Keratometry

Abstract

AbstractThe technique of measuring the corneal power by keratometry is reviewed. The importance of the keratometer index for the translation of the radius into dioptric power is explained, and evidence is given that the conventional index of 1.3375 gives a corneal power which is higher than the physiological value. The effect of spherical aberration on the keratometer reading is shown by numerical examples using exact ray tracing. Ray tracing is used to analyze tomographically obtained elevation data to calculate the effective corneal power as a function of pupil size. The ray-traced corneal power at 3 mm pupil was found to be about 1 D lower than the standard keratometer diopter value. Finally, it is shown how the Stiles–Crawford effect moderates the effect of spherical aberration for large pupil sizes.