The TMS-1 corneal topography measurement applied to calibrated ellipsoidal convex surfaces.

The purpose of this report is to assess the accuracy of the TMS-1 videokeratoscope (Computed Anatomy Inc.) by using convex ellipsoidal surfaces. The ellipsoids were calibrated using Form Talysurf analysis, which allowed for subsequent calculation of the vertex radius and p value. The videokeratoscope was used to examine the same ellipsoids. The data provided by the instrument software were used to plot a graph of r2 verses y2, where r is the measured radius at y, the distance from the corneal point being measured to the optical axis of the instrument. The intercept on the ordinate of this graph gives the vertex radius, and the slope give the p value. The results arising from the Talysurf and the TMS-1 techniques were compared. The TMS-1 videokeratoscope gave readings for the vertex radius that were generally higher than those of the Talysurf analysis. The vertex radius was up to 0.09 mm greater. The p value results were similar by the two methods for p values of approximately 0.8; however, the TMS-1 results were higher, and the discrepancy increased as the p value approached that of a paraboloid. Although the videokeratoscope may be useful in comparative studies of the cornea, there must be some doubt about the absolute values displayed as the surface becomes increasingly aspheric.

The effect of red and neutral density filters on the degree of eccentric fixation.

The reported shift of fixation to a more central position when a red filter is placed before an eye which fixates eccentrically, has never been satisfactorily explained. To further investigate this phenomenon, 14 subjects with eccentric fixation were recruited. A Wratten No. 92 filter was placed before the eccentrically fixating eye, and any change of fixation was assessed by the after-image transfer test. A neutral density (ND) filter was then used, to mimic the reduced retinal illumination caused by the red filter. The results suggest that there is a significant reduction in the eccentricity of fixation, when placing the red filter before an eccentrically fixating eye, and also when placing the ND filter. No significant difference in the effects produced by either filter was found. This suggests that it is the reduction in retinal illuminance caused by the red filter which contributes towards such improvements in fixation.