Desenvolvimento de um instrumento computadorizado para medida do poder refrativo da córnea (videoceratógrafo)

Desenvolvimento de um instrumento computadorizado para medida do poder refrativo da córnea (videoceratógrafo)


L. A. V. Carvalho,
S. A. Tonissi,
A. C. Romão,
L. E. Santos,
F. Yasuoka,
A. C. Oliveira,
P. Schor,
W. Chamon,
J. C. Castro


Arquivos Brasileiros de Oftalmologia

versão impressa ISSN 0004-2749versão On-line ISSN 1678-2925

Arq. Bras. Oftalmol. vol.61 no.6 São Paulo dez. 1998



We have developed the first Brazilian Videokeratograph (or less accurately, Corneal Topographer).


Placido discs painted on a conic pattern are reflected by the cornea. The images are magnified and focused on a CCD behind the cone. The CCD’s signal is sent to a frame grabber mounted on a compatible IBM PC. By means of digital image processing algorithms, border distances of the Placido discs are extracted from the digitized images. These values are put into an algorithm with mathematical models of the corneal curvature.


As in other topographers a color coded diopter plane map is plotted on the computer screen. For a set of 9 spheres of radius: 5.50, 5.75, 6.00, 6.25, 6.75, 7.00, 7.25, 7.50, 7.75 mm an error of 0.1 diopter for refractive power and 0.02 mm for curvature, was obtained. 5,760 points were analyzed for each sphere. Exams of 20 corneas were compared and an error of 0.2 diopters was obtained relative to an Eyesys topographer.


We may affirm that with our instrument results very similar to those of the topographer used for comparison are obtained.

Keywords: Corneal topography; Keratometry; Placido Discs


1 Schanzlin DJ, Robin JB. Corneal Topography-Measuring and Modifying the Cornea. Springer-Verlag 1991;vii.
2 Placido A. Novo Instrumento de Exploração da Cornea. Periodico d'Oftalmológica Practica 1880;5:27-30.
3 Troutman RC, Buzard KA. Corneal Astigmatism. Mosby Year Book, St. Louis.
4 Le Grand Y, El Hage SG. Physiological Optics, Springer Series in Optical Sciences, Springer-Verlag, 1980; capitulo 13.
5 Gullstrand A. In Helmholtz Treatise on Physiological Optics, Ed. 3, vol. 1.
6 Mandell RB, York MA. A New Calibration System for Photokeratoscopy. Am J Optom 1969;46(6):410-7.
7 Stone J. The Validity of Some Existing Methods of Measuring Corneal Contour Compared with Suggested New Methods. Brit J Physiol Opt 1962;19:205-30.
8 Knoll HA, Stimson R. New Photokeratoscope Utilizing a Hemispheral Object Surface. J Optical Soc Am 1957;47:221-2.
9 Mammone RJ, Gersten M, Gormley DJ, Koplin RS, Lubkin VL. 3D Corneal Modeling System. IEEE Trans Biomedical Eng 1990;37:66-73.
10 Wang J, Rice DA, Klyce SD. A New Reconstruction Algorithm for Improvement of Corneal Topographical Analysis. Refract. Corneal Surg 1989;5:379-87.
11 Doss JD, Hutson RL, Rowsey JB. Method for Calculation of Corneal Profile and Power Distribution. Arch Ofthalmol 1981;99:1261-5.
12 van Saarlos, Paul P, Constable IJ. Improved Method for Calculation of Corneal Topography for Any Photokeratoscope Geometiy. American Academy of Optometry 1991;68:960-5.
13 Halstead MA, Barsky BA, Klein SA, Mandell RB. Geometric Modeling of the Cornea Using Videokeratography. Mathematical Methods for Curves and Surfaces 1995;213-23.
14 Halstead MA, Barsky BA, Klein SA, Mandell RB. A Spline Surface Algorithm for Reconstruction of Corneal Topography from a Videokeratographic Reflection Pattem. Optom and Vis Sci 1995;72:821-7.
15 Wilson SE, Verity SM, Conger DL. Accuracy and Precision of the Corneal Analysis System and the Topographic Modeling System. Cornea 1992;1 l(l):28-35.
16 Klyce SD. Computer-Assisted Corneal Topography, High Resolution Graphics Presentation and Analyses of Keratoscopy. Invest Ofthalmol Vis Sci 1984;25:426-35.
17 Mandell RB, St Helen R. Mathematical Model of the Corneal Contour. Brit J Physiol Optics 1971;26:183-97.
18 Roberts C, Caracterization of the Inherent Error in a Spherically-Biased Corneal Topography System in Mapping a Radially Aspheric Surface. Refract Corneal Surg 1994;10:103-11.
19 Paranaguá E, Carvalho LAV, Castro JC. Construção de um Topógrafo de Cornea III Fórum Nacional de Ciência e Tecnologia em Saüde, Campos de Jordão-Outubro de 1996.
20 Koch DD, Foulks GN, Moran CT, Wakil JS. The Corneal Eyesys system: accuracy analysis and reproductivity of first generation prototype. J Refract Corneal Surgery 1989;6:423-9.