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B.I. Gramatikov, K. Tarczy-Hornoch, D.S. Nassif, D.G. Hunter, D.L. Guyton; Improved Fixation Target for Accommodation in a Focus-detection Photoscreener . Invest. Ophthalmol. Vis. Sci. 2003;44(13):4834.
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Purpose: Screening at a near fixation distance for uncompensated refractive error may be a better method to detect amblyopia-prone children than measurement of distance refractive error. A double-pass focus detection device using a differential bulls-eye photodetector appears useful for this purpose. But the fixation target must stimulate natural accommodation in normal subjects, a requirement not met by the original monochromatic (785 nm) point source target in our Pediatric Vision Screener. We wished to develop a fixation target eliciting more reliable accommodation. Methods: A bulls-eye photodetector, conjugate via a beam splitter to an 830 nm point light source (a Super Luminescent Diode (SLD) at a fixation distance of 40 cm), was used to assess the goodness of double-pass focus of subjects' eyes viewing monocularly. A white LED, rear-illuminating a random black-line figure, was mounted near the optical axis at a distance from the eye which compensated for the measured longitudinal chromatic aberration of the eye, such that the 830 nm point source and white LED target appeared equally clear at the same best focus for each eye tested. The white LED target subtended a visual angle of 0.6° and was displaced 6° horizontally from the direction of the 830 nm target. The goodness of focus with each color target was then determined for one eye each of 5 young adult subjects, ages 26 to 31, using added trial lenses from -5.00 D to +5.00 D on each side of the highest power plus lens that gave best subjective focus. The resulting "focus curves" were analyzed. Results: The chromatic refractive difference between the two targets measured 1.00 D for all subjects, to the nearest 0.25 D. The white LED target was therefore positioned at 28.6 cm from the eye, 1.00 D closer than the 830 nm target at 40 cm. All focus curves for each target showed some accommodation when minus lenses were added to the highest power plus lens that gave best subjective focus. Subjectively, for all subjects, accommodating to the white LED target was reported easier and more natural than accommodating to the 830 nm target. With the white LED target, areas under the focus curves were 46 % +/- 33 %(SD) larger than with the 830 nm target (p=0.022). Conclusions: A white LED "accommodative" fixation target is superior to an 830 nm SLD point source for stimulating accommodation in a focus-detection photoscreener, at least in young adults under monocular conditions at near fixation.
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