ASSESSMENT OF THE POWER AND HEIGHT OF RADIAL ASPHERES REPORTED BY A COMPUTER-ASSISTED KERATOSCOPE

PURPOSE: The two purposes of this study were (a) to assess the accurac y with which a keratoscope, the Topographic Modeling System (TMS-1), c alculated the heights and powers of rotationally symmetric, radially a spheric test surfaces and (b) to determine whether the TMS-1 used an a xial solution for radius of curvature to determine the power of a sphe re that would produce the same semichord as would the test surface on a keratograph. METHODS: The TMS-1 heights and powers were studied for four test surfaces that had radial profiles similar to those of normal corneas, The powers of the surfaces were calculated from the local ra dius of curvature derived from the sur faces' manufacturing formulas. The heights and powers that would result from an axial solution were c alculated in a TMS-1 simulator, TMS-1 data were compared with data fro m the surfaces' formulas and with data from the simulation. RESULTS: T he TMS-1 data were almost identical to the heights and powers calculat ed from the simulated axial solution. The TMS-1 data were similar to t he heights and powers calculated from the mathematical formulas from t he apex to 2 mm from the apex but differed by up to 85 mu m of height and 10 diopters of power in the periphery. CONCLUSIONS: The TMS-1 appe ared to use the axial solution that does not calculate power from loca l radius of curvature, Clinicians should use caution when inferring co rneal shape from power maps based on an axial solution, especially out side the central 2-mm radius of a normal cornea, because such power do es not depict corneal curvature.