Corneal Topography using a New Moiré Image-based System

Abstract

Aim: To describe a new device, the Maastricht topographer (MT), which uses the principle of moire interference, to measure corneal shape. Methods: The Maastricht Topographer (MT) projects two sinewave gratings onto the tearfilm in quick succession, and by local analysis of grating intensity in the image so formed, reconstructs the true corneal shape in terms of height from a reference plane. A total of 262 144 data points are measured in an area 21.5 × 14.5 mm. During a one year period, patients referred for topography had measurements made by the MT and a videokeratoscope. Three cases were selected to demonstrate the advantages of the MT. Results: Image acquisition by the MT was straightforward, with alignment and focusing being of less importance than in videokeratoscopes. It provided detailed topographic information from the whole corneal area, and from irregular or non-reflective surfaces. The initial reconstruction of the true corneal shape was expressed in terms of height, from which measurements of slope, curvature and power were derived directly. Conclusions: Height reconstructions made by the MT are particularly useful in certain corneal pathologies and in evaluating the corneal profile after photorefractive keratectomy. Measurement of the ablation surface immediately postoperatively is essential in the study of ablation profiles and postoperative wound healing.