title: Non-cycloplegic spherical equivalent refraction in adults: comparison of the double-pass system, retinoscopy, subjective refraction and a table-mounted autorefractor.
abstract: OBJECTIVE
  To evaluate the accuracy of spherical equivalent (SE) estimates of a double-pass system and to compare it with retinoscopy, subjective refraction and a table-mounted autorefractor.
  
  
  METHODS
  Non-cycloplegic refraction was performed on 125 eyes of 65 healthy adults (age 23.5±3.0 years) from October 2010 to January 2011 using retinoscopy, subjective refraction, autorefraction (Auto kerato-refractometer TOPCON KR-8100, Japan) and a double-pass system (Optical Quality Analysis System, OQAS, Visiometrics S.L., Spain). Nine consecutive measurements with the double-pass system were performed on a subgroup of 22 eyes to assess repeatability. To evaluate the trueness of the OQAS instrument, the SE laboratory bias between the double-pass system and the other techniques was calculated.
  
  
  RESULTS
  The SE mean coefficient of repeatability obtained was 0.22D. Significant correlations could be established between the OQAS and the SE obtained with retinoscopy (r=0.956, P<0.001), subjective refraction (r=0.955, P<0.001) and autorefraction (r=0.957, P<0.001). The differences in SE between the double-pass system and the other techniques were significant (P<0.001), but lacked clinical relevance except for retinoscopy; Retinoscopy gave more hyperopic values than the double-pass system -0.51±0.50D as well as the subjective refraction -0.23±0.50D; More myopic values were achieved by means of autorefraction 0.24±0.49D.
  
  
  CONCLUSIONS
  The double-pass system provides accurate and reliable estimates of the SE that can be used for clinical studies. This technique can determine the correct focus position to assess the ocular optical quality. However, it has a relatively small measuring range in comparison with autorefractors (-8.00 to +5.00D), and requires prior information on the refractive state of the patient.
authors.affiliation: Centre for Sensors, Instruments and Systems Development (CD6), Technical University of Catalonia, Terrassa, Barcelona 08222, Spain.
authors.forename: Meritxell
authors.initials: M
authors.lastname: Vilaseca
authors.forename: Montserrat
authors.initials: M
authors.lastname: Arjona
authors.forename: Jaume
authors.initials: J
authors.lastname: Pujol
authors.forename: Elvira
authors.initials: E
authors.lastname: Peris
authors.forename: Vanessa
authors.initials: V
authors.lastname: Martínez
keywords: accuracy; autorefraction; double-pass system; optical quality; repeatability; retinoscopy; subjective refraction; trueness