title: Estimation of the refractive index of rigid contact lenses on the basis of back vertex power measurements.
abstract: OBJECTIVE
  To assess the possibility of estimating the refractive index of rigid contact lenses on the basis of measurements of their back vertex power (BVP) in air and when immersed in liquid.
  
  
  METHODS
  First, a spreadsheet model was used to quantify the magnitude of errors arising from simulated inaccuracies in the variables required to calculate refractive index. Then, refractive index was calculated from in-air and in-liquid measurements of BVP of 21 lenses that had been made in three negative BVPs from materials with seven different nominal refractive index values. The power measurements were made by two operators on two occasions.
  
  
  RESULTS
  Intraobserver reliability showed a mean difference of 0.0033±0.0061 (t = 0.544, P = 0.59), interobserver reliability showed a mean difference of 0.0043±0.0061 (t = 0.707, P = 0.48), and the mean difference between the nominal and calculated refractive index values was -0.0010±0.0111 (t = -0.093, P = 0.93). The spreadsheet prediction that low-powered lenses might be subject to greater errors in the calculated values of refractive index was substantiated by the experimental results.
  
  
  CONCLUSIONS
  This method shows good intra and interobserver reliabilities and can be used easily in a clinical setting to provide an estimate of the refractive index of rigid contact lenses having a BVP of 3 D or more.
authors.affiliation: Department of Optometry & Visual Science, City University, London, United Kingdom. rpearson@contacts.fsnet.co.uk
authors.forename: Richard
authors.initials: R
authors.lastname: Pearson
mesh_terms: D003261:Contact Lenses; D006801:Humans; D055620:Optical Phenomena; D012031:Refractometry