Index of Refraction Error
The assumed index of refraction (IR) of the normal cornea is based on the relationship between the anterior and posterior corneal curvatures. This relationship changes in eyes treated with ablative laser procedures. Ophthalmologists long believed that IR error did not occur in eyes that have undergone RK. This situation leads to an overestimation of the corneal power by approximately 1 D for every 7 D of correction obtained and results in hyperopic refractive surprise. A recent study showed that in eyes treated with RK, there is greater flattening of the posterior curvature than of the anterior curvature. Both manual keratometers and computerized corneal topographers that measure only the front surface curvature convert the radius of curvature (r) obtained to diopters (D), usually by using an IR value of 1.3375. The following formula can be used to convert diopters to radius:
To convert r to D, use
Excerpted from BCSC 2020-2021 series : Section 3 - Clinical Optics. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.