of the prior art, and provides a simple and effective calibration method based on an equivalent coaxial spherical optical system, which can finish the determination of lens model parameters without complex experimental instruments and complicated calculation processes.
In order to solve the technical problems, the technical scheme of the invention is as follows:
A lens parameter calibration method based on an equivalent coaxial spherical optical system comprises the following specific steps:
Adjusting the position of the point light source on the optical axis to enable the image of the point light source in the camera to be a point and recording the position at the moment as an ideal imaging position;
the distance L of the ideal imaging position to the camera image detector is measured.
Placing the calibration plate at a recorded ideal imaging position, shooting an image in the camera at the moment, and calculating the side length of a square lattice in the image of the calibration plate according to the pixel size of a CCD (charge coupled device) of the camera; calculating the magnification beta of the equivalent lens model at the moment compared with the square edge length of the real object of the calibration plate;
moving the point light source along the optical axis by a certain distance delta 1, shooting an image of a light spot formed by the camera at the moment, and calculating a light spot radius r 1 according to the pixel size of a CCD (charge coupled device) of the camera;
Moving the point light source along the optical axis by another distance delta 2, shooting an image of a light spot formed by the camera at the moment, and calculating a light spot radius r 2 according to the pixel size of the CCD of the camera;
imaging the camera by using a parallel light source, shooting an image of a light spot formed by the camera at the moment, and calculating a light spot radius y' according to the pixel size of a CCD (charge coupled device) of the camera;
solving the following equation set to obtain lens parameters R, f', d:
d＝L-l-l′
l＝x+f
l′＝x′+f′
Wherein R represents the range of light rays which can enter the lens equivalent optical system corresponding to a certain fixed lens aperture size on the main surface of the object; f represents the focal length of the object; f' represents an image Fang Jiaoju; d represents the distance from the object principal surface to the image principal surface; y represents the distance from the point light source to the optical axis; x represents the distance from the ideal imaging position to the focus of the object; x' represents the distance of the object focus from the camera image detector plane; l' denotes the distance of the image side main surface from the camera image detector.
Compared with the prior art, the invention has the beneficial effects that:
1. the method adopts the coaxial spherical equivalent model which is closer to the real situation of the camera lens, and particularly the model has main surface interval, thereby being beneficial to improving the accuracy of ray tracing.
2. The invention designs the calibration method with higher feasibility, the calibration experiment can be completed by using the point light source, the parallel light source and the calibration plate, and the invention has less measurement data and simple experimental operation.
3. The lens model used in the method can utilize the object-image conjugate relation to carry out ray tracing after the parameters are determined, and the calculation is simple.
Drawings
Fig. 1 is a schematic diagram of a lens equivalent optical system.
Fig. 2 is a schematic diagram of calibration experimental parameters based on a lens equivalent optical system.
Fig. 3 is a schematic diagram of the ray tracing principle of the equivalent optical model.
Detailed Description
In order to better understand the above technical solution, the following further details of the technical solution of the present invention will be described with reference to the accompanying drawings and specific embodiments of the specification:
It will be understood by those skilled in the art that, unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. It will be further understood that terms, such as those defined in commonly used dictionaries, should be interpreted as having a meaning that is consistent with their meaning in the context of the prior art and will not be interpreted in an idealized or overly formal sense unless expressly so defined herein.
The camera parameter calibration method adopts a lens model based on an equivalent optical system, and referring to fig. 1, the main components of the camera parameter calibration method mainly comprise a main focus (F, F '), a main point and a node (H, H') of the system, which are collectively called as a base point of the system. When the coaxial system consisting of a plurality of spherical surfaces is taken as a whole, and imaging of each surface is not studied one by one, the imaging characteristic of the system can be represented by the base point of the system. Regardless of the specific arrangement of the coaxial spherical system, the position of the conjugate point and the imaging magnification can be calculated by using a Gaussian formula or a Newton formula as long as the system base point is determined through calibration, so that the light ray is tracked.
When a point light source is placed at a point on the optical axis which is two times away from the focal length of the camera lens, and the position of the point light source on the optical axis is adjusted, a position exists so that the image of the point light source in the camera is also a point, the point light source is positioned at an ideal imaging position, namely a point B, and the position of the point light source at the moment is recorded. The point light source is moved by delta to the point a with respect to the ideal imaging position, and R is the range of light rays on the object principal surface that can enter the lens equivalent optical system corresponding to a certain fixed lens aperture size. The emergent light is obtained according to the incident light drawing, the light reaching the point M from the point A and the point B passes through the equivalent optical system, and then is emitted from the point M ' at the same height and reaches the point A ' and the point B ' on the plane of the image detector respectively, at the moment, the image on the image plane is changed into a light spot from one point, and the radius of the light spot is the distance r between the points A ' and B ' in the drawing. Geometric analysis is carried out on the lens equivalent optical model to obtain the following geometric relationship between the moving distance delta of the point light source and the radius r of the light spot on the image plane:
Light parallel to the optical axis can form a light spot with a certain size on an image detector (CCD) after passing through a camera lens, and the radius y' of the light spot can be measured by shooting an image of the light spot. The range of parallel rays which can pass through the lens is represented by R, and the ratio of the two rays can represent the magnification of the equivalent model of the lens:
From the newton formula:
In the experiment, the distance L between the ideal imaging position of the object and the camera image detector can be obtained by measuring, so that the distance d between the object principal plane H and the image principal plane H' in the lens equivalent optical system can be obtained:
And placing a calibration plate with known side length of the square pattern at an ideal imaging position point B, shooting and measuring the side length of the square pattern in the image to obtain the magnification beta.
The distance l from the ideal imaging position of the point light source to the main surface at different positions can be obtained by measuring the spot radius r i of the point light source at different positions delta i, and the parameters f, f' and d of the equivalent optical system of the lens are determined, so that the parameter calibration of the lens is completed.
The light rays in all directions emitted by a certain point light source A are converged at one point A', and any light ray emitted by the point light source can be traced by combining the characteristic of the vertical axis magnification beta= +1 of the main plane. The ideal imaging position a 'can be determined by newton's formula:
The foregoing describes embodiments of the present invention in detail with reference to the accompanying drawings. Furthermore, it should be noted that the above-mentioned embodiments are merely for illustrating the technical solution of the present invention, and not for limiting the same, and although the present invention has been described in detail with reference to examples, it should be understood by those skilled in the art that modifications and equivalents may be made to the technical solution of the present invention without departing from the spirit and scope of the technical solution of the present invention, and all such modifications and equivalents are intended to be encompassed in the scope of the claims of the present invention.
Claims (1)
1. A lens parameter calibration method based on an equivalent coaxial spherical optical system is characterized in that: the equivalent coaxial spherical optical system comprises an object side focal point, an image side focal point, an object side main surface and an image side main surface; based on the focus of the object space and the image space, the main surface of the object space and the main surface of the image space, the position of the conjugate point and the imaging magnification are calculated by using a Gaussian formula or a Newton formula, the light is traced, the calibration of the object space focal length, the image space focal length, the entrance pupil and the main surface interval parameters of the lens is completed, and the specific steps for completing the calibration comprise:
Adjusting the position of the point light source on the optical axis to enable the image of the point light source in the camera to be a point and recording the position at the moment as an ideal imaging position;
placing the calibration plate at a recorded ideal imaging position, shooting an image in the camera at the moment, and calculating the side length of a square lattice in the image of the calibration plate according to the pixel size of a CCD (charge coupled device) of the camera; calculating the magnification beta of the equivalent lens model at the moment compared with the square edge length of the real object of the calibration plate;
moving the point light source along the optical axis by a certain distance delta 1, shooting an image of a light spot formed by the camera at the moment, and calculating a light spot radius r 1 according to the pixel size of a CCD (charge coupled device) of the camera;
Moving the point light source along the optical axis by another distance delta 2, shooting an image of a light spot formed by the camera at the moment, and calculating a light spot radius r 2 according to the pixel size of the CCD of the camera;
imaging the camera by using a parallel light source, shooting an image of a light spot formed by the camera at the moment, and calculating a light spot radius y' according to the pixel size of a CCD (charge coupled device) of the camera;
Solving the following equation set to obtain lens parameters R, f', d:
d＝L-l-l′
l＝x+f
l′＝x′+f′
Wherein R represents the range of light rays which can enter the lens equivalent optical system corresponding to a certain fixed lens aperture size on the main surface of the object; f represents the focal length of the object; f' represents an image Fang Jiaoju; d represents the distance from the object principal surface to the image principal surface; y represents the distance from the point light source to the optical axis; x represents the distance from the ideal imaging position to the focus of the object; x' represents the distance of the object focus from the camera image detector plane; l' represents the distance from the image side main surface to the camera image detector; l denotes the distance of the ideal imaging position to the camera image detector.
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Citations (2)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| CN106017312A (en) * | 2016-03-22 | 2016-10-12 | 武汉武大卓越科技有限责任公司 | Structured light triangulation automatic calibrating system and calibrating method |
| CN110060303A (en) * | 2019-03-18 | 2019-07-26 | 英特科利(江苏)医用内窥影像技术有限公司 | A kind of two step scaling methods of light-field camera |
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| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| CN106017312A (en) * | 2016-03-22 | 2016-10-12 | 武汉武大卓越科技有限责任公司 | Structured light triangulation automatic calibrating system and calibrating method |
| CN110060303A (en) * | 2019-03-18 | 2019-07-26 | 英特科利(江苏)医用内窥影像技术有限公司 | A kind of two step scaling methods of light-field camera |
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