为实现基于Placido盘的角膜地形图仪中图像的有效采集, 根据人眼角膜的特点以及所选用的CCD面阵参数, 设计了一套对称式消色差物镜及准直照明透镜系统。利用初级像差理论及PW法计算成像镜头的初始结构, 根据近轴光线追迹公式计算准直照明透镜参数, 利用Zemax光学软件进行系统优化。成像镜头结构由2组4片镜片组成, 有效焦距为20 mm, 后工作距离为19.2 mm, 相对孔径为1/3, 全视场角为8°, 光学总长控制在20 mm以内。在镜头分辨率66 lp·mm－1处, 所有视场的调制传递函数值均大于0.3, 全视场畸变量小于0.5%。该系统具有整体结构简单、紧凑、易加工、成本低、成像质量好等特点, 其性能很好地满足了整机的要求。

In order to collect images effectively with a corneal topography based on the Placido disk, a system with a symmetrical achromatic lens and a collimating illumination lens is designed according to characteristics of human corneal and CCD plane array parameters. The initial structure of the imaging lens is calculated based on the theory of primary aberration and the PW method. The collimating illumination lens parameters is designed by paraxial ray trace equations. The imaging system is optimized with the Zemax software. The imaging lens structure is composed of four lenses in two groups, its effective focal length is 20 mm, back working distance is 19.2 mm, full field of view is 8°, relative aperture is 1/3, and maximum optical total track is 20 mm. The modulation transfer function in all fields is larger than 0.3 at resolution of 66 lp·mm－1 and the full field-of-view distortion is less than 0.5%. The system is characterized by its simplicity, compactness, easy processing, low cost and high imaging quality, which meets the requirements of the whole system very well.