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H. Song, Y. Chui, Y. Zhao, X. Qi, S. A. Burns; Measurement of Depolarization of Light in the Retina Using an Adaptive Optics Scanning Laser Ophthalmoscope. Invest. Ophthalmol. Vis. Sci. 2007;48(13):4253.
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© ARVO (1962-2015); The Authors (2016-present)
To investigate light tissue interactions in the retina using high magnification adaptive optics and near infrared imaging. While most light returning from the retina retains polarization, light which has been multiply scattered can be depolarized. To date there is little understanding of how this affects imaging at the microscopic level. For this reason, we modified an adaptive optics scanning laser ophthalmoscope (AOSLO) to measure the Stokes vector of light returning from the retina, when it is illuminated with light of known polarization state.
We used the Indiana AOSLO to image the cones in near infrared light (790 nm). The illuminating light was vertically linearly polarized, and a polarization analyzer was placed before the detector. The analyzer consisted of a zero-order quarter wave plate followed by a linear grid polarizer, and then an avalanche photodiode. By rotating the quarter wave plate and using a least squares technique we measured the Stokes vector (polarization state) of light returned from the retina for each point in the image. Images of the retina of a field size of 1.3 deg were digitized at 512 x 512 pixels. Wavefront sensing and control were performed using a 678 nm light at a constant polarization state. The system was tested with specular and diffusing targets, as well as with human eyes.
Specular reflections from glass bead targets were polarization preserving as expected. When paper was imaged, the image was primarily made up of depolarized light, although small bright regions of the paper were highly polarization preserving. In the human images, cones were readily imaged in the perifoveal regions. The image intensity for different features depended on the relation between input polarization and the quarter wave plate angle. With the larger confocal aperture cone photoreceptors preserved more than 50% of the polarization while light from regions shadowed by blood vessels (and therefore expected to primarily consist of multiply scattered light) was 90% depolarized.
We have adapted a high resolution AOSLO for Stokes vector measurements of light returning from the retina. Forming images based on different polarization properties allows us to change the relative contrast of different target features in the retina.
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