title: Comparison of polycarbonate and steel test surfaces for videokeratography.
abstract: BACKGROUND
  Assessing video imaging systems for measuring corneal topography often requires test surfaces. Steel bearings have been employed, but manufacturers caution that high reflectance (> 90%) relative to the eye (< 10%) may compromise test findings. The differences between steel and polycarbonate test surfaces are quantified in this study.
  
  
  METHODS
  Images of a steel and a polycarbonate sphere of known radius of curvature were obtained with the Tomey/Computed Anatomy Topographic Modeling System (TMS, Cambridge, Mass). Analysis was performed on the raw video data files and the resultant surface curvature estimates.
  
  
  RESULTS
  The raw video images differed sufficiently to affect image processing. Polycarbonate yielded consistently better images. Many steel images (approximately 25%) contained data points that could not be processed; calculated surface contour was more variable for these. Differences were less obvious when these images were removed from the pool.
  
  
  CONCLUSIONS
  Results support the manufacturer's caution against the use of steel surfaces for testing or calibration of the TMS instrument. Problems appear due to the fundamental differences in the intensity distributions of video images captured from high- and low-reflectance surfaces.
authors.affiliation: Centre for Contact Lens Research, University of Waterloo, Ontario, Canada.
authors.forename: R J
authors.initials: RJ
authors.lastname: Potvin
authors.forename: D
authors.initials: D
authors.lastname: Fonn
authors.forename: L
authors.initials: L
authors.lastname: Sorbara
mesh_terms: D002138:Calibration; D003315:Cornea / Q000033:anatomy & histology*; D006801:Humans; D007091:Image Processing, Computer-Assisted / Q000379:methods*; D008953:Models, Anatomic; D011076:Polycarboxylate Cement; D013232:Steel
chemical_list: D011076:Polycarboxylate Cement; D013232:Steel; C005506:polycarbonate