The new designs, described in Optometry and Vision Science ... To increase the power of the peripheral prism, the bi-part double Fresnel prism combines two prism segments angled to each other.

These temporary prisms are called Fresnel prisms. They let you try out prism lenses before getting permanent prisms. They're also useful if your diplopia is likely to be temporary or is changing.

Boston, Mass. -- Researchers from the Schepens Eye Research Institute of Massachusetts Eye and Ear and Harvard Medical School have designed three new eyeglasses using high-power prisms to ...