title: Accuracy of Topcon CM-1000 videokeratoscope on spherical test surfaces.
abstract: BACKGROUND
  Many videokeratoscopes use mathematical formulas to calculate corneal radii; calculations depend on slope, curvature, coordinate position, or focal properties of the surface. Accuracy of each type of videokeratoscope must be evaluated. A controversy exists about whether axial or tangential methods best provide a precise description of corneal shape; therefore results with the Topcon CM-1000 using both methods were evaluated.
  
  
  METHODS
  Measurements were done on black polymethylmethacrylate (PMMA) spherical calibrated surfaces. Lenses were first aligned and measured and then misaligned in different directions and measured. Results for each position were compared with the zero or alignment position.
  
  
  RESULTS
  Accuracy of the CM-1000 was high even under extreme misalignment conditions. Tolerance to misalignment was high (about 300 mm). Misalignment-induced variations in the output results were small (usually less than 0.05 mm). However, important variations (more than 1.00 diopter [D]) were found for the lowest measured radius (6 mm). In some cases, small differences between axial and tangential radii for the same point could be found.
  
  
  CONCLUSIONS
  With the exception of extremely low radii of curvature, the CM-1000 was accurate for measuring spherical surfaces. Further investigation remains to be done on aspheric surfaces and in clinical practice.
authors.affiliation: Departamento Interuniversitario de Optica de Alicante, San Vicente del Raspeig, Spain.
authors.forename: E
authors.initials: E
authors.lastname: Pérez-Yern
authors.forename: A
authors.initials: A
authors.lastname: Fimia-Gil
authors.forename: F
authors.initials: F
authors.lastname: Mateos
authors.forename: L
authors.initials: L
authors.lastname: Carretero
mesh_terms: D002138:Calibration; D003315:Cornea / Q000033:anatomy & histology*; D019781:Corneal Topography / Q000295:instrumentation*; D006801:Humans; D008433:Mathematics; D008953:Models, Anatomic; D019904:Polymethyl Methacrylate; D015203:Reproducibility of Results; D013499:Surface Properties; D014741:Video Recording
chemical_list: D019904:Polymethyl Methacrylate