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Widefield ocular fundus imaging is conventionally performed in a reflection geometry. In this configuration, back-reflections from inner retinal layers, such as the nerve fiber layer, the inner limiting membrane, or even the anterior walls of large blood vessels, are often encountered, and may obscure the visibility of deeper features. Moreover, spectroscopic quantification of endogenous chromophores is complicated since the final image is a summation of reflections from several fundus layers (i.e. no single absorption pathlength can safely be assumed). Researchers have sought to model the fundus reflections, however the models are sensitive to the populations used and particular imaging platform. In theory, unwanted superficial reflections could be avoided and light path modeling could be simplified by adopting a transmission imaging geometry. We present an alternative transillumination fundus imaging strategy based on deeply penetrating near-infrared (NIR) light delivered transcranial near the subject’s temple. A portion of this light diffuses through bone and illuminates the posterior eye not from the front, as with conventional methods, but rather mostly from behind. As such, we image light transmitted through the fundus rather than back-reflected off multiple fundus layers. This single-pass measurement geometry simplifies absorption pathlength considerations and provides complementary information to fundus reflectometry. The use of NIR light enables imaging as deep as the choroid. Importantly, the technique is compatible with reflection-based techniques and we have shown that it works well with a commercial non-mydriatic fundus camera. Combining information from these two illumination approaches should improve spectroscopic analysis of the fundus.
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