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K. Irsch, B. I. Gramatikov, Y.-K. Wu, D. L. Guyton; Polarization Modulation Using Wave Plates to Enhance Foveal Fixation Detection in Retinal Birefringence Scanning for Strabismus Screening Purposes. Invest. Ophthalmol. Vis. Sci. 2009;50(13):4755.
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© ARVO (1962-2015); The Authors (2016-present)
To enhance foveal fixation detection while bypassing the deleterious effects of corneal birefringence in binocular retinal birefringence scanning (RBS) for strabismus screening purposes, a new RBS design was developed incorporating a double-pass spinning half wave plate (HWP) combined with a fixed double-pass wave plate (WP) into the optical system.
The incorporation of the spinning HWP effectively enables differential polarization detection with only one detector, and together with a fixed WP the differential polarization signal can be detected essentially independently of various amounts and orientations of corneal birefringence that occur in the population. Utilizing the measured corneal birefringence from a data set of 300 human eyes, an algorithm and related computer program were developed in MATLAB for optimizing the properties of both wave plates to statistically maximize the foveal fixation signal, while having the greatest independence from left and right eye corneal birefringence. The computer model was verified with experimental human data using an intermediate RBS-based eye fixation monitor.
Foveal fixation detection was optimized with the HWP spun 9/16 as fast as the circular scan, with the fixed WP having a retardance of 45° and fast axis at 90°. With this optimized RBS design, a significant statistical improvement of 7.3 times in signal strength, i.e. FFT power, was achieved for the available data set compared with the previous RBS design. Validation experiments confirmed the model’s ability to find the optimum amount of double-pass retardance necessary to yield maximal signal strength for the investigated range of corneal birefringence.
The computer-model-optimized RBS design greatly enhances foveal fixation detection. Combining a binocular eye fixation monitor based on this new RBS design with bull’s-eye focus detection promises to be both robust and sensitive in screening infants automatically and reliably for both of the primary causes of amblyopia: strabismus and defocus.
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