title: Measuring non-spherical optical surfaces.
abstract: BACKGROUND
  More lens designs for the presbyopic market are emerging and the use of novel constructions to generate varifocal optics in contact lenses is increasing. A number of designs incorporate aspheric shapes, of known p-values on the front and back surface of contact lenses. Frequently, the back optic zone radius quoted by the manufacturer is a nominal reference making verification of these lenses difficult. The purpose of this study was to determine whether a videokeratoscope or an optical microspherometer could verify these complex surfaces.
  
  
  METHODS
  Fifty-four concave test surfaces of known vertex curvatures (7.30 to 8.10 mm, in 0.1 mm steps) and known p-values (1.0 to 0.0, in 0.1 steps) were produced and measured with two different systems.
  
  
  RESULTS
  The optical microspherometer was able to verify the vertex radius to a high degree of accuracy (+/- 0.072[2SD]) without bias (mean difference = 0.008 +/- 0.01(2SE]). However, verification of the p-value by utilising Baker's equation demonstrated a poor degree of accuracy (+/-0.574) and bias (mean difference=0.134, +/-0.078). The degree of accuracy for the measurement of the vertex radius of the concave surface, when using the videokeratoscope, was also very high (+/-0.09) but with a demonstration of bias (mean difference=-0.017, +/-0.012). The videokeratoscope was able to describe the p-value for the surface to a high degree of accuracy (+/-0.136) but did demonstrate bias (-0.052, +/-0.018). These results for the measurement of the p-value for the optical microspherometry were statistically significantly different from that of the control measurement (p = 0.0379) and that for the videokeratoscope (p = 0.0042).
  
  
  CONCLUSIONS
  To determine the degree of asphericity of a concave optical surface to an acceptable degree of accuracy a videokeratoscope can be used in preference to an optical microspherometer. Both instruments demonstrated a high degree of accuracy when measuring the vertex radius.
authors.affiliation: The Victorian College of Optometry, University of Melbourne, Victoria, Australia. c.woods@optometry.unimelb.edu.au
authors.forename: C A
authors.initials: CA
authors.lastname: Woods