title: Automated compensation of defocusing errors in videokeratography.
abstract: OBJECTIVE
  The reproducibility of videokeratography measurements is mainly dependent on the accuracy of manual adjustment in the focal plane. Videokeratoscopes having small Placido cones show a considerable amount of error when the required working distance between cornea and keratoscope is not maintained. The advantages of small cones (optimal illumination and the reduction of anatomically caused shadows) are in no proportion to the disadvantage-poor depth of focus, resulting in poor reproducibility.
  
  
  METHODS
  The Tubingen Color Ellipsoid Topometer compensates with software and hardware for defocusing errors by means of a triangulation measurement. An 8.0 mm sphere (42 D) was measured in a distance range of +/- 1.5 mm around the focal plane with and without compensation. Five measurements were acquired at each position to evaluate the reproducibility of the calculated data.
  
  
  RESULTS
  Defocusation in the range of +/-1.5 mm leads to deviation smaller than 0.05 mm (0.25 D). Without compensation, the maximal deviation is increased up to 0.7 mm (3.0 D), and thus is considerably greater.
  
  
  CONCLUSIONS
  Automated compensation of defocusing errors in videokeratography actively compensates for defocusation errors, enhancing precision and avoiding measuring artifacts.
authors.affiliation: Department of Ophthalmology, University of Tübingen, Germany.
authors.forename: T
authors.initials: T
authors.lastname: Oltrup
authors.forename: B
authors.initials: B
authors.lastname: Jean
authors.forename: T
authors.initials: T
authors.lastname: Bende
mesh_terms: D003315:Cornea / Q000473:pathology*; D006801:Humans; D007091:Image Processing, Computer-Assisted / Q000379:methods*; D009885:Ophthalmology / Q000295:instrumentation; D012030:Refractive Errors / Q000175:diagnosis*; D015203:Reproducibility of Results