Prisms are an optical aid that redirect where an image is viewed and can be a very beneficial tool in helping people with low vision.

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Prisms are a type of lens prescribed by an optometrist with a special feature that redirects the way the light reaches our eyes. It does not have refractive power like a traditional lens, however it performs a very important function of bringing images to our eyes in specific directions that are otherwise difficult to be seen. With a triangular shape, prisms are available in four directions known as base up, base down, base in and base out. Wherever the base is located indicates that the light is being redirected in the opposite direction. For example, a base down prism helps move the image upwards.

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What are prisms used for?

There are many optical uses for prisms in various fields, including vision therapy, and these lenses are also very instrumental in the field of low vision. Prisms are a helpful feature that eye doctors use to customize the prescription glasses to the patients’ individual needs and capabilities. Different applications of the use of prisms are detailed below.

High Magnification

High Magnification

When a person requires strong magnification due to a variety of reasons that are applicable in the low vision world, it could cause difficulty for the eyes to focus. This becomes especially relevant when using strong magnification and trying to look at a target that is close up. For example, when reading and using a high magnification, you need to bring the text very close to your face in order to read. This task requires the eyes to focus extremely inwards which can cause eye strain, and the magnification makes this inward motion challenging. Sometimes it’s too hard to meet these demands and therefore people resort to using magnification in only one eye, like a monocle. This is where prisms come into play because they are an optical tool used to help bring an image to a certain area. Prisms can be incorporated into high magnification glasses to help the eyes focus inwards together, and in a more comfortable way, despite the strong magnification.

Prisms accomplish a number of advantages in high magnification glasses, including:

  • Preventing eye strain and providing comfortable reading.
  • Establishing clear vision.
  • Despite the high magnification and close up vision, prisms enable both eyes to work together properly, known as binocularity.
High Magnification

High Magnification

Visual field loss is a prevalent issue in low vision. For example, up to half the amount of people who suffered from a stroke, have a visual field loss and they may or may not be aware of it. There are many people who suffer from the loss of visual field in certain directions due to a vast range of causes, such as:

  • Glaucoma
  • Age related macular degeneration
  • Diabetic retinopathy
  • Hypertension
  • Stroke
  • Brain tumor
  • Head injury
  • Multiple sclerosis

When a person suffers a visual field loss, it is recommended that they are evaluated by a low vision optometrist who can decide the best way to help them maximize their visual abilities and reclaim their independence. Anyone with a visual field loss is at a greater risk of falling or sustaining injuries due to the fact that their vision is obstructed. There is no reason to suffer such risks alone, low vision optometrists can help. Depending on where the location is of the blind spot, otherwise known as a scotoma, the optometrist can add a prism to your glasses in order to help your eyes see images in the area that has suffered a loss of visual field. The eye doctor will help you learn how to adjust to the prisms so that you can enjoy better vision and overall functionality. For example, in the early stages of age related macular degeneration, prisms can be instrumental in enabling the patient to attain vision in the areas that have lost vision.

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Double Vision

Double Vision

Many people with low vision suffer from double vision, also known as diplopia, due to a wide variety of reasons, including brain injuries or other neurological conditions. Prisms are used to help people with double vision in a way that can enhance their entire lifestyle by being able to see clearly and to accomplish day to day tasks. When there is double vision, each eye is focusing in a different direction. The eye doctor will prescribe prisms to direct both eyes to focus on the same target which can correct the double vision. Correcting diplopia with prisms can treat the symptoms of double vision including headaches, eye strain and general discomfort, in addition to enabling the person to reclaim clear vision and their independence.

Type of Prisms

There are different kinds of prisms, each with their own benefits, depending on every individual’s needs. There are two main categories of prisms:

Built in Prisms

The traditional way of implementing prisms is when the optical lab makes the lens with a prism built into it. A prism is a triangular feature of the lens in which the base of the triangle is the thick side of the prism and the point of the triangle, known as the apex, is the area that the light is being directed to.

For example, this is a base down prism:

The baseline is the thickest part of the prism and the light is being directed upwards so the base down prism redirects the image upwards.


High Magnification
High Magnification


It is important to consult with a professional when choosing frames for glasses that have prisms, as certain considerations must be taken into account for the thickness of the prism.

Press-On Prisms

These prisms, also known as Fresnel prisms, are temporary and cost-effective as they are placed on the back surface of the lens and are not built into the lens. These prisms are made out of flexible vinyl and do not add any noticeable weight or thickness to the lens. It can be easily cut and placed on the lens in an adjustable way so that it’s easy to alter the prism, as needed.

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Common Questions

Prisms are employed in vision therapy to alter the way light enters the eye and thereby change the visual perception. They can help in training the eyes to work together, correct alignment issues, or improve specific visual skills. By bending light, prisms create a shifted image that can challenge the visual system and promote the development of new visual strategies and responses. A neuro optometrist specialized in vision therapy may use prisms to rehabilitate various visual disorders.
While both prisms and lenses manipulate light, they do so in different ways. A lens focuses light to correct refractive errors like nearsightedness or farsightedness, while a prism bends or refracts light without focusing it. Prisms can shift an image up, down, or sideways without magnifying it, used to correct alignment issues or in vision therapy.
In treating binocular vision disorders, prisms can be used to align the images seen by both eyes. This alignment helps the eyes work together properly, facilitating binocular vision and improving depth perception. Neuro optometric rehabilitation may include the use of prisms for conditions such as double vision or convergence insufficiency to help retrain the visual system.
Prism lenses can be categorized into various types, including Fresnel prisms, which are thin and lightweight; ophthalmic prisms, which are ground into eyeglass lenses; and prism bars used in vision therapy. The specific type and orientation (base up, down, in, or out) depend on the eye condition being treated.
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Prism lenses can be extremely helpful for patients with low vision, allowing them to regain their clear vision and independence. It is important to see an eye doctor for a low vision eye exam and to receive a prescription customized to your daily needs. To schedule your low vision consultation, you can reach out to your nearest Amplify EyeCare practice either via a call or in-person visit. Our team of eye care professionals is ready and equipped to provide you with the care you need.

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