title: Power vector analysis of the optical outcome of refractive surgery.
abstract: OBJECTIVE
  To demonstrate the power vector method of representing and analyzing spherocylindrical refractive errors.
  
  
  METHODS
  School of Optometry, Indiana University, Bloomington, Indiana, USA.
  
  
  METHODS
  Manifest and keratometric refractive errors were expressed as power vectors suitable for plotting as points in a 3-dimensional dioptric space. The 3 Cartesian coordinates (x, y, z) of each power vector correspond to the powers of 3 lenses that, in combination, fulfill a refractive prescription: a spherical lens of power M, a Jackson crossed cylinder of power J0 with axes at 90 degrees and 180 degrees, and a Jackson crossed cylinder of power J45 with axes at 45 degrees and 135 degrees. The Pythagorean length of the power vector, B, is a measure of overall blurring strength of a spherocylindrical lens or refractive error. Changes in refractive error due to surgery were computed by the ordinary rules of vector subtraction.
  
  
  RESULTS
  Frequency distributions of blur strength (B) clearly demonstrate the effectiveness of refractive surgery in reducing the overall blurring effect of uncorrected refractive error.
  
  
  CONCLUSIONS
  Power vector analysis also revealed a reduction in the astigmatic component of these refractive errors. Paired comparisons revealed that the change in manifest astigmatism due to surgery was well correlated with the change in keratometric astigmatism. Power vectors aid the visualization of complex changes in refractive error by tracing a trajectory in a uniform dioptric space. The Cartesian components of a power vector are mutually independent, which simplifies mathematical and statistical analysis of refractive errors. Power vectors also provide a natural link to a more comprehensive optical description of ocular refractive imperfections in terms of wavefront aberration functions and their description by Zernike polynomials.
authors.affiliation: School of Optometry, Indiana University, Bloomington, Indiana 47405, USA. thibos@indiana.edu
authors.forename: L N
authors.initials: LN
authors.lastname: Thibos
authors.forename: D
authors.initials: D
authors.lastname: Horner
mesh_terms: D001251:Astigmatism / Q000601:surgery*; D003315:Cornea / Q000601:surgery*; D003627:Data Interpretation, Statistical; D003941:Diagnostic Techniques, Ophthalmological; D006801:Humans; D009216:Myopia / Q000601:surgery*; D012029:Refraction, Ocular; D014792:Visual Acuity