Reliability and validity of videokeratoscopic measurements

When a test surface or a cornea is carefully and properly positioned with respect to the Corneal Modeling System (CMS) videokeratoscope, measurements are highly repeatable, but the accuracy of the measurements is dependent on the shape of the surface. For spherical and toroidal surfaces, the CMS provides suitably accurate descriptions of the topography of the surface. For aspheric surfaces the CMS renders less accurate, but still clinically acceptable, topographic descriptions.