title: Refractive indices used by the Haag-Streit Lenstar to calculate axial biometric dimensions.
abstract: OBJECTIVE
  To estimate refractive indices used by the Lenstar biometer to translate measured optical path lengths into geometrical path lengths within the eye.
  
  
  METHODS
  Axial lengths of model eyes were determined using the IOLMaster and Lenstar biometers; comparing those lengths gave an overall eye refractive index estimate for the Lenstar. Using the Lenstar Graphical User Interface, we noticed that boundaries between media could be manipulated and opposite changes in optical path lengths on either side of the boundary could be introduced. Those ratios were combined with the overall eye refractive index to estimate separate refractive indices. Furthermore, Haag-Streit provided us with a template to obtain 'air thicknesses' to compare with geometrical distances.
  
  
  RESULTS
  The axial length estimates obtained using the IOLMaster and the Lenstar agreed to within 0.01 mm. Estimates of group refractive indices used in the Lenstar were 1.340, 1.341, 1.415, and 1.354 for cornea, aqueous, lens, and overall eye, respectively. Those refractive indices did not match those of schematic eyes, but were close in the cases of aqueous and lens. Linear equations relating air thicknesses to geometrical thicknesses were consistent with our findings.
  
  
  CONCLUSIONS
  The Lenstar uses different refractive indices for different ocular media. Some of the refractive indices, such as that for the cornea, are not physiological; therefore, it is likely that the calibrations in the instrument correspond to instrument-specific corrections and are not the real optical path lengths.
authors.affiliation: Institute of Health & Biomedical Innovation and School of Optometry & Vision Sciences, Queensland University of Technology, Brisbane, Australia.
authors.forename: Marwan
authors.initials: M
authors.lastname: Suheimat
authors.forename: Pavan K
authors.initials: PK
authors.lastname: Verkicharla
authors.forename: Edward A H
authors.initials: EA
authors.lastname: Mallen
authors.forename: Jos J
authors.initials: JJ
authors.lastname: Rozema
authors.forename: David A
authors.initials: DA
authors.lastname: Atchison
mesh_terms: D058452:Axial Length, Eye / Q000033:anatomy & histology*; D001699:Biometry / Q000379:methods*; D005123:Eye / Q000033:anatomy & histology; D006801:Humans; D016014:Linear Models; D008953:Models, Anatomic; D012029:Refraction, Ocular / Q000502:physiology*; D012031:Refractometry / Q000379:methods*
keywords: Lenstar; air thickness; axial length; partial coherence interferometry; refractive index