¿1,3375 index?

Abstract

To convert the measurements obtained from the corneal surface into dioptric value, the B&L keratometer uses the general lens formula (n’-n/R) and assumes an n’ of 1,3375 (compared to the actual corneal refractive index of n’=1,376). This is a fictional value, which includes an allowance for the small, yet significant, negative power of the posterior corneal surface.