Evaluating the anterior corneal surface using an improved null-screen system

In this work, a conical corneal null-screen topographer is presented. The device is designed to adapt to the human morphology of the face and measures the human cornea assuming that its shape is an aspherical surface. The evaluation of the performance of the conical null-screen corneal topographer includes calibration of the device and evaluation and compensation of the distortion introduced by the lens used to acquire the images. For the calibration of the device we perform the evaluation of a spherical reference surface of 7.8 mm radius of curvature and 11 mm in diameter. Here we obtain an rms difference in sagitta between the evaluated surface and the best fitting sphere of about 2.1 μm. We present examples of surface topography measurements on some human corneas. Elevation, sagittal curvature, and meridional curvature maps can be calculated. Other geometrical parameters such as the radius of curvature and the conic constant are obtained. Results of the corneal surface topography were compared with commercial Placido-based corneal topography device.