The Bausch and Lomb keratometer does not measure the tangential radius of curvature

The purpose of this research was to determine if the conventional keratometer measures tangential or sagittal radius. A Bausch and Lomb keratometer was modified to enable an assessment of vertex radius and p-value to be made on convex conicoidal buttons of known surface form. Calculation of these two parameters was achieved assuming that the keratometer was measuring tangential radius and then repeated assuming a sagittal measurement. The resulting values could then be compared with those obtained by Form Talysurf analysis and by the use of an autokeratometer designed to measure sagittal radius. There was good agreement between the three instruments when the modified Bausch and Lomb keratometer was assumed to measure sagittal radius. We conclude that the Bausch and Lomb keratometer makes a radius measurement closer to the sagittal than the tangential radius of a conicoid of revolution.