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Georges Goetz, Daniel Palanker, Tomas Cizmar; Holographic display system for photovoltaic retinal prosthesis. Invest. Ophthalmol. Vis. Sci. 2013;54(15):352. doi: https://doi.org/.
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© ARVO (1962-2015); The Authors (2016-present)
To restore sight to patients blinded by degenerative retinal diseases using photovoltaic stimulation of the surviving inner retinal neurons. We present a holographic display system that is used to deliver visual information with high efficiency by holographic control of light incident onto a subretinal silicon photodiode array. The pulses of near-infrared (NIR, 880-915nm) light are converted into bi-phasic pulses of electric current in each pixel.
For animal studies, a spatial light modulator (SLM) illuminated by a collimated laser beam is mounted on a slit lamp. A concave 100mm lens is used as the Fourier lens and the holographic image is projected onto the retina using a 60mm convex lens, while its corneal curvature is canceled by a contact lens. Holograms are computed using the Gerchberg-Saxton algorithm.
We show that the holographic approach allows creating an image with contrast over 200:1 for contour images and power transmission efficiency to the image of ~60%, independent of the pattern projected. To avoid image degradation by the remaining ~40% of light in the 0th diffraction order, the SLM is illuminated with a diverging beam, thereby defocusing the 0th order while the 1st order modulation comprises a Fresnel lens cancelling the beam divergence. The use of an LCD screen can provide comparable contrast and similar maximum efficiency. However, depending on the pattern projected, the transmission efficiency of LCD system can decrease to below 1% for images with large dark areas. An inherent limitation of holographic control of coherent light is the presence of speckles. Due to the very fast response of photodiodes in the implant, speckles cannot be smeared out by consecutive display of different holograms. However, it is possible to make the speckles small enough that they do not cause shadowing issues on the diodes. For both, holographic and LCD imaging, it is possible to create patterns with far greater resolution than what the implant can resolve.
Holographic display provides a compact, efficient, high-brightness solution for delivery of visual information to a photovoltaic retinal prosthesis, with power supply and image processing provided by a smartphone-sized device.
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