Error model analysis and correlation correction of Hartmann's focimeter

Yang Jinlong; Jia Hongzhi; Jin Tao; Xiang Huazhong; Zhang Dawei; Zhuang Songlin

doi:10.12086/oee.2021.200238

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Yang J L, Jia H Z, Jin T, et al. Error model analysis and correlation correction of Hartmann's focimeter[J]. Opto-Electron Eng, 2021, 48(3): 200238. doi: 10.12086/oee.2021.200238

Citation:

Yang J L, Jia H Z, Jin T, et al. Error model analysis and correlation correction of Hartmann's focimeter[J]. Opto-Electron Eng, 2021, 48(3): 200238. doi: 10.12086/oee.2021.200238

Error model analysis and correlation correction of Hartmann's focimeter

School of Optical-Electrical and Computer Engineering, University of Shanghai for Science and Technology, Shanghai 200093, China

More Information

Corresponding author: Jin Tao, E-mail: jintao@usst.edu.cn

Received Date 28 June 2020

Revised Date 18 September 2020

Published Date 15 March 2021

Abstract

Abstract

In order to reduce the error and improve the measurement accuracy, a more detailed error model is established for the Hartmann method of focal power measurement in this paper. It focuses on the analysis of several problems that cause the error of refraction problems, including the dispersion error of the light source, the inaccurate of the photodetector's central positing, the tilt of lens, misalignment between incidence axis and main axis of lens, and the incident light and the lens are not perpendicular. At last, it is concluded that the inaccuracy of the center extraction on the photodetector will cause a large error to the final result. For all these reasons, a method of dual bilinear interpolation combined with a fitting method to find the centroid is proposed, proving its effectiveness and accuracy.
- Hartmann diaphragm /
- focimeter /
- error model /
- image processing /
- centroid calculation

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References

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Overview

Overview

Overview: The focus meter mainly adopts the Hartmann diaphragm measurement method. The parallel light passes through the lens and the Hartmann diaphragm plate on the photodetector image. The spot image formed without the lens has a corresponding change in position, according to the position of the spot. The change in distance calculates the optical parameters of the lens' top power. This method can not only effectively reduce the error caused by human but also accurately measure the optical parameters of the lens. However, there is no complete error model for autofocus focal meters. Therefore, the relatively perfect error model analysis of the Hartmann measurement method is beneficial to the error correction of the subsequent focal meter measurement. The inaccuracy of the actual diopter measurement is caused by the dispersion error of the light source, the inaccurate of the photodetector's central positing, the tilt of lens, and the misalignment between the incidence axis and the main axis of the lens. Through the analysis of the error model, the inaccurate positioning of the CMOS photodetector centroid will lead to larger errors. Therefore, a dual bilinear method is proposed. Combining the interpolation method with the fitting method, one can find the center of mass, and it's the method is effectiveness and accuracy. The conclusion is obtained through analysis. The light source is the green LED light source. Ignoring the distance between the lens and the CMOS photodetector, the Abbe number correction can be eliminated and the complexity of the algorithm can be reduced, resulting in a small error in the final measurement result. For the errors caused by the lens tilt and the misalignment between the incidence axis and the main axis of the lens. The larger misalignment above the large diopter error, and it affects the accuracy of the experimental results. In the experiment, it is necessary to keep the lens and the incident light is perpendicular to each other. In practice, the error model described in this article can be used for correction. Moreover, if the location of the spot center of mass on the CMOS photodetector is inaccurate, a larger error will occur. Therefore, combining the fitting method in this article with the double bilinear interpolation can improve the accuracy of the spot center of mass and ensure the high positioning of the center of mass, ensuring the accuracy of the diopter measurement results.