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Eli Peli; Multiplexing Prism: A novel optical element for visual field expansion. Invest. Ophthalmol. Vis. Sci. 2014;55(13):4128.
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© ARVO (1962-2015); The Authors (2016-present)
Prisms for visual field loss shift images into the seeing field; however, they also block part of that field (apical scotoma), resulting in field substitution rather than expansion. When fitted unilaterally in binocular patient, the other eye sees the images that fall into the apical scotoma, giving true expansion which is dependent on binocular visual confusion, tolerable in peripheral vision. However, binocular confusion is impossible in monocular patients or in conditions with no overlap of the seeing fields of the two eyes. A new optical element, called a multiplexing prism (MxP), addresses these limitations of conventional prisms in a number of applications.
MxPs provide simultaneous shifted and unshifted views (i.e., multiplexed) by inserting flat areas between the prismatic segments of a Fresnel prism. This results in monocular visual confusion and thus monocular field expansion. The ratio of flat and prismatic area controls the relative contrast of the two views. Prototype MxPs of varying ratios (1:1, 1:2, etc.) and in powers of 30, 40 and 57 prism diopters (Δ) were developed and evaluated. Novel designs were evaluated for field expansion of patients with: tunnel vision with one or two functional eyes, for hemianopic patients with only one eye, and for patients that lost vision in one eye. The temporal field of normally sighted people can be expanded by placing MxP at the far temporal edge of the spectacle lenses for athletic, road safety, or military purposes. Conventional prisms and MxPs were perimetrically compared for these designs.
In all tested conditions the conventional prism resulted in field substitution (field expansion with equivalent field loss elsewhere) while the MxPs provided field expansion without an apical scotoma. The unshifted view through the prism was natural and continuous. When viewing directly through the MxP, visual confusion was apparent, but in peripheral locations it was not noticeable. With 57Δ MxP, the nasal periphery of a monocular patient was expanded up to 100° and the temporal periphery of normally sighted up to 145°. This higher field expansion was achieved due to minification effects of the prisms.
MxP is a new device that may be applied to provide field expansion in a wide range of conditions. While early perimetric results are promising, testing in more natural conditions (with monocular rivalry) is needed to establish the utility of the MxPs.
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