title: Loss of resolution in a corneal topography system.
abstract: BACKGROUND
  Videokeratoscopes employing a spherically biased reconstruction algorithm are known to inaccurately model radially aspheric surfaces. The purpose of this study was to quantify the effect of a spherically biased reconstruction on the accuracy and resolution of corneal shape measurements.
  
  
  METHODS
  Corneal asphericity and apical radius of curvature values were calculated from the local radius of curvature data produced by the EyeSys Corneal Analysis System running version 2.1.08 of the software on 40 normal eyes. A theoretical model was constructed to simulate the effect of the spherically biased reconstruction.
  
  
  RESULTS
  A significant difference was found between the asphericity values from this study and previous studies of corneal shape (P < 0.05 all cases). A large compression of the range of asphericity values was also found. Theoretical simulation demonstrated that these changes could be explained by the inherent error in the reconstruction algorithm alone.
  
  
  CONCLUSIONS
  The measured asphericity value was lower by a factor of 3 than accepted values for normal corneas. There was a similar reduction in the range of asphericities recorded. The consequence is a loss of resolution by a factor of 3 when assessing corneal shape change as measured by corneal asphericity. This may compromise longitudinal clinical studies or be important when one is making measurements on different corneal topography systems.
authors.affiliation: Centre for Applied Vision Research, The City University, 311-321 Goswell Road, London EC1V 7DD, UK. C.C.Hull@city.ac.uk
authors.forename: C C
authors.initials: CC
authors.lastname: Hull
mesh_terms: D000328:Adult; D003315:Cornea / Q000033:anatomy & histology*; D019781:Corneal Topography / Q000295:instrumentation*; D004868:Equipment Failure; D005260:Female; D006801:Humans; D008297:Male; D015203:Reproducibility of Results