Measurement and correction of misalignments in corneal topography using the null-screen method

Misalignments are a common problem in corneal topography reconstruction. To determine the topography of the surface using a corneal topographer based on the null-screen method, it is necessary to measure errors caused by mechanical misalignment of the topographer, which could influence the results of this technique but are not related to the quality of the corneal surface. To evaluate the variability in measurements, in this work, we simulate some misalignments in the optical system, for the simulation we design a semi-radial null-screen to test a reference spherical surface to identify the variations in results introduced by misalignment errors of the corneal topographer with respect to the test surface. According to the simulations, the accuracy of the null-screen method ranges from 0.81 µm to 2.84 µm for typical tilt, decentering, and defocusing errors. Experimental results for the testing of a spherical surface are shown. After removing the misalignments, we found that the variations are approximately 3.5 μm rms value measured with respect to the best-fitting sphere, and the radius of curvature differs approximately 0.06% from the design value.