title: Corneal powers measured with a rotating Scheimpflug camera.
abstract: OBJECTIVE
  Keratometry measures the anterior corneal curvature only. Corneal power is calculated by multiplication with the keratometric refractive index, which takes into account the average negative posterior corneal power. The aim of this study was to calculate and compare various expressions for total corneal power assessed with Scheimpflug camera techniques, which also measure the posterior corneal curvature.
  
  
  METHODS
  We used the Pentacam rotating Scheimpflug camera to measure the equivalent power, total corneal refractive power (based on Snell's law ray tracing), and simulated keratometry (keratometric refractive index=1.3375) over the central 3.0 mm zone in 951 eyes. The keratometric refractive index of the equivalent power and the total corneal refractive power was calculated as the ratio between these values and the anterior corneal curvature.
  
  
  RESULTS
  The equivalent power, total corneal refractive power, and simulated keratometry all differed statistically significantly (analysis of variance, p<0.001) and averaged 42.26 (±1.46) dioptres (D), 42.78 (±1.51) D and 43.42 (±1.49) D. The calculated keratometric refractive indices for equivalent and total corneal refractive power averaged 1.3284 (±0.0009) and 1.3324 (±0.0015). The error of using these calculated keratometric refractive indices rather than the measured values for equivalent and total corneal refractive power averaged 0 (±0.11 D) and -0.01 D (±0.19).
  
  
  CONCLUSIONS
  Pentacam rotating camera assessment of total corneal power over the central 3.0 mm zone differed significantly for simulated keratometry, equivalent power and Snell's law ray tracing.
authors.affiliation: Regions Hospital Randers, Randers, Denmark.
authors.forename: Kristian
authors.identifier: http://orcid.org/0000-0002-5346-9350
authors.initials: K
authors.lastname: Næser
authors.affiliation: Fondazione GB Bietti IRCCS, Rome, Italy.
authors.forename: Giacomo
authors.initials: G
authors.lastname: Savini
authors.affiliation: Regions Hospital Randers, Randers, Denmark.
authors.forename: Jesper Flethøj
authors.initials: JF
authors.lastname: Bregnhøj
mesh_terms: D003315:Cornea / Q000000981:diagnostic imaging*; D019781:Corneal Topography / Q000295:instrumentation*; D004867:Equipment Design; D005260:Female; D006801:Humans; D008297:Male; D008875:Middle Aged; D009216:Myopia / Q000175:diagnosis* / Q000503:physiopathology; D010781:Photography / Q000295:instrumentation*; D012029:Refraction, Ocular; D015203:Reproducibility of Results; D012189:Retrospective Studies
keywords: Cornea; Optics and Refraction; Vision