title: The effective corneal refractive surface as a function of a point in visual space: a three-dimensional analysis.
abstract: OBJECTIVE
  To develop a three-dimensional optical model of the anterior segment which will provide a basis for understanding the effects of corneal and adnexal problems on vision.
  
  
  METHODS
  A three-dimensional optical model of the anterior segment was developed to calculate the effective corneal refractive surface (ECRS), which is a specific area of cornea that refracts incident light rays arising from an object in visual space through the physical pupil (PP); light rays refracted by cornea outside the ECRS do not traverse the PP. This model incorporated the patient variables of central anterior chamber depth (A) and central corneal curvature (K). A vector analysis was combined with three-dimensional analytic geometry to create a unified solution for all object locations in visual space. A computer program in BASIC on a PC was developed to perform the calculations.
  
  
  RESULTS
  The ECRS was circular for objects on the line of sight, but with increasing displacement from the fixation point it became oval and eventually crescent shaped. The size of the ECRS increased as the distance of the object from the eye increased, but it rapidly reached a limiting value at about 200 mm. The size of the ECRS increased with PP diameter, but showed only a small dependence on A and minimal dependence on K.
  
  
  CONCLUSIONS
  This comprehensive optical model of object space and the anterior segment of the eye allows clinicians to better understand the effects of corneal light transmission on vision.
authors.affiliation: Department of Ophthalmology and Visual Sciences, Texas Tech University Health Sciences Center, Lubbock, TX, USA.
authors.forename: Kenn A
authors.initials: KA
authors.lastname: Freedman
authors.forename: Sandra M
authors.initials: SM
authors.lastname: Brown
mesh_terms: D000867:Anterior Chamber / Q000033:anatomy & histology; D003315:Cornea / Q000033:anatomy & histology* / Q000502:physiology / Q000601:surgery; D019781:Corneal Topography; D006801:Humans; D021621:Imaging, Three-Dimensional; D008954:Models, Biological; D012029:Refraction, Ocular / Q000502:physiology; D014794:Visual Fields / Q000502:physiology*