Comparative study of the elevation topography of complex shapes

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Abstract
To compare the accuracy and precision of the elevation topography from two commercially available instruments using videokeratoscopy or rasterstereography. University of Ottawa Eye Institute, Ottawa General Hospital, Ontario, Canada. Repeated measurements of elevation topography of six calibrated surfaces were done with the PAR Corneal Topography System (CTS) and the Tomey Topographic Modeling System (TMS-1). The shapes simulated normal (A: aspheric, B: spherocylindric) and postsurgical corneas (C: hyperopic, D: myopic, E: central island, F: phototherapeutic keratectomy). Surface shapes were described by parametric equations. Equation parameters associated with each elevation measurement were determined by best-fit analysis. Measurement precision was assessed by the standard deviation of the difference between the fitted and the measured data. Fitted parameters were compared with nominal values obtained from an independent calibration of each surface. The root mean square error (RMSE) of the deviation of the fitted from the nominal surfaces was used to evaluate the accuracy of each instrument. The accuracy of the CTS exceeded that of the TMS-1 for all surfaces measured. The RMSE values (micron) were (A: 0.1, 6.5), (B: 0.3, 3.8), (C: 1.1, 11.8), (D: 5.0, 43.0), (E: 1.2, 3.2) and (F: 2.2, 17.5) for the CTS and TMS-1, respectively. The differences in the measured data from the fit surface were generally smaller with the TMS-1. Quantitative analysis of elevation measurements showed that the CTS represented surface topography more accurately than the TMS-1.