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Optical Prism Designs Expands Vision Field for the Sight-Impaired

Photonics.com
May 2016
BOSTON, May 24, 2016 — A series of novel optical designs may address the limitations of current peripheral prism eyeglasses and help further expand the field of vision for patients with hemianopia, a condition in which the visual fields of both eyes are cut in half.

Current high power prism segments embedded in a regular spectacle lens can expand the upper and lower visual fields of patients by as much as 30°. However, this technology does not expand the central area of the visual field.

By embedding the current prism in a spectacle lens that has prismatic power in the opposite direction, the image shifting effect is increased by the summation of the power of both prism types.
By embedding the current prism in a spectacle lens that has prismatic power in the opposite direction, the image shifting effect is increased by the summation of the power of both prism types. This design allows for up to 36° of expansion to the visual field on the patient's blind side. This design permits 5° of scanning range to the blind side with full effect. Courtesy of Massachusetts Eye and Ear.

With the goal of increasing the visual field provided by current technology, researchers from the Schepens Eye Research Institute of Massachusetts Eye and Ear and Harvard Medical School explored new optical techniques to create even higher power image shifting devices designed to bend the light farther than the 30° limit of conventional prisms. They developed, implemented and tested three novel eyeglass designs aimed at increasing the prism power to reduce the central gap in the vision field while maintaining wide lateral expansion.

One design involved inserting the peripheral prisms into carrier lenses that included yoked prisms in the opposite directions. By embedding the current prism in a spectacle lens that had prismatic power in the opposite direction, the image shifting effect increased by the summation of the power of both prism types. This design allows for up to 36° of expansion to the visual field on the patient's blind side and permits five degrees of scanning range to the blind side.

A second design involved the use of bi-part double Fresnel prisms. Two Fresnel segments are combined, attached at the base and angled to each other to increase the power of the peripheral prism. This design allows for up to 43° of expansion to the visual field on the patient's blind side and an increase to 14° of scanning range to the blind side.

To increase the power of the peripheral prism, the bi-part double Fresnel prism combines two prism segments angled to each other.
To increase the power of the peripheral prism, the bi-part double Fresnel prism combines two prism segments angled to each other. This design allows for up to 43° of expansion to the visual field on the patient's blind side and an increase to 14 degrees scanning range into the blind side. Courtesy of Massachusetts Eye and Ear.

For their third design, the researchers created Fresnel prism-like segments from nonparallel periscopic mirror pairs (reflective prisms). The pair of angled mirrors was used to deflect the image from the blind side to the seeing side — not unlike prism correction. Due to the mirror-based design, this device is distortion-free and does not suffer from the color splitting effect of prisms, which can reduce image clarity. The mirror-based periscopic prism is not yet fully manufactured, but may allow for up to 40° of expansion to the visual field on the patient's blind side and permit a much wider scanning range.

"The new optical devices can improve the functionality of the current prism devices used for visual field expansion and may find use in various other field expansion applications such as a mobility aid for patients with tunnel vision," Dr. Eli Peli said. "It's not just that we need a device with a higher angle of light shifting to let patients see farther. We also want the new devices to provide the additional range of vision when the patient scans their eyes in both directions. The current prism devices support such flexibility only when scanning into the seeing side."

A research team led by Dr. Peli has been developing prism devices for hemianopia patients for more than 15 years. In 2013, they introduced peripheral prism glasses, which optically shift objects from the blind side of the visual field to the seeing side, expanding the visual fields of patients by as much as 30°. The researchers intend to fully design and implement the mirror-based periscopic prism and begin testing all three novel designs in patients with hemianopia.

The research was published in Optometry and Vision Science (doi: 10.1097/OPX.0000000000000820)


GLOSSARY
prism
A transparent optical element having at least two polished plane faces inclined relative to each other, from which light is reflected or through which light is refracted.
lens
A transparent optical component consisting of one or more pieces of optical glass with surfaces so curved (usually spherical) that they serve to converge or diverge the transmitted rays from an object, thus forming a real or virtual image of that object.
ophthalmology
The branch of medicine involved in the study of the anatomy, functions, diseases and treatments of the eye.
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