Development of a wide field height eye topographer: validation on models of the anterior eye surface

The aim of this research was to develop a corneal topographer that determines the shape of the entire anterior surface of an eye without assumptions, and with uniformly high accuracy in the center and periphery.

Based upon a double projection of two sine wave gratings and analysis of the distortion of the sine wave gratings due to the corneal-scleral shape, point-by-point measurements of surface elevation were obtained with a sample density equal to the pixel density of the CCD-detector. Using this principle, a prototype topographer, called the Maastricht Shape Topographer (MST), was developed. The accuracy and reproducibility of the instrument were evaluated using bispheric models of the anterior surface of the eye.

The average accuracy of height measurements was +/- 0.55 micron in the 10-mm central area and +/- 22.50 microns in the periphery (14 to 19 mm). Reconstruction accuracy of the radius of curvature was +/- 0.0155 mm (+/- 0.88 D) in the center and +/- 0.0313 mm in the periphery (sclera). Average height reproducibility standard error was 0.0282 micron in the center and 2.6156 microns in the periphery.

With the MST, unambiguous shape measurements of the entire anterior surface of the eye are possible, with accuracy up to clinically accepted standards. MST is able to measure height over a wide area of 20 mm, with a 6-mm depth of field. The tested prototype of the device can be further improved by the use of custom-made optics in order to increase signal to noise ratio in the periphery of the image. This height topographer could offer a reliable method in cases where shape is of paramount importance, e.g., in (scleral) contact lens fitting and refractive surgery.