Assessment of aspheric surfaces by placido systems

Purpose. Spheric objects have been used as standards in computerized corneal topography studies. However, since it is well known that the human cornea is an aspheric surface, aspheric standards provide a better approximation of corneal contour and can be used to evaluate the accuracy and precision of these machines. Methods. Using two placido based photovideokeratoscopes, EyeSys and TMS, radius of curvature readings at 4mm and 8mm diameters were obtained for four polymethylmetharcrylate (PMMA) objects designed with micron accuracy by Computer Numeric Control (CNC) from Bausch & Lomb. Measurements from the placido systems were compared against reference data provided by Bausch & Lomb using Rank Hobson Taylor Talysurf profilometry (accuracy = ±0.00110 mm). Results. Overall precision measurements range from 0.0260 to 0.059D for EyeSys and 0.132D to 0.346D for TMS. The accuracy of the 2 systems in assessing aspheric surfaces varies depending on the shape of the given ball and chord diameter from which the values were taken. Conclusions. This study suggests that EyeSys is highly precise in measuring diopteric power on oblate and prolate surfaces. In addition, EyeSys is more accurate in measurements at central regions on oblate surfaces than TMS. TMS more accurately measured radius of curvature at the 4mm chord diameter on the prolate surfaces than EyeSys. In both systems, the error of measurements was found to increase from center to periphery and this may be due to spherically biased reconstruction algorithms employed in these systems.