Corneal topography and photokeratoscopy

A keratometer measures the radius of curvature of the two principal meridians that correspond to the steepest and flattest meridians, by projecting an illuminated circle onto the corneal surface from a fixed distance. A photokeratoscope allows a permanent record to be made and further evaluated. It takes a picture primarily of the intermediate zone of the cornea. By measuring the distance between reflected rings and comparing them to the actual distance between the rings on the instrument, the curvature of the cornea can be ascertained at any given point.