Metrological calibration of ophthalmometers

Ophthalmometers are very important for clinical ophthalmology and usually used to measure the radius of curvature, direction of principal meridian and keratometric dioptre of the human cornea. In order to realize the calibration and quality evaluation of ophthalmometers, standard devices based on reflection principle are developed. It consists of both radius and axial standard device. The radius standard device is used to calibrate the radius of curvature and keratometric dioptre of ophthalmometers. It comprises six lenses with three different radius value and two kinds of spherical surface. The axial standard device is used to calibrate the axis of ophthalmometers which comprises two axial model eyes with square cylinder body. The front surface is toroidal and back surface is plane. With optical processing and mechanical positioning, four axial alignments of 0°, 45°, 90° and 135° are realized. By measurement and evaluation, the expanded uncertainty of the standard device for radius of curvature is 0.002mm and for axis is 0.5° both with coverage factor of 2. It meets the calibration accuracy which is required in the international standard of ophthalmometers. Moreover, the experiment results prove that the developed standard device is easy to operate and feasible. It can not only realize the calibration very well but also provide manufacturers technical support for quality control and improvement.