The effect of videokeratoscope faceplate design on radius of curvature maps

A computer model using finite ray tracing methods was developed to simulate a videokeratoscope analysing an average cornea. Different faceplate designs were tested using five points on the faceplate subtending angles at the corneal vertex between 15 degrees and 75 degrees in 15 degrees intervals. Image quality was assessed by adding the geometrical blurs of the five image points. Differences (error) between true sagittal radius of curvature, and sagittal radius of curvature calculated by the van Saarloos algorithm, were calculated for selected surfaces at the same corneal points. Differences equal to or bigger than 0.02 mm were regarded as clinically significant. The faceplate surface that provided the sharpest image for an average cornea was a cylinder with the base 120 mm away from the corneal vertex and a diameter of 26 mm. Changing the faceplate design results in clinically significant differences for an average cornea. Image quality is not proportional to corneal surface reconstruction accuracy.