title: Comparison of three videokeratoscopes in measurement of toric test surfaces.
abstract: OBJECTIVE
  We compared the accuracy of the Computed Anatomy TMS-1 (1.41), the EyeSys Laboratories Corneal Analysis System (2.1), and the Visioptic EH-270 (3.0) videokeratoscopes in measuring toric surfaces. These non-rotationally symmetric aspheric surfaces served as models of corneal astigmatism.
  
  
  METHODS
  Precision diamond-turned toric surfaces modeling 0.00 diopter (D) to 7.00 D of astigmatism were fabricated. A three-dimensional contact profiler was developed to calibrate the aspheric surfaces. Videokeratoscopic data taken at "best focus" were compared to the theoretical shape to quantify device measurement errors.
  
  
  RESULTS
  The Computed Anatomy system measurement accuracy shows no statistically significant correlation between measurement error and surface toricity (r2 < 0.13). Measurement error increased linearly with surface astigmatism for the EyeSys Laboratories system (0.12 D rms error per D of astigmatism, r2 > 0.96, p < 0.001 and the Visioptic system (0.03 D error per D of astigmatism, r2 = 0.88, p < 0.001).
  
  
  CONCLUSIONS
  This study found systematic performance differences among the three machines. Under ideal alignment conditions, the Computed Anatomy TMS-1 is more accurate at detecting astigmatism. The EyeSys Laboratories Corneal Analysis System apparently underestimates the amount of surface astigmatism because of excessive data smoothing. The Visioptic EH-270 errors are primarily in the central zones and may be due to ring localization errors.
authors.affiliation: Optical Sciences Center, University of Arizona, Tucson 85721, USA.
authors.forename: J E
authors.initials: JE
authors.lastname: Greivenkamp
authors.forename: M D
authors.initials: MD
authors.lastname: Mellinger
authors.forename: R W
authors.initials: RW
authors.lastname: Snyder
authors.forename: J T
authors.initials: JT
authors.lastname: Schwiegerling
authors.forename: A E
authors.initials: AE
authors.lastname: Lowman
authors.forename: J M
authors.initials: JM
authors.lastname: Miller
mesh_terms: D000465:Algorithms; D001251:Astigmatism / Q000175:diagnosis*; D003196:Computer Graphics; D003315:Cornea / Q000473:pathology; D006801:Humans; D007091:Image Processing, Computer-Assisted / Q000295:instrumentation*; D008953:Models, Anatomic; D019721:Ophthalmoscopes; D012029:Refraction, Ocular; D012984:Software; D014741:Video Recording / Q000295:instrumentation*