Accuracy and repeatability of the Topcon K-3500 autokeratometer on calibrated convex surfaces.

The vertex (apical) radius and the p value of 30 conicoidal convex surface plastic buttons were measured using the Topcon KR-3500 autokeratometer. The vertex radius was found using the equations derived in the introduction. The results were compared to those derived by the Form Talysurf technique, which was considered to give a precise indication of both the vertex radius and p value. The results revealed a good agreement between the two methods of measurement. The autokeratometer displayed a tendency to underread the vertex radius, compared to the Talysurf result, but the maximum difference was 0.02 mm (95% confidence interval). For the p value, the autokeratometer could over- or underread with a maximum difference of 0.035. The largest coefficient of repeatability for the two observers using the Topcon autokeratometer was found to be 0.033 mm for vertex radius and 0.068 for the p value.