多波段鱼眼镜头相机实验室几何定标方法

Laboratory Geometric Calibration Method for Multi-band Fisheye Lens Camera

鱼眼镜头因其独特的曲率和光学特性,使用非相似成像模型拟合后,仍有接近10像素的径向残余畸变。研究了多波段鱼眼相机的定标原理、定标流程,在非相似成像模型基础上,提出了基于分离式精密二维转台的鱼眼镜头定标方法,并通过相机姿态位置精密调整与方位转台360°旋转,光斑像素位置始终不变的方法,确定相机主点。为分析色差对鱼眼镜头径向畸变的影响,采取不同波段单独定标的方法。利用不同时刻的太阳角度数据来验证镜头定标系数,结果显示天顶角与方位角的均方根误差分别为0.226°和0.487°。该方法有效实现了鱼眼镜头相机在不同波段的几何定标,提高了像素点与入射光线角度之间的匹配精度。

The significant distortion introduced by fisheye lenses,while expanding the field of view,poses a new scientific problem:the projection process cannot be described using traditional pinhole photography models.To obtain the mapping relationship between pixel points and the angle of incident light,it is necessary to reconstruct the imaging model based on the unique nonlinear distortion characteristics of fisheye cameras.However,due to their unique curvature and optical characteristics,even the common universal models cannot completely eliminate radial residual distortion,with errors reaching nearly 10 pixels.Also,due to the chromatic aberration characteristics of fisheye lenses,there are differences in the refraction of light in different bands.This article studies the calibration principle and calibration process of multi-band fisheye cameras,proposing a fisheye lens calibration method based on a separated precision two-dimensional turntable.The rotary indexing table and the vertical turntable have good repeated positioning accuracy,which are 2 s and 10.8 s respectively,and the orthogonal error of the two rotation axes is less than 10 s.Both drive the camera and collimator to rotate,so that the light spot covers the entire field of view of the lens.To simplify the coordinate conversion process,it is necessary to adjust the collimator to align its optical axis perpendicular to the rotation axis of the rotary indexing table.According to the theory that an ideal lens can focus parallel light from an infinite distance on the main point,fine-tune the camera's position and posture until the camera rotates around the Z-axis from 0°to 360°,and the position of the spot on the image remains unchanged.At this point,the centroid coordinates of the spot are the pixel coordinates of the main point.On this basis,a fifth-degree polynomial is used to fit the residuals and describe the camera projection process together with an equisolid angle projection.The five bands from visible light to near-infrared are calibrated separately to improve the geometric calibration accuracy.Based on experimental results,the influence of lateral chromatic aberration on radial distance in geometric distortion was analyzed and discussed.It was found that the maximum difference in radial distance between different bands at the same incident angle is 8 pixels.Combined with the edge resolution of the lens,which is approximately 0.11°,this difference will result in an angle error of approximately 0.88°.Therefore,when the fisheye lens is applied to different bands,it is necessary to independently calibrate each band to improve the accuracy of geometric calibration.In addition,to analyze the reliability of the data,this article calculated the uncertainty of five main influencing factors during the calibration process.T he r esults a re a s f ollows:the measurement error of the two-dimensional precision turntable is 13 s,the accuracy of spot centroid extraction is 0.034 pixels,the main point positioning error is 1.5 pixels,the focal length fitting error is 0.14μ,and the distortion compensation coefficient fitting error is 1×10^(-9).To verify the accuracy of the calibration results,in Hefei,Anhui,China,with latitude and longitude of 117.1661°E and 31.9039°N,a flat and open field,the sun was imaged on the afternoon of November 2,2023,and the morning of November 3,2023,covering the time from 9 am to 16 pm.The camera parameters were adjusted to keep the sun image within the dynamic range.With the help of a theodolite,considering a 4°magnetic declination angle in the Hefei area,align the camera aperture scale with the north direction,and adjust the camera tripod according to the state of the spirit level to keep the camera in a horizontal state.Calculate the zenith angle and azimuth angle of the sun at different times using astronomical algorithms,and use geometric calibration results to calculate corresponding positions.Compare the difference between the two to verify the accuracy of the model.In the collected data,the observed range of solar zenith angle is 47.22°to 77.22°,with root mean square errors of zenith angle and azimuth angle being 0.226°and 0.487°,respectively.Based on the above analysis and experimental verification,the proposed camera calibration method can establish an accurate mapping relationship between the incident angle of light and pixel coordinates,simplifying the calibration process.In addition,this article discusses the influence of lateral chromatic aberration on radial distance in geometric distortion and proposes that different bands should be calibrated separately to achieve higher geometric calibration accuracy.