title: Verification of aspheric contact lens back surfaces.
abstract: OBJECTIVE
  To suggest a tolerance level for the degree of asphericity of aspheric rigid gas-permeable contact lenses and to find a simple method for its verification.
  
  
  METHODS
  Using existing tolerances for the vertex radius, tolerance limits for eccentricity and p values and were calculated. A keratometer-based method and a method based on sag measurements were used to measure the vertex radius and eccentricity of eight concave progressively aspheric surfaces and six concave ellipsoidal surfaces. The results were compared with a gold standard measurement made using a high-precision mechanical instrument (Form Talysurf).
  
  
  RESULTS
  The suggested tolerance for eccentricity and p value and is +/-0.05. The keratometer method was very accurate and precise at measuring the vertex radius (mean deviation +/- SD from Talysurf results, -0.002 +/- 0.008 mm). The keratometer was more precise than and similar in accuracy to the sag method for measurement of asphericity (mean deviation of keratometer method results from Talysurf results, 0.017 +/- 0.018; mean deviation of sag method results from Talysurf results using five semichords, -0.016 +/- 0.032).
  
  
  CONCLUSIONS
  Neither method was precise enough to verify the asphericity within the suggested tolerance. The keratometer can be efficiently used to verify the back vertex radius within its International Organization for Standardization tolerance and the back surface asphericity within an eccentricity/p value tolerance of +/-0.1. The method is poor for progressive aspheres with large edge blending zones. Deriving the eccentricity from sag measurements is a potential alternative if the mathematical description of the surface is known. The limiting factor of this method is the accuracy and precision of individual sag measurements.
authors.affiliation: Department of Optometry, University of Bradford, Richmond Road, Bradford/West Yorkshire BD7 1DP, UK. h.dietze@bradford.ac.uk
authors.forename: Holger H
authors.initials: HH
authors.lastname: Dietze
authors.forename: Michael J
authors.initials: MJ
authors.lastname: Cox
authors.forename: William A
authors.initials: WA
authors.lastname: Douthwaite
mesh_terms: D003261:Contact Lenses; D004867:Equipment Design; D006801:Humans; D008962:Models, Theoretical; D019904:Polymethyl Methacrylate
chemical_list: D019904:Polymethyl Methacrylate