Design of imaging keratometer with annular object and charge-coupled device detector

A novel imaging keratometer by the aid of modern optoelectronic technology is proposed. The optical system consists of an annular object, a first imaging subsystem, a second imaging subsystem, and a CCD detector. The measurement range of corneal refraction is from 30D to 60D (5.5-11 mm in radius of curvature), and the accuracy reaches 0.156D, 0.072D, and 0.036D, respectively, for the corneal surface of the steepest, the radius of curvature of 7.8 mm, and the corneal surface of the flattest. The accuracy of corneal astigmatism is verified to be 0.05D for cylindrical refraction of 0.75D. Compared with the traditional keratometer, the proposed keratometer possesses advantages of high accuracy.