Ray-Tracing Analysis of Corneal Power From Scheimpflug Data
Purpose
To describe a method by which mapped corneal elevation data can be used for ray tracing analysis of the effective corneal power.
Methods
Scheimpflug data exported in an elevation matrix by the Pentacam software was triangulated into a polygonal format and imported into the Zemax optical engineering software. By ray tracing analysis the focal length of the cornea was determined as the distance giving the sharpest point-spread-function at the image plane. The corneal power was estimated as the reciprocal of the observed focal length ‘reduced to air’. The reproducibility of the procedure was evaluated from double experiments in 10 cases and the effect of the pupil size was evaluated in 20 cases referred for cataract or refractive lens surgery and finally compared with standard methods.
Results
The ray traced corneal power was found to be highly reproducible with a maximum error of 0.023 D between repeated measurements. Assuming 3 mm pupil the mean ray traced corneal power of 42.34 D was significantly higher than the mean Pentacam TNP variable of 41.91 D (p < 0.01, by paired t-test) but not significantly different from the mean Pentacam TCRP variable of 42.38 D (p > 0.05, by paired t-test). The mean standard ‘K-reading’ of 43.36 D was found to be 1.02 D higher than the ray traced power and higher than all the other metrics of corneal power (p < 0.001). The estimated corneal power increased 0.31 D when the pupil size increased from 3 to 5 mm as a result of spherical aberration.
Conclusion
Exact ray tracing can be used on mapped tomography data to analyze the corneal power. This technique was found to be highly reproducible and may be a promising tool in the analysis of the true refraction of the cornea of any shape.