Last year I blogged briefly about the optics of prisms and lenses relative to vision therapy. This was prompted in part by the pilgrimage of a segment of the Orthodox Jewish Community to England in search of a solution to difficulties related to learning, as detailed in Ami Magazine. Independently, Dr. Warshowsky and I wrote letters to the editor of the magazine that were published, pointing out that the use of prism has been commonplace in our practices for many years, and can be prescribed by any Doctor of Optometry around the world. All one needs to know is the uses and applications of prisms for which, frankly, aiding learning and reading difficulties is only one. Naturally, packaging it as part of a special proprietary prescription, for which one has to travel great distances and obtain only through a specific laboratory adds mystique and appeal.
What is a prism? The most common form is a triangular surface that has a thicker part known as the base, coming to a peak at its opposite side known as the apex. When looking through a prism, the image seen is displaced away from the base and in the direction of the apex. One prism diopter will displace an image one centimeter at a viewing distance of one meter.
In the example below, if the word “HELLO” is printed above a line, the portion of the image viewed through the prism with its base down will be displaced upward, toward the apex. Prism also expands space in the direction of the apex and compresses space toward its base.
Technically speaking, all lenses have inherent prism power, which is what enables them alter the path of incoming light. Lenses can be considered as a series of progressive prisms arranged base-to-base or apex-to-apex.
Additional prism power may be incorporated throughout the lens either by grinding the thickness of lens differentially, or by decentering the optical centers of the lens relative to the patient’s interpupillary distance. But in reality, every lens power has a prism effect through each eye when the patient looks away from the optical centers.
Optometrists who are aware of these induced effects are prepared to deal with the visual sensitivities experienced by individual patients. Prescribing prism takes into account the power of the patient’s prescription, and how the doctor wishes to influence the patient’s looking patterns, perceptions, visual performance, and behavior. Prism has a wide array of potential applications, from improving reading efficiency when used in a standard configuration to influencing individuals on the autistic spectrum when prescribed in yoked form.
Earlier today I was admiring Karen French’s book on The Hidden Geometry of Life: The Science and Spirituality of Nature.
An irony suddenly emerged. The surge in awareness of prisms in the ultra-Orthodox community is symbolized by the Star of David. Colloquially known as the Jewish Star, it is in the configuration of a pair of opposing prisms.