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J. M. Miller, E. M. Harvey, V. Dobson; Infant Keratoscopy Ring Keratoscope Error Analysis. Invest. Ophthalmol. Vis. Sci. 2010;51(13):5280.
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New Title: Infant keratoscopy ring keratoscope error analysis
Handheld, camera mounted ring keratoscopes measure infant astigmatism at working distances exceeding 100 mm. The corneal reflection of the ring is recorded and analyzed. The system is calibrated at best-focused vertex distance. Handheld instruments have uncontrolled vertex distances, resulting in measurement errors.We evaluated magnification errors in un- and collimated light source ring keratoscopes.
Two ring keratoscopes were compared when mounted on a telecentric camera lens with best focus at 152 mm. One keratoscope, using uncollimated light sources, had a ring of 12 IRLEDs mounted in a 185 mm diameter ring. The other keratoscope used 12 IRLEDs with collimating lenses at the same equivalent angle. Six steel balls were imaged in five positions over a 25 mm range centered at best focus. Vertex distance induced keratometric power measurement errors for each ball was determined.
For uncollimated sources, there was a linear relation between mean keratometric diopter (D) power measurement error and vertex distance variation that increased with increasing keratometric power (Table). This error averaged 0.2 D/mm defocus and. The error rate was linearly related to keratometric power. Regression analysis indicated that defocus errors have smaller effect on keratometric astigmatism (0.004 D/mm error change per D Keratometric power, p=0.009), resulting in relatively accurate measurement of astigmatism. For example, a 44 D cornea having 4 D astigmatism, imaged 12 mm either side of best focus, would have defocus errors of +/- 2.4 D (12 mm *0.2 D/mm) in mean corneal power, but only +/- 0.19 D (12 mm * 0.004 (D/mm)/D * 4D) error in astigmatism measurement. These systematic errors are not observed with collimated sources.
When uncollimated light sources are used in ring keratoscopy, corneal power measurement varies with vertex distance. Use of collimated light sources permits accurate measurement of both mean keratometric power and astigmatism in handheld keratoscopy.
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