title: Topographer reconstruction of the nonrotation-symmetric anterior corneal surface features.
abstract: OBJECTIVE
  The purpose of this study is to demonstrate the performance of a topographer (the VU topographer, prototype development, VU University Medical Center, Amsterdam, The Netherlands) that uses a color-coded stimulus pattern to reconstruct both the rotation-symmetric and nonrotation-symmetric shape features of the anterior corneal surface.
  
  
  METHODS
  Spherical surfaces, toric surfaces, the Rand surface (surface with peripheral corrugations), and sample eyes were measured. A ring topographer (Keratron, Optikon 2000, Rome, Italy) and the Haag-Streit ophthalmometer (Haag Streit, Bern, Switzerland) were used for comparison.
  
  
  RESULTS
  All three instruments produced similar values for the radii of curvature of spherical surfaces with a tolerance of 0.02 mm. The Keratron gave underestimated values for the astigmatic power of toric surfaces (>0.25 D for toric surfaces with astigmatism >9 D). Because it eliminates skew ray error, only the VU topographer was able to reconstruct the correct shape of the Rand surface in contrast with ring topographers. The effect of skew ray error was also observed in the surface reconstruction of a radially keratotomized (RK) eye. There was height difference of 2.75 +/- 1.25 microm between the output of the VU topographer and the output of the Keratron.
  
  
  CONCLUSIONS
  The VU topographer is just as accurate in reconstructing the rotation-symmetric features of the anterior corneal surface as the ring topographers but is superior in recovering the nonrotation-symmetric shape features.
authors.affiliation: Department of Physics and Medical Technology (FMT), VU University Medical Center, Amsterdam, The Netherlands. va.sicam@vumc.nl
authors.forename: Victor Arni D P
authors.initials: VA
authors.lastname: Sicam
authors.forename: Rob G L
authors.initials: RG
authors.lastname: VAN der Heijde
mesh_terms: D003315:Cornea / Q000033:anatomy & histology*; D019781:Corneal Topography / Q000295:instrumentation*; D004867:Equipment Design; D006801:Humans; D015203:Reproducibility of Results