To which index of refraction is the keratometer calibrated?

The keratometer, a device used to measure the curvature of the cornea, is calibrated to an index of refraction of 1.3375. This specific value is significant because it closely approximates the refractive index of the tear film that covers the cornea. When performing measurements on the cornea, the keratometer assumes this index in order to calculate the curvature accurately.

Using the correct refractive index is crucial for obtaining precise measurements, which are essential in fitting contact lenses and for assessing corneal health. Since the keratometer's primary function involves evaluating the curvature in relation to the tear film, the value of 1.3375 ensures that the readings translated to keratometry values reflect the actual corneal surface conditions accurately.

The other indexes of refraction provided do not align with the calibration standard used for keratometers, which is why they are not applicable in this context.