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Kristina Irsch, Boris I. Gramatikov, Yi-Kai Wu, David L. Guyton; Polarization modulation improves retinal birefringence scanning. Journal of Vision 2008;8(17):61. doi: https://doi.org/10.1167/8.17.61.
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To increase the signal-to-noise ratio in binocular retinal birefringence scanning (RBS) for strabismus screening purposes, we add a double-pass spinning half wave plate (HWP) to the optical system. In RBS, polarized near-infrared light is directed onto the retina in a circular scan, with a fixation point in the center, and the polarization-related changes in light retro-reflected from the ocular fundus are analyzed by means of differential polarization detection.1 Due to the radially symmetric arrangement of the Henle fibers about the fovea, a characteristic frequency that is a multiple of the scanning frequency appears in the obtained periodic signal when the scan is exactly centered on the fovea, which is the case with central fixation. The incorporation of the spinning HWP effectively enables differential polarization detection with only one detector. The orthogonal polarization states are measured on alternate scans rather than simultaneously, and the differential signal is calculated digitally by subtracting the acquired signal from itself, shifted by one period. By spinning the HWP at a fractional frequency of the scan, the RBS signal can be modulated so that a strong “multiple of half” frequency component is generated, whereas internal reflections causing optical noise still repeat over a complete scanning cycle. Thus, the RBS signal doubles in amplitude, whereas much of the background noise is eliminated with differential subtraction. Combined with our technique for focus detection, this new design promises to provide an effective screening instrument for automatic identification of infants at risk for amblyopia, the leading form of vision loss in childhood.
GuytonD. L.HunterD. G.PatelS. N.SandruckJ. C.FryR. L. (2000). Eye fixation monitor and tracker. U.S. Patent 6027216.
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