Feb. 06, 2019

What Is Prism Correction in Eyeglasses

What Is Prism Correction in Eyeglasses? Feb. 06, 2019 Prism correction is used in eyeglasses for some people who have diplopia, or double vision. This is when someone sees two separate images of one object. The prism helps align the two images so that only one is the image is seen. How does prism correction work? To see clearly, light enters each eye passing through the cornea to fall on the retina, the light-sensitive tissue lining the back of the eye. With normal binocular vision, the eyes work together, and the brain sees a single image. This is because light falls on the same part of the retina of each eye. With double vision, two images are seen because the light falls in different places on the retina of each eye. A prism used in eyeglasses bends light before it travels through the eye. The light is redirected so it will fall correctly on the retina in each eye. The brain then does its usual work of fusing the two retina images together to produce one, clear picture. Who might benefit from prism correction? A prism can be used for a double vision from eye misalignment when

caused by:

Eye muscle problems, such as myasthenia gravis, Graves’ disease, or strabismus (crossed eyes or wall eyes) Neurological (brain-related) issues, such as head injuries, stroke, migraine, or tumour. Nerve-related problems, like multiple sclerosis or diabetes mellitus Double vision has many causes, and a prism isn't helpful for all types of double vision. Because it can be a sign of a serious health condition, it's important to find out what's causing double vision. See a doctor immediately if your double vision: is new, is caused by eye misalignment, or happens for unknown reasons. How is a prism prescribed? You may need to do one or more of these tests to get a prism for your glasses:

Hirschberg test.

This basic test helps check for strabismus. A penlight is aimed at the eyes. When the patient looks at the light, the corneal reflection should appear in the centre of the pupil in each eye. If the reflection is off-centre in either or both eyes, there may be strabismus. Off-centre corneal reflection of light in the left eye, showing possible strabismus Krimsky test.

This test helps to measure the degree of misalignment. This is like the Hirschberg test, but with a prism. While looking at the penlight, prisms of different strengths are placed in front of the eye. When a prism brings the reflection in each eye to the centre, the correct prism measurement has been found. Cover tests. There are three types of cover tests. With the cover/uncover test, one eye is covered and one is left uncovered.

The doctor looks for any movement in the eye that is uncovered. The prism and alternate cover test alternate the cover over both eyes while a prism is placed in front of one eye.

This helps measure the offset (difference) between the two eyes and determine what prism is needed to fix your double vision. With the Maddox test, each eye sees a vertical or horizontal line. The offset of these images allows the doctor to determine what type of double vision you have. Prisms are also used to measure the distance between the two images.

Your prism prescription and eyeglasses Like normal eyeglasses, a prism is measured and prescribed with a unit of measurement called prism diopters (from 0.5PD, 1.0PD, 1.5PD, and so on). Depending on your double vision, the prism is placed vertically, horizontally or diagonally in one or both lenses of your


Sometimes, a temporary Fresnel prism (a thin press-on vinyl sticker) is fitted over the front of your eyeglasses. With a Fresnel prism lens, the prism is slightly visible. This isn't ideal for long-term use, but it does allow you to test drive a prism and see how it works. Fresnel prisms are also used when the prism prescription isn't stable and may change soon. If prism correction is needed for a longer period, it can be ground into (made part of) the lens of your eyeglasses. Eyeglasses with a prism look like any other glasses without a prism, although the lens on one side may be thicker and more noticeable.

What’s All The Fuss About Prism? All eye doctors (optometrists and ophthalmologists) use different types of lenses to improve your eyesight and vision. Regardless if you are near-sighted (myopia), far-sighted (hyperopia), or have astigmatism…You may need lenses to bend light differently to sharpen your focus! However, sometimes the brain’s inability to use both eyes together can lead to a problem with binocular vision. As a result, this can create trouble focusing (accommodation), turning the eyes inward (convergence), outward (divergence), or double vision (diplopia).

How Do Prism Lenses Work? Therefore, to assist the brain in turning the eyes in proper directions, eye doctors will prescribe a type of lens known as PRISM to fix the problem. In addition to compensating for the problem, the prism can be used as a therapeutic way to assist the brain in learning how to fix the problem on its own. This is something brain scientists call neuroplasticity. So what are prisms and how do they work? In essence, optical lenses bend and focus light, known as refraction. Prism lenses, however, refract light a bit differently. As a result, this produces a different result on sight and vision. Remember from science class, a prism looks like a pyramid with a base and a pointed top. Light passing through a prism will bend towards the base, while the image of the object viewed with the prism moves toward the peak. Due to this shift, it appears the object we look at has moved in that direction. Using prisms in different powers and orientation, eye doctors can make a change in the direction of what we see. As a result, you can experience a single vision when there was double vision before.

So Do I Need Prism Lenses?

The path the signal travels to the brain is changed dramatically when prisms are used in the same direction on both eyes, a term called yoked. Therefore, the brain changes the way it uses this new information. This can have a dramatic impact on visual processing! Using prism this way, the eye doctors at EyeXcel treat children with binocular vision issues affecting development or learning. In addition, adults who have suffered a brain injury or stroke can benefit from the prism. It also may be helpful for the athlete wanting better vision skills to go to the next level. Our job is to find the right direction and amount of prism that improves how a person’s brain uses visual information. So, if you are experiencing double vision, problems pointing your eyes in the right direction or focusing on the task at hand, make an appointment with the doctors. Let’s find out if prism just might finally be the solution you’ve been needing!