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J. M. Miller, J. Schwiegerling, E. DeHoog, V. Dobson, C. E. Clifford-Donaldson, E. M. Harvey; Open-Field Keratometer for Infant Vision Research. Invest. Ophthalmol. Vis. Sci. 2009;50(13):3987.
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© ARVO (1962-2015); The Authors (2016-present)
To describe a novel design of an infant keratometer that incorporates open-field binocular viewing and on-axis illumination with a grid pattern, for study of higher-order aberrations of the eye and determination of corneal astigmatism.
Infrared (940 nm) light emitting diodes in a grid array reflect off a partially (10%) IR-reflective mirror to illuminate the cornea. The cornea is imaged is through the first mirror and two additional IR mirrors with a telecentric lens fitted to a Sony Digital Camcorder. The system is hand-held, weighs 1.5 kg, and is battery powered. The optical head is completely enclosed and the working distance (optical faceplate to eye) is 25 mm. Calibration is performed using a novel telecentric calibration device which presents known image size reference points and image rays limited to those parallel to the optical axis. Images were analyzed from 5 steel balls having equivalent corneal reflecting power of 35.4, 42.5, 44.7, 53.1 and 70.9 Diopters (using the keratometric conversion of 337.5 Diopters /mm mirror radius).
Two grid reference points were selected and used for all image analysis, resulting in corresponding images sizes of 17.8, 14.5, 13.8, 12.9 and 8.6 pixels). As keratometric image size is proportional to corneal reflecting power, this relation should be linear. Regression analysis demonstrated a correlation coefficient of r-squared of 0.93. The telecentric calibration image demonstrated that for the images analyzed, telecentric imaging was obtained, allowing for head motion without associated magnification change.
The instrument offers several potential advantages for infant vision research including open-field viewing and on-axis measurement of the cornea. The open-field view allows better interaction with the infant. On-axis measurement directly over the pupil allows measurement of the portion of the cornea actually used for image formation. By incorporating a grid illumination design, we anticipate being able to measure higher-order aberrations of the eye if they are of sufficient magnitude to be detected with a relatively small number of centroids. Use of a rectangular grid permits mathematical approaches for Shack-Hartmann sensor analysis to be used in estimating these aberrations.
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