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F. Zhou, D.T. Miller, L.N. Thibos, A. Bradley; Validation of a Combined Corneal Topographer and Aberrometer Based on a Shack-Hartmann Wavefront Sensor . Invest. Ophthalmol. Vis. Sci. 2003;44(13):4071.
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© ARVO (1962-2015); The Authors (2016-present)
Purpose: Our aim was to validate the repeatability and accuracy of the 2nd generation Indiana Corneal Topographer /Aberrometer based on Shack-Hartmann wavefront sensing technology. Methods: The combined aberrometer /topographer measures whole-eye aberrations in a conventional way using light reflected from the fundus. Corneal topography and surface aberrations were measured by light reflected from the anterior cornea. The primary improvement of our 2nd generation SH Corneal Topographer over that described previously (ARVO 2000) was the use of a custom-designed, aberration free objective lens with focal length = 36mm and f # = 1.0 and real time wavefront analysis. Refractive aberrations of the cornea were determined from surface topography by computer ray tracing of an optical model of test eyes. Measurements were obtained on five model eyes with known ellipsoid surfaces (apical radius r = 7.3 to 8.3mm, conic shape factor p = 0.5 to 1.3) and two pupil sizes (5.0 and 6.2 mm). Repeatability was evaluated from nine measurements taken continuously within seconds and averaged to get means for each model eye. Accuracy was evaluated by comparing theoretical prediction with the measured surface profile (sag), r and p values. Whole-eye aberrations computed from surface topography were then compared with theoretical predictions and with measured whole-eye aberrations. Results: Maximum sag errors for 6.2 mm pupil ranged from 0.15~1micron for five model eyes, which is within the manufacturer's tolerance specifications. For 5mm pupil, the range was from 0.15~0.48micron . The RMS sag errors ranged from 0.09~0.36micron (mean = 0.18micron). Mean difference between measured apical radius r and manufacturer’s specification was 0.0094 mm and for p-values was 0.033. For 6.2 mm pupil, estimates of spherical aberration coefficient Z(4, 0) inferred from corneal topography differed from theoretical predictions over the range 0.0031~0.049micron (mean =0.0207micron). Differences between whole-eye measurements and calculations from surface topography ranged from 0.0117~0.04micron (mean = 0.026micron). Mean differences were about half these values for a 5.0 mm pupil. Variability between repeated measurements was small compared to these differences. Conclusions: SH corneal topographer can accurately measure the topography of model eyes with aspheric surfaces typical of human corneas. Refractive aberrations derived from measurements of topography agree with whole-eye aberrometry to within a small fraction of a wavelength.
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