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Luis A V Carvalho, Valeria M Carvalho; CHROMATIC CORNEAL TOPOGRAPHY: COMPARISON WITH CONVENTIONAL PLACIDO DISC AND POSSIBLE ADVANTAGES FOR KERATOCONIC EYES. Invest. Ophthalmol. Vis. Sci. 2014;55(13):2470.
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© ARVO (1962-2015); The Authors (2016-present)
A chromatic placido disc was developed using previously developed techniques  and adapted to the optical head of a conventional corneal topographer. The objective was to compare conventional monochromatic placido disc corneal topography with color-coded (RGB - image processing) topography for regular shaped corneas and also for very distorted corneas (keratoconus).
40 right eyes of 40 healthy subjects, 20 males and 20 females, ages ranging from 12 to 62 years (signed consent - Helsinki convention), were measured using both a gold standard corneal topographer (Eyesys) and our chromatic Placido system (www.wavetek.com.br) and in-house software [2, 3]. Initial measurements on the Eyesys were used by 2 experts to classify the corneas in 2 groups: 20 severe keratoconus and 20 regular corneas.
RMSE for axial diopter values for calibration spheres when both systems were compared was 0.02 D, clearly demonstrating that for smooth surfaces both systems were equivalent. The RMSE for astigmatic surfaces was 0.05D and for Keratoconic corneas was 0.5D.
The RMSE deviation for spheres and regular corneas was much smaller than those for the keratoconic corneas. Our claim is that there must be some degree of overlapping of neighboring discs on the conventional monochromatic Placido images that is avoided by color-coded image segmentation. Results obtained here indicate that chromatic corneal topography may have certain advantages over conventional topography for distorted surfaces.  Belini T, Trevelin LC, Moreira FMA, Bagnato VS, Schor P, Carvalho LA, Development and preliminary results of a chromatic illumination system for indirect ophthalmoscopes. Arquivos Brasileiros de Oftalmologia, v. 72, p. 146-151, 2009.  Carvalho LA, Stefani M, Romão AC, Carvalho LAV, Castro J, Tonissi S, Schor P, Chamon W, Videokeratoscopes for dioptric power measurement during surgery. Journal of Cataract and Refractive Surgery, EUA, v. 28, p. 2006-2016, 2002.  Carvalho LA, On the Absolute Accuracy of Placido based Videokeratographs for Measuring the Optical Aberrations of the Cornea. Optometry and Vision Science, EUA, v. 81, n. 8, p. 616-628, 2004.
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