title: Precision of orbscan II assessment of anterior corneal curvature and asphericity.
abstract: To determine the number of measurements required to produce a representative mean result for corneal apical radius and asphericity and to examine the repeatability of this mean result.
  
  
  
  The Bausch & Lomb Orbscan II corneal topographer was used to take 10 measurements of right eyes of 20 normal individuals. These were used to determine the number of measurements required to obtain a representative result. The apical radius and p-value were derived from the instrument software for the entire surface and also calculated from the raw data for a single meridian.
  
  
  
  Both apical radius and asphericity stabilized as the number of responses contributing to the mean increased. Repeat measurement for the entire surface of a three-measurement mean result was +/-0.090 mm for apical radius and +/- 0.151 for the p-value. The results for a single meridian, on repeat measurement, will lie within +/- 0.069 mm and +/- 0.051, respectively.
  
  
  
  Little improvement in stability or repeatability was observed after a three-measurement mean. The error and repeatability of the measurement appear to be better in this investigation when only a single meridian is analyzed. Thus, instrument software might be improved by providing analysis of apical radius and asphericity for a single, selectable meridian.
authors.affiliation: Department of Optometry, University of Bradford, West Yorks, United Kingdom. w.a.douthwaite@bradford.ac.uk
authors.forename: William Arthur
authors.initials: WA
authors.lastname: Douthwaite
authors.forename: Annette
authors.initials: A
authors.lastname: Parkinson
mesh_terms: D000328:Adult; D003315:Cornea / Q000033:anatomy & histology*; D019781:Corneal Topography / Q000592:standards*; D005260:Female; D006801:Humans; D008297:Male; D015203:Reproducibility of Results; D055815:Young Adult