title: Accuracy of Orbscan II slit-scanning elevation topography.
abstract: OBJECTIVE
  To establish the accuracy of Orbscan II (Orbtek Inc.) slit-scanning elevation topography in analyzing the anterior surface of complex test objects.
  
  
  METHODS
  Discipline of Ophthalmology, University of Auckland, Faculty of Medical and Health Sciences, Auckland, New Zealand.
  
  
  METHODS
  Six test objects were created from 2 materials: standard calibration poly(methyl methacrylate) (PMMA) (Orbtek Inc.) and a research PMMA material. The test objects were produced with spherical (radii of curvature 6.00 mm, 7.67 mm, and 8.88 mm), aspherical (apical radius 7.67, eccentricity 0.5, Q -0.25), and toroidal (7.67/7.92 mm radii of curvature) surfaces. The accuracy of the test surfaces was established by Form Talysurf Analysis. A single calibrated Orbscan II device was used to obtain 20 separate anterior elevation maps of each test object. The data obtained from Orbscan II, at 0.2 mm intervals along the chosen meridian, were directly compared with the Talysurf values for each test surface.
  
  
  RESULTS
  Orbscan II measurements of all test objects were statistically significantly different from the Talysurf values (P <.001). The test objects produced from standard calibration material were more accurately measured by Orbscan II than the objects produced from the research material. Data obtained by Orbscan II from the central 3.5 mm of all test objects were more accurate than peripheral data when compared with the Talysurf values (P =.001).
  
  
  CONCLUSIONS
  Orbscan II anterior surface elevation measurements differed significantly from Form Talysurf Analysis of complex test surfaces. However, the magnitude of the errors in the measurement of standard test objects was small, less than 0.20 microm centrally and 0.70 microm peripherally. Clinically, if similar accuracy of measurement is confirmed in the human eye, anterior surface elevation maps can be considered accurate representations of corneal shape.
authors.affiliation: Discipline of Ophthalmology, Faculty of Health Sciences, University of Auckland, Auckland, New Zealand.
authors.forename: Gerard
authors.initials: G
authors.lastname: Cairns
authors.forename: Charles N J
authors.initials: CN
authors.lastname: McGhee
authors.forename: Michael J
authors.initials: MJ
authors.lastname: Collins
authors.forename: Helen
authors.initials: H
authors.lastname: Owens
authors.forename: Greg D
authors.initials: GD
authors.lastname: Gamble
mesh_terms: D003261:Contact Lenses; D003315:Cornea / Q000033:anatomy & histology*; D019781:Corneal Topography / Q000379:methods* / Q000592:standards; D006801:Humans; D008953:Models, Anatomic; D019904:Polymethyl Methacrylate; D015203:Reproducibility of Results
chemical_list: D019904:Polymethyl Methacrylate