THE BAUSCH-AND-LOMB KERATOMETER DOES NOT MEASURE THE TANGENTIAL RADIUS OF CURVATURE

The purpose of this research was to determine if the conventional kera tometer measures tangential or sagittal radius. A Bausch and Lomb kera tometer was modified to enable an assessment of vertex radius and p-va lue to be made on convex conicoidal buttons of known surface form. Cal culation of these two parameters was achieved assuming that the kerato meter was measuring tangential radius and then repeated assuming a sag ittal measurement. The resulting values could then be compared with th ose obtained by Form Talysurf analysis and by the use of an autokerato meter designed to measure sagittal radius. There was good agreement be tween the three instruments when the modified Bausch and Lomb keratome ter was assumed to measure sagittal radius. We conclude that the Bausc h and Lomb keratometer makes a radius measurement closer to the sagitt al than the tangential radius of a conicoid of revolution.