Fig. 3. Global and local radius of curvature (ROC) and power. For the scale, radius of curvature or power can be selected. One is converted to the other using the standard keratometric index. A. Global/axial/sagittal measurements are made relative to the visual axis and therefore have a spherical bias. B. The same map expressed in terms of local/instantaneous/tangential measurements is more accurate over irregularities and in the corneal periphery. (Corbett MC, Rosen ES, O'Brart DPS: Corneal Topography: Principles and Applications. London, BMJ Books, 1999.) |