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H. Song, Z. Zhong, X. Qi, A.E. Elsner, S.A. Burns; Dependence of High Resolution Cone Images on Polarization . Invest. Ophthalmol. Vis. Sci. 2006;47(13):4061.
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© ARVO (1962-2015); The Authors (2016-present)
To probe macular photoreceptors using polarized light and high contrast, high magnification imaging. In the visible portion of the spectrum, light guided by photoreceptors back through the pupil largely retains polarization. For near infrared light, .there is decreased absorption of light by the RPE and choroid. This light, because of multiple scattering, has a lower degree of polarization. If light which has penetrated these deeper retinal layers is collected by the cones and guided towards the pupil, then cone images should have a low preservation of polarization in the near infrared. To test this, we used a near infrared, adaptive optics scanning laser ophthalmoscope (AOSLO) to measure the polarization of the returning light for both tightly confocal images and less confocal images.
We used the Indiana AOSLO to image the cones in near infrared light (830 nm). The illumination light was linearly polarized, and an analyzer was placed before the detector. Images of the retina were digitized at 512 x 512 pixels and a field size of 1.3 deg. Intensity and contrast were measured for both small confocal (100 microns or 1.2x the size of the Airy disc) and large confocal (1 mm) apertures. Wavefront sensing and control were performed with a separate wavelength (678 nm) and an invariant polarization state.
Cones were readily imaged both in the parafoveal and perifoveal regions. Image intensity was strongly dependent on the relation between input polarization and the analyzer position. In general the image could be attenuated by as much as 65%, suggesting that the preservation of polarization was greater than this. When small regions containing individual bright cones were analyzed, the polarization dependent modulation increased to more than 85%. Increasing the confocal aperture diameter includes more widely scattered light, and in this condition cone average background modulation decreased, but the cones continued to show large preservation of polarization.
We conclude that light returning from the cone photoreceptors in near infrared light is highly polarization preserved. This is consistent with these images forming from light that has entered the photoreceptor waveguides, traversed the cones, and been directly reflected either within the outer segments or in a coupled mode at the base of the cone outer segments. This portion of the light has not been multiply scattered.
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