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Diego Rativa Millan, Brian Vohnsen; Measuring Individual Cone Directionalities Using Scanning Laser Ophthalmoscopy. Invest. Ophthalmol. Vis. Sci. 2012;53(14):5602.
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Cone photoreceptors have a limited acceptance angle due to their waveguide structure that changes from fovea to parafovea cones. To determine their individual directionality a scanning laser ophthalmoscope (SLO) can be used which is the purpose of this study. The obtained values are analyzed and compared to the model results for light coupling between a focused scanning beam and an individual cone waveguide as well as a fiber-based photoreceptor simulator.
We have used a non-commercial SLO to acquire retinal images of both authors at a frame rate of 47fps while translating a movable entrance pupil of ∼ 1.6mm across the plane of the subject’s pupil. A full and fast pupil scan method is obtained, such that 100 steps of 70μm during a lapse time of 2 seconds allows obtaining an accurate fit of the apodization effect caused by the finite acceptance angle of the cones. Directionality values are obtained both for individual and groups of cones at different retinal eccentricities.
The individual directionality values range from 0.08 to 0.12mm–2 at 2.5° of eccentricity and increase approximately up to 0.35 mm–2 at 20° where it is expected that cone diameters are matched to the spot size of the incident beam. When a region containing a group of cones is analyzed the directionality decreases to approximately 0.12mm–2 remaining approximately constant at different eccentricities. The difference between the averaged and individual directionality values may plausible be explained by the difference in pointing direction for each cone that when averaged creates a broadening of the collective acceptance angle.
We have demonstrated a fast SLO imaging method to measure individual and collective cone-photoreceptor directionalities. As demonstrated by theory and experimental values, the directionality is highly dependent on the light coupling mechanism of the incident beam to the photoreceptor.
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