Citation: | Chen Peng, Wang Cheng, Zheng Gang, et al. Optimization design and realization of a keratometer[J]. Opto-Electronic Engineering, 2019, 46(1): 180373. doi: 10.12086/oee.2019.180373 |
[1] | Gutmark R, Guyton D L. Origins of the keratometer and its evolving role in ophthalmology[J]. Survey of Ophthalmology, 2010, 55(5): 481-497. doi: 10.1016/j.survophthal.2010.03.001 |
[2] | 王英丽.角膜曲率计浅析(上)[J].中国眼镜科技杂志, 2016(21): 136-137. doi: 10.3969/j.issn.1004-6615.2016.21.055 Wang Y L. Analysis of corneal curvature meter(A)[J]. China Glasses Science-Technology Magazine, 2016(21): 136-137. doi: 10.3969/j.issn.1004-6615.2016.21.055 |
[3] | 王英丽.角膜曲率计浅析(下)[J].中国眼镜科技杂志, 2017(5): 172-174. doi: 10.3969/j.issn.1004-6615.2017.05.066 Wang Y L. Analysis of corneal curvature meter(B)[J]. China Glasses Science-Technology Magazine, 2017(5): 172-174. doi: 10.3969/j.issn.1004-6615.2017.05.066 |
[4] | 陶瑛, 范冬娟, 康玉霜.角膜曲率计检定中常见问题及解决方法探讨[J].中国眼镜科技杂志, 2013(7): 124-126. doi: 10.3969/j.issn.1004-6615.2013.07.050 Tao Y, Fan D J, Kang Y S. Discussion on problems and solutions in the verification of keratometer[J]. China Glasses Science-Technology Magazine, 2013(7): 124-126. doi: 10.3969/j.issn.1004-6615.2013.07.050 |
[5] | 蔡建奇, 刘毅.角膜曲率计测量原理和检测方法探讨[J].中国计量, 2010(1): 89-90. doi: 10.3969/j.issn.1006-9364.2010.01.045 Cai J Q, Liu Y. Discussion on principle and detection method of keraometer[J]. China Metrology, 2010(1): 89-90. doi: 10.3969/j.issn.1006-9364.2010.01.045 |
[6] | 徐唐, 秦爱玲, 李一壮, 等.角膜屈光力新公式与近视眼准分子激光角膜原位磨镶术后的角膜屈光力[J].南京大学学报(自然科学), 2011, 47(1): 91-96. Xu T, Qin A L, Li Y Z, et al. A new formula of corneal refractive power and the corneal refractive powers of myopia eyes after laser in situ keratomileusis[J]. Journal of Nanjing University (Natural Sciences), 2011, 47(1): 91-96. |
[7] | 李炳震, 梁晨, 冬雪川, 等.四种不同方法测量角膜曲率比较研究[J].中国实用眼科杂志, 2014, 32(4): 450-455. doi: 10.3760/cma.j.issn.1006-4443.2014.04.013 Li B Z, Liang C, Dong X C, et al. Comparison and evaluation of four different techniques of keratometric measurements[J]. Chinese Journal of Practical Ophthalmology, 2014, 32(4): 450-455. doi: 10.3760/cma.j.issn.1006-4443.2014.04.013 |
[8] | Miller J M. A handheld open-field infant keratometer (an american ophthalmological society thesis)[J]. Transactions of the American Ophthalmological Society, 2010, 108: 77-95. |
[9] | He Y Q, Wang Y, Wang Z Q, et al. Design of imaging keratometer with annular object and charge-coupled device detector[J]. Applied Optics, 2013, 52(35): 8532-8539. doi: 10.1364/AO.52.008532 |
[10] | 闫洁, 孟鹏花, 赵俊奇.人眼角膜曲率测量系统的研究[J].应用基础与工程科学学报, 2011, 19(S1): 254-261. Yan J, Meng P H, Zhao J Q. Research of curvature measuring system of eyes cornea[J]. Journal of Basic Science and Engineering, 2011, 19(S1): 254-261. |
[11] | 赵俊奇, 段培华, 郭智勇, 等.人眼角膜曲率参数亚像素测量系统的设计[J].中北大学学报(自然科学版), 2011, 32(3): 362-366. doi: 10.3969/j.issn.1673-3193.2011.03.022 Zhao J Q, Duan P H, Guo Z Y, et al. Design of subpixel algorithm of dioptric paramater measurement system for eye cornea[J]. Journal of North University of China (Natural Science Edition), 2011, 32(3): 362-366. doi: 10.3969/j.issn.1673-3193.2011.03.022 |
[12] | 赵俊奇, 郭智勇, 陈安世, 等.一种基于图像处理的人眼全自动角膜曲率计研究[J].中国生物医学工程学报, 2011, 30(1): 100-104. doi: 10.3969/j.issn.0258-8021.2011.01.017 Zhao J Q, Guo Z Y, Chen A S, et al. Auto-ophthalmometer of eye based on image processing[J]. Chinese Journal of Biomedical Engineering, 2011, 30(1): 100-104. doi: 10.3969/j.issn.0258-8021.2011.01.017 |
[13] | 何远清, 刘永基, 翟奕.成像角膜曲率计的光学设计[J].中国光学, 2014, 7(6): 956-961. He Y Q, Liu Y J, Zhai Y. Optical design of imaging keratometer[J]. Chinese Optics, 2014, 7(6): 956-961. |
[14] | 郑少林, 刘永基, 王肇圻, 等.新型成像角膜曲率仪的光学系统设计[J].光学学报, 2013, 33(5): 0522004. Zheng S L, Liu Y J, Wang Z Q, et al. Design of optical system for a novel imaging keratometer[J]. Acta Optica Sinica, 2013, 33(5): 0522004. |
Overview: The paper summarizes previous research results on corneal curvature measurement. Based on previous studies, a simplified corneal curvature measurement system is proposed. The system uses the corneal reflection principle to measure the curvature radius of the corneal anterior surface. The specific measurement method employs six pointolites which arranged in a regular hexagon to emit parallel light to the surface of a cornea and are imaged by the cornea. Then the image is captured by a telecentric optical system in the object space to a CMOS camera. In order to obtain the corneal curvature, the distance between two pointolites located on the regular hexagonal diagonal in the corneal reflection images are calculated by using the center of gravity algorithm. We introduce the measurement range, measurement accuracy and other design indexes of the system, as well as the related parameters of the optical imaging system. We also performed image quality analysis of the imaging system. In the aspect of error analysis, we also made some theoretical analysis. We believe that the measurement error of the system is mainly determined by the accuracy of image algorithm to the image processing of corneal reflection, and we have given the theoretical calculation results in this aspect. We built a corneal curvature measurement system to further verify our theoretical analysis. We firstly calibrate the parameters of the system and then do repeated validation to get the repeated error. In addition, we measured the glass spheres with different curvature radius to verify that the system could accurately measure the curvature radius within the measurement range. Finally, we measured the corneal curvature radius of human eyes. The results showed that the system can quickly and accurately measure the corneal curvature radius of human eyes. We select the LED with a wavelength of 850 nm as the illumination source. The main consideration is that the near-infrared optical system can penetrate well. Although the human eye is not very sensitive to light at 850 nm, it has a certain effect on stabilizing the eye position of the test subject. In addition, the 850 nm LED process is mature, easy to obtain, and inexpensive. The whole optical system is simple, the device is few, the difficulty of assembly and adjustment is greatly reduced, and the imaging system adopts the object-distance optical path, which avoids the problem that the center of gravity of the image changes and the measurement result deviates.
Principle of the measurement system. (a) Principle of the corneal curvature measurement; (b) Arrangement of the measuring cursor
Principle of the imaging system
The schematic illustration of the optical system. Where, ① Eye; ② Collimating lens; ③ Aperture; ④ Measuring cursor; ⑤ Beam splitting cube; ⑥ LED to fix eyes; ⑦ and ⑧ Cylindrical lens; ⑨ Stop. The marked angle numbers are intersection angles between the parallel light and the main optical axis. The figure shows only two of the six cursors
The design structure of the optical system
MTF curves of imaging system corresponding to different curvature radius. (a) R=5.5 mm; (b) R=7.18 mm; (c) R=11 mm
Field curvatures and distortions curves of imaging system corresponding to different curvature radius. (a) R=5.5 mm; (b) R=7.18 mm; (c) R=11 mm
Spot diagram curves of imaging system corresponding to different curvature radius. (a) R=5.5 mm; (b) R=7.18 mm; (c) R=11 mm
The relationship between the radius of curvature of the cornea and the distance between two points of light on the hexagonal diagonal in the corneal reflection image
Corneal reflex image. (a) Corneal reflex image of artificial eye; (b) Corneal reflex image of human eye
The results of simulated eye
Reflection images of a spherical mirror with different curvature. (a) R=5.5 mm; (b) R=6 mm; (c) R=7 mm; (d) R=9 mm; (e) R=10 mm; (f) R=11 mm