Abstract
AIMS—To investigate the ability of a telecentric keratometer to describe the asphericity and curvature of convex ellipsoidal surfaces and human corneas. METHODS—22 conicoidal convex surfaces and 30 human corneas were examined by conventional keratometry. Additional keratometric measurements were made when the surface was tilted in the horizontal plane relative to the instrument optical axis. This resulted in a series of radius measurements derived from different regions of the surface. These measurements were used to determine the apical radius and the p value of the horizontal meridian of each surface. The results were compared with those derived from measurements using the EyeSys videokeratoscope and form Talysurf analysis. The method was repeated on 30 human corneas and the results compared with those of a videokeratoscope. RESULTS—For the aspheric buttons, the keratometric and the EyeSys results tended to give higher values for both apical radius and the p values than those of the Talysurf analysis. The best agreement was between the Talysurf and the keratometer where the results were not significantly different. For the human corneas, the apical radii were significantly different comparing the keratometer with the videokeratoscope but the p values were not significantly different. CONCLUSION—The keratometric method for assessing curvature and asphericity appears to hold promise as a method for quantifying the corneal topography.
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Figure 1 .
A keratometer with collimated mires is being used to measure the radius of curvature of a convex surface. For diagrammatic simplicity, the surface being examined is spherical. The chief ray, centred on the telescope objective lens, defines the region of the cornea which is being used to generate the mire image (size i) which is the object for the keratometer telescope. The corneal region being measured is centred on the point distance h1 from the keratometer optical axis.
Figure 2 .
The keratometer is tilted 18° in relation to the aspherical surface major axis. It is assumed that the instrument will measure the sagittal radius along the keratometer telescope optical axis. This is an oversimplification. A region of the surface further from the major axis and another region closer to the axis will form the mire images.
Figure 3 .
Diagram showing the hollow cross mire target of the Zeiss telecentric keratometer. The numbers refer to the angular subtense of the centre of each block from the instrument optical axis.
Figure 4 .
Typical radius squared versus distance squared graphs for one of the 22 aspherical surfaces. (A) The scatter plot derived from the EyeSys VK measurements. (B) The scatter plot derived from the Zeiss keratometric measurements. The Talysurf analysis for this surface gave an apical radius of 7.800 mm and a p value of 0.212.
Figure 5 .
(A) A graph of the mean apical radius ((Talysurf + EyeSys)/2) versus the apical radius difference. (B) A graph of the mean apical radius ((Talysurf + keratometer)/2) versus the apical radius difference. (C) A graph of the mean p value ((Talysurf + EyeSys)/2) versus the p value difference. (D) A graph of the mean p value ((Talysurf + keratometer)/2) versus the p value difference. The bias is the mean difference and the error is the standard deviation of the difference.
Figure 6 .
(A) Typical radius squared versus distance squared graphs for one of the human corneas. The scatter plot derived from the EyeSys VK measurements. (B) The scatter plot derived from the Zeiss keratometric measurements.
Selected References
These references are in PubMed. This may not be the complete list of references from this article.
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