title: Videokeratoscopes for dioptric power measurement during surgery.
abstract: OBJECTIVE
  To evaluate 2 versions of a computerized surgical videokeratoscope that measures a central region of the cornea of approximately 4.0 to 6.0 mm in diameter to provide information on dioptric power and astigmatism.
  
  
  METHODS
  Grupo de Optica Oftalmica Universidade de São Paulo, and Departamento de Oftalmologia da Escola Paulista de Medicina, São Paulo, Brazil.
  
  
  METHODS
  The first videokeratoscope system with 10 Placido rings is based on a 15 000 fiber-optic illuminated disk attached to the objective lens of a surgical microscope. With this system, the light intensity can be adjusted during surgery for better contrast of the Placido image. The second system with 14 Placido rings is based on a neon light source behind the Placido disk. With both systems, a 480 x 640 pixel resolution charge-coupled device camera and an IBM-compatible personal computer frame grabber were used. Image processing was used for boundary detection of the rings. An axial curvature algorithm based on the spherical surfaces was used to calculate the dioptric power for each examination. Measurements of 4 real spherical surfaces and 4 simulated aspheric surfaces (ellipsotoric surfaces with different apical radii and different shape factors) were performed.
  
  
  RESULTS
  Twenty corneas of 10 healthy volunteers were measured on both videokeratoscope systems and an EyeSys System 2000 corneal topographer. The root- mean-square error for the spherical and simulated aspheric surfaces was, respectively, less than 0.20 diopter (D) and 1.30 D for the 10-disk videokeratoscope system and less than 0.16 D and 1.40 D for the 14-disk system. The mean deviation in corneal measurements with both systems was 0.09 mm for the radius of curvature, 0.51 D for dioptric power, and 5 degrees for cylinder.
  
  
  CONCLUSIONS
  The results indicate that the 2 surgical videokeratoscopes are sufficiently precise to aid the anterior segment surgeon in reducing residual astigmatism in cataract surgery and keratoplasty.
authors.affiliation: Instituto de Física de São Carlos, Grupo de Optica, Universidade de São Paulo, São Carlos, Brazil. lavcf@if.sc.usp.br
authors.forename: Luis A
authors.initials: LA
authors.lastname: Carvalho
authors.forename: Marcos
authors.initials: M
authors.lastname: Stefani
authors.forename: Antonio Carlos
authors.initials: AC
authors.lastname: Romão
authors.forename: Luis
authors.initials: L
authors.lastname: Carvalho
authors.forename: Jarbas C
authors.initials: JC
authors.lastname: de Castro
authors.forename: Silvio
authors.initials: S
authors.lastname: Tonissi
authors.forename: Paulo
authors.initials: P
authors.lastname: Schor
authors.forename: Wallace
authors.initials: W
authors.lastname: Chamon
mesh_terms: D000328:Adult; D001251:Astigmatism / Q000175:diagnosis; D019781:Corneal Topography / Q000295:instrumentation / Q000379:methods*; D003936:Diagnosis, Computer-Assisted; D005260:Female; D006801:Humans; D007432:Intraoperative Period; D008297:Male; D018715:Microscopy, Video; D008962:Models, Theoretical; D013508:Ophthalmologic Surgical Procedures; D055095:Optics and Photonics