为了实现检测角膜的表面形貌, 设计了基于Placido盘的角膜地形图仪投射照明系统。首先, 根据人眼的明视觉光谱光视效率、CCD元件光谱响应曲线以及仪器对面光源照度与发光均匀性的要求, 进行了LED背光源设计。其次, 根据角膜地形图仪检测原理, 比较了球形和椭球形Placido盘的利弊, 在背光源照明下分析了这两种面型Placido盘的光照情况。分析显示, 椭球形Placido盘的最大辐照度优于球形盘。采用反向逆推法建立数学模型, 利用Matlab软件求解确定了盘上黑白条纹环的分布。最后, 根据上述设计参数进行了Placido盘加工, 搭建了投影照明实验装置, 并利用标准模型眼对所获得的投射照明系统进行检测。实验结果表明: 在主波长为625 nm的AlGalnP红色LED面光源照明的情况下, 采用所设计的24环椭球型Placido盘成像到CCD上的黑白条纹环满足等间隔均匀分布的要求, 检测精度可达到0.05 m-1, 较好地满足了角膜地形图仪对投射照明系统的高对比度、高精度的要求。

In order to detect the cornel surface topography, a projection lighting system of corneal topography was designed based on Placido disk. Firstly, LED backlight was designed according to the visual spectral luminous efficiency of human eyes, the spectral response curve of CCD element and requirements for illumination and uniformity of the light source opposite to the instrument. Secondly, based on detection principles of corneal topography, the advantages and disadvantages of spherical Placido disk were compared with those of ellipsoidal Placido disk. Lighting conditions of the two Placido disks were analyzed in the backlight illumination. The analysis shows that the maximum irradiance of ellipsoidal Placido disc is larger than the sperical one. The mathematical model of the ellipsoidal disc was established with inverse extrapolation, thus the distributions of black and white loops on the disk were determined by solution with Matlab software. Finally, a ellipsoidal Placido disk was processed in terms of the design parameters. Furthermore, the experimental devices of projection and lighting were set up and then detected with the standard model eyes. Experimental results show that in the illumination of AlGalnP red LED area light source with dominant wavelength of 625 nm, white and black loops on CCD projected from achievable 24-ring ellipsoidal Placido disk meets the requirement of uniform distribution with equal interval. The detection precision can reach 0.05 m-1, meeting the requirements of high contrast and precision for projection lighting system of corneal topography.