Design of optical system for high accuracy imaging keratometry

Zhang Xueying; Wang Jinsong; Huang Guolin; Xu Pengfei

doi:10.12086/oee.2019.180392

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Zhang Xueying, Wang Jinsong, Huang Guolin, et al. Design of optical system for high accuracy imaging keratometry[J]. Opto-Electronic Engineering, 2019, 46(1): 180392. doi: 10.12086/oee.2019.180392

Citation:

Zhang Xueying, Wang Jinsong, Huang Guolin, et al. Design of optical system for high accuracy imaging keratometry[J]. Opto-Electronic Engineering, 2019, 46(1): 180392. doi: 10.12086/oee.2019.180392

Design of optical system for high accuracy imaging keratometry

College of Optical and Electrical Engineering, Changchun University of Science and Technology, Changchun, Jilin 130022, China

Fund Project: Supported by Jilin Province Key Technology R & D Project Fund (20180201025GX)

More Information

Corresponding author: Wang Jinsong, E-mail:soldier_1973@163.com

Received Date 23 July 2018

Revised Date 13 September 2018

Published Date 01 January 2019

Abstract

Abstract

In order to reduce the alignment deviation of the imaging keratometer along the optical axis and improve the measurement accuracy of corneal diopter, a high precision imaging keratometer optical system was designed. The optical system includes imaging system and low coherence interferometry system. The imaging system consists of imaging objective, cornea, and measurement target ring, wherein the imaging objective lens adopts a double telocentric optical path design. The low-coherence interferometry system uses the grating scale to measure the displacement of the scanning mirror, and then locates the vertices of the cornea and the measuring target ring by low-coherence interference signals, achieving accurate measurement between the apex of the cornea and the distance of the measuring target ring. The imaging objective has a modulation transfer function greater than 0.4 at a maximum spatial frequency of 70 lp/mm, the distortion is less than 0.05%. The simulation results show that the system has a compact structure, good imaging quality and simple operation. It meets the demand for high precision measurement of corneal refractive power by an imaging keratometer.
- high precision /
- imaging keratometer /
- double telecentric light path /
- low coherence interference

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References

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Overview

Overview

Overview: In order to reduce the alignment deviation of the imaging keratometer along the optical axis and improve the measurement accuracy of corneal diopter, a high precision imaging keratometer optical system was designed. The optical system includes imaging system and low coherence interferometry system. The imaging system consists of imaging objective, cornea, and measurement target ring. Imaging objective lens consists of double telocentric light path, which has large depth of field, low distortion and constant magnification in a certain object distance range. The magnification of the image stays the same with any object distance changes within a certain object distance range. The change of image distance does not affect the size of the image as well. It also has advantages of being insensitive to the object distance and the image distance change contribute to the image obtained by different human eyes and the magnification stability when the human eye moves slightly because of the relatively large depth of field. It is convenient and quick to align in the direction of the optical axis during measurement. The low-coherence interferometry system uses the grating scale to measure the displacement of the scanning mirror, and then locates the vertices of the cornea and the measuring target ring by low-coherence interference signals, achieving accurate measurement between the apex of the cornea and the distance of the measuring target ring. The use of low coherence interferometry solves the problem of using the double telocentric lens in the imaging keratometer which is hardly determining the distance from the apex of the cornea to the measuring target ring accurately, and improves the measurement accuracy of this distance. The design completed system has the following parameters, the total length of the imaging objective system is 85 mm, the object height is 14 mm, the image height is 6.2 mm, the magnification is -0.4, the depth of field is 8 mm, the spectral range is 900 nm~980 nm, and the maximum spatial frequency is 70 lp/mm. The modulation transfer function at mm is greater than 0.4, and the distortion is less than 0.05%. The parameters meet the design requirements of corneal index. The low-coherence interferometry system uses a broadband light source with a center wavelength of 850 nm, a 2×2 fiber coupler with a split ratio of 99:1, and a linear delay line with a scan speed of 120 mm/s to achieve precise positioning of the cornea. The system has compact structure, good imaging quality and simple operation, and meets the high-precision measurement requirements of imaging keratometer for corneal diopter. In summary, the system has practical significance for realizing high-precision corneal curvature measurement.