The power of a prism is related to the angle formed by its sides, not its thickness. Nevertheless, when ground into a spectacle lens, a prism can make one edge of the lens quite thick. An alternative is a Fresnel prism (Figure 1-7), in which the angled surface is broken up into a series of much smaller prismatic surfaces at the same angle.
Fresnel Press-On prisms, fabricated out of flexible plastic, can be applied to a spectacle lens with the base in any orientation. The optical quality of a press-on prism is not as good as a prism ground into a spectacle lens. However, prism ground into a spectacle lens is expensive and increases the weight and thickness of the lens. Moreover, often a prism is required only temporarily, especially in adults (for example, when used on a trial basis in the office), or may need to be changed in strength frequently. Fresnel Press-On prisms can be quite useful in these situations.
Figure 1-7 Fresnel prism. A Fresnel prism is a collection of small prisms placed parallel to each other. Both prisms have the same power but the Fresnel prism is thinner.
(Redrawn from Duane TD, ed. Clinical Ophthalmology. Hagerstown, MD: Harper & Row; 1976: vol 1, chap 52, fig 52-2.)
Excerpted from BCSC 2020-2021 series : Section 3 - Clinical Optics. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.