Accuracy of the Tomey Topographic Modeling System in measuring surface elevations of asymmetric objects

Background Most studies have assessed the accuracy of videokeratographic sy stems using spheres, ellipsoids, or toric surfaces. Most human corneas are asymmetric to some degree and many pathological corneas are markedly asymme tric. To date, little work has been done to ascertain the accuracy of video keratographic systems for measuring the shape of asymmetric objects. Purpos e. The purpose of this study is to determine the accuracy with which the To mey Topographic Modeling System can evaluate the topography of asymmetric s urfaces. Methods. Calibrated ellipsoidal test objects were tilted with resp ect to the videokeratometric axis to create asymmetric test surfaces with k nown characteristics. Results. Root mean squared error of all the measured surface elevations varied from 0.7 mu m to 11.3 mu m. Although there was a trend for greater error with the more asymmetric surfaces, the trend was no t statistically significant. Accuracy was not dependent on apical radius. M easurement error increased toward the periphery. Conclusions. For the Tomey Topographic Modeling System, the accuracy in measurement of smooth, asymme tric surfaces is comparable the accuracy in measurement of symmetric surfac es.