title: Peripheral light-focusing: measurement reliability and correlations with ocular dimensions.
abstract: OBJECTIVE
  When a beam of light is directed toward the temporal limbus from an angle of about 120 degrees temporal to the primary line of sight, an illuminated patch appears on the sclera at the nasal limbus. An optical apparatus was developed for measuring the angular range of this optical phenomenon, often called the temporal catchment angle. It is possible that the temporal catchment angle or its correlates could serve as a key risk factor for the development of pterygia or cortical cataract.
  
  
  METHODS
  We built an apparatus that allows variation in the angle of a beam directed toward the temporal limbus. Two examiners measured the angular catchment range over which peripheral light focusing occurred for the right eyes of 30 subjects. Other anterior segment dimensions, including palpebral aperture height, refractive error, central and peripheral corneal power, anterior chamber depth, and corneal diameter, were measured.
  
  
  RESULTS
  There was no statistically or clinically significant difference between the repeated measures of the examiners. The mean temporal catchment angle was 17.1 degrees (SD, 6.5). The intraobserver and interobserver 95% limits of agreement were 6.2 and 9.7 degrees, respectively. The mean posterior limit was 121.0 degrees (SD, 5.5), and the mean anterior limit was 104.1 degrees (SD, 6.5) from the primary line of sight. The temporal catchment angle was positively correlated with central corneal power (R(2) = 0.18; p = 0.02), peripheral corneal power (R(2) = 0.18; p = 0.01), anterior chamber depth (R(2) = 0.29; p = 0.002), and corneal diameter (R(2) = 0.37; p = 0.002). Using multivariate linear regression analysis, we determined that central corneal power in the horizontal meridian and corneal diameter, in combination, best predicted the magnitude of the temporal catchment angle (R(2) = 0.37; p = 0.0001).
  
  
  CONCLUSIONS
  We have developed a reliable method of measuring the temporal catchment angle using a new head-mounted instrument. The temporal catchment angle was highly correlated with central corneal power and corneal diameter, in combination. The temporal catchment angle or its correlated ocular dimensions could be used to investigate risk factors for pterygia and cortical cataract.
authors.affiliation: University of Arizona Department of Ophthalmology, Tucson, AZ, USA.
authors.forename: J Daniel
authors.initials: JD
authors.lastname: Twelker
authors.forename: Shauna C
authors.initials: SC
authors.lastname: Harbison
authors.forename: Ian L
authors.initials: IL
authors.lastname: Bailey
mesh_terms: D003315:Cornea / Q000033:anatomy & histology / Q000502:physiology*; D004867:Equipment Design; D006801:Humans; D007908:Lens, Crystalline / Q000502:physiology*; D008027:Light; D016850:Limbus Corneae / Q000502:physiology; D009907:Optometry / Q000295:instrumentation*; D012044:Regression Analysis; D015203:Reproducibility of Results