Extent and effect of surface tilt on the data display of the EyeSys videokeratoscope

Aims - This study examined the occurrence, magnitude, and the consequences of a possible tilt between the corneal surface and optical axis of the EyeSys videokeratoscope. Methods - Initially, a theoretical model was developed to calculate the angle of tilt. The predictions of the model were verified empirically using a convex conicoid surface and were found to predict the tilt to within 0.5 degrees of the actual tilt. The likely effects of the tilt on the corneal power were also examined. The angle of tilt was then measured on the human cornea and the effect of neutralising the tilt on the videokeratoscopic data display was observed. Results - The angle of tilt was found to lie between 1 degrees and 6 degrees in a temporal direction. Conclusion - When the corneal tilt on the human subjects was neutralised, then a reduction in the nasal/temporal asymmetry was observed.