Acta Photonica Sinica, Volume. 53, Issue 8, 0811001(2024)

Laboratory Geometric Calibration Method for Multi-band Fisheye Lens Camera

Caixia WANG1...2, Hongyao CHEN1,*, Xiaolong SI1, Xin LI1, Jiawei LI1, Liming ZHANG1 and Shiwei BAO1 |Show fewer author(s)
Author Affiliations
  • 1Anhui Institute of Optics and Fine Mechanics, Hefei Institutes of Physical Science, Chinese Academy of Sciences, Hefei 230031, China
  • 2University of Science of and Technology of China, Hefei 230026, China
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    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. The results are as follows: 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 0.14 μ, 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.

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    Caixia WANG, Hongyao CHEN, Xiaolong SI, Xin LI, Jiawei LI, Liming ZHANG, Shiwei BAO. Laboratory Geometric Calibration Method for Multi-band Fisheye Lens Camera[J]. Acta Photonica Sinica, 2024, 53(8): 0811001

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    Paper Information

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    Received: Dec. 25, 2023

    Accepted: Feb. 26, 2024

    Published Online: Oct. 15, 2024

    The Author Email: CHEN Hongyao (hychen@aiofim.ac.cn)

    DOI:10.3788/gzxb20245308.0811001

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