title: The effect of axial length on laser spot size and laser irradiance.
abstract: OBJECTIVE
  To determine the effect of the axial length of the eye on laser spot size and irradiance.
  
  
  METHODS
  Experimental study using a calibrated Gullstrand-type model eye.
  
  
  METHODS
  The model eye, which was fitted with a scale of half circles in the center of the artificial fundus, was first examined using 2 different fundus imaging systems, then with a setup of a slitlamp, 2 indirect condensing laser lenses, and a laser unit with a spot size of up to 7 mm. The axial length of the model eye was set to different values ranging from 20 to 31 mm, and the magnifications of the fundus imaging systems and the laser lenses were calculated and compared for a treatment spot with a diameter of 4 mm. The laser irradiance for treating the spot at different axial lengths was also recorded.
  
  
  RESULTS
  Whereas the magnification of a fundus imaging system is inversely related to the axial length, the laser spot size is directly related to axial length when using indirect condensing laser lenses. Therefore, the changes of magnification produced by axial ametropia are mostly compensated, so that the intended size of the treatment spot is obtained even in eyes with a high axial ametropia. The laser irradiance, on the other hand, has a significant variation for the observed range of the axial length.
  
  
  CONCLUSIONS
  Axial length has a significant effect on laser spot size and laser irradiance.
  
  
  CONCLUSIONS
  The effect of axial length on laser spot size and laser irradiance may be ignored when administering photodynamic therapy with verteporfin but has to be considered for transpupillary thermal treatment of choroidal neovascular lesions.
authors.affiliation: Department of Ophthalmology, University of Vienna Medical School, Allgemeines Krankenhaus, Währinger Gürtel 18-20/8i, A-1090 Vienna, Austria. michael.stur@akh-wien.ac.at
authors.forename: M
authors.initials: M
authors.lastname: Stur
authors.forename: S
authors.initials: S
authors.lastname: Ansari-Shahrezaei
mesh_terms: D020256:Choroidal Neovascularization / Q000188:drug therapy / Q000209:etiology; D005123:Eye / Q000033:anatomy & histology*; D006801:Humans; D007834:Lasers; D008268:Macular Degeneration / Q000150:complications; D008954:Models, Biological; D009887:Ophthalmoscopy; D010778:Photochemotherapy; D010781:Photography / Q000379:methods; D011166:Porphyrins / Q000627:therapeutic use; D000077362:Verteporfin
chemical_list: D011166:Porphyrins; D000077362:Verteporfin