Overview of Fresnel Prisms
In the 1960s, a principle known as the Fresnel principle, which had been around for over 150 years, began to be used to treat a condition called strabismus, which is a misalignment of the eyes. In 1970, a group of optical scientists in San Rafael, California started creating a set of special types of lenses called Fresnel press on prisms.
Fresnel prisms are made from thin plastic materials. They contain a range of strengths from 0.5-30 prism dioptres and were first developed based on the ideas of Augustine Fresnel, a specialist in physics from France, in 1921. Originally, these types of lenses were made to be used in lighthouses.
However, these prisms are nowadays used extensively in eye care, particularly in the areas of neuro-ophthalmology, which is the branch of medicine dealing with the neurologic and medical aspects of vision, and strabismus. They are used to widen the binocular field which is the amount of view that can be seen with both eyes, relieve a condition known as diplopia which is when you see two images of a single object, or widen the field of vision. Fresnel prisms are lightweight and easy to manage, making them a preferred choice among doctors.
They are highly used to temporarily relieve the diplopia in cases of nerve palsies that can come secondary to injury, blowout fractures of the eye, decompensated phoria (a condition where the eyes drift when in rest position), divergence insufficiency or convergence insufficiency (both of which are conditions where the eyes do not perfectly align).
Fresnel prisms are most often used as stuck-on prisms. In addition, historically these prisms have also been used in stroke patients and patients with homonymous hemianopia, a condition where the patient loses half of the field of view in each eye. The goal with these patients is to move the peripheral image towards the center of vision.
Anatomy and Physiology of Fresnel Prisms
Fresnel prisms are formed from a lightweight, see-through plastic sheet that has numerous angled lines on one side. These prisms aren’t as heavy as the typical prisms with the same strength. Picture them as many tiny plastic prisms, placed on a slim plastic stand and arranged next to each other so that the bottom of one prism is connected to the top of the next one. Fresnel prisms don’t magnify as much as regular prisms, so the enlarging effect you might see with typical prisms is much less noticeable with Fresnel prisms.
Why do People Need Fresnel Prisms
Fresnel prisms, which are a type of lens, are often used as a treatment for a variety of eye problems. These include squints, a lazy eye often referred to as amblyopia, issues with the nerves in the eye, and nystagmus, which causes uncontrolled movements of the eyes.
For squints, these prisms help the eye move correctly to focus on an object, reducing the squint. In the case of early onset strabismus, which is another term for squinting, these prisms can improve the results of surgery or even allow the eyes to align without needing surgery.
For a mild lazy eye, Fresnel prisms can be used like a weak eyepatch on the stronger eye, helping the lazy eye work harder and improve its function.
For nystagmus, these prisms are particularly useful in severe cases where the individual has to move their head to a specific position to see clearly. The prism lens helps the eyes line up properly, reducing the need for a significant head turn. This works by directing the visual focus towards the point where the eye shaking is minimal.
Fresnel prisms can also help people with nerve issues in their eyes by temporarily helping to avoid double vision and helping to maintain normal vision during the recovery process. The strength of these prisms can be adjusted as the eye recovers.
In cases of incomitant strabismus, where the degree of squint changes in different directions of gaze, the prism can be used on a specific part of the lens for correction. If different strengths are required for various directions, the Fresnel prism can be easily cut and fitted over glasses as needed.
Further, for patients with vision loss in half of the visual field or those with vision in only one eye, Fresnel prisms can help increase the visual field by shifting the focus towards the non-seeing eye.
They can also assist bedridden patients in reading or watching television without needing to raise their heads. The prism changes the angle of the image, making it easier to see.
In rehabilitation, these prisms can also be used to help patients become more aware of their neglected side. This can be especially helpful in stroke patients who tend to lean towards one side. The prisms, when placed in a certain direction, can aid in alleviating balance issues related to movements.
For patients with limited vision field in one eye, a standard Fresnel prism may not be sufficient. In such cases, a new device called a multiplex prism can be used for better results.
When a Person Should Avoid Fresnel Prisms
While prescribing Fresnel prisms, which are a type of glasses lens used to correct vision problems, has no clear reasons it should not be done. However, doctors should take into account several factors before issuing a prescription.
Some patients may adapt to the prism in a way that can increase an underlying eye misalignment. This is if “prism adaptation” occurs without being noticed by the doctor.
In specific situations, where a patient’s eye tends to diverge irregularly in different directions (a condition called “incomitant strabismus”), Fresnel prisms can only help correct the eye misalignment in one direction. So, these prisms may not assist with correcting double vision (seeing two images of a single object) in other viewing angles. In some cases, it may even worsen the double vision.
In a condition known as “dragged fovea,” where the patient’s fovea (a part of the eye responsible for sharp, central vision) is dislocated, Fresnel prisms can help reduce double vision in the center. But as the peripheral (side) vision overpowers the central vision, the double vision may return within a few seconds. If the eye specialists continue to add prism, this could lead to an actual misalignment of the eye due to and muscle length changes.
Equipment used for Fresnel Prisms
Fresnel prism trial sets are very important when it comes to testing how the eyes move and work together. These sets use special devices called prisms to help accurately find any large differences or abnormalities in eye position. They come in two types – the stronger, more rigid prisms and the flexible stick-on prisms. The flexible ones are made from a eyeglass-quality material called polyvinyl chloride (PVC).
If a patient comes in with a squint or is seeing double, they need to have a thorough assessment. In these cases, there’s a selection of specialized eye-testing tools that will be used. These include a stick-on Fresnel prism, eye charts for visual clarity that use both eyes (binocular single vision charts), charts to identify any double vision (diplopia charting), charts to test the strength and coordination of the eye muscles (Hess charting), and equipment to test the full range of peripheral or ‘side’ vision (Goldman perimetry). These tools all come together to help the eye doctor understand what might be causing the patient’s symptoms.
Who is needed to perform Fresnel Prisms?
Fresnel prisms are a type of eyewear, often recommended for certain eye conditions. Depending on the condition, they can either be a temporary or permanent solution. The decision is usually made by two types of eye specialists – orthoptists, who deal with eye movement disorders, or ophthalmologists, who are doctors specializing in eye and vision health. Optometrists, professionals who check your vision and prescribe glasses, can also suggest Fresnel prisms, but only after discussing your case with an ophthalmologist.
The decision about the power of the Fresnel prisms, is based on a specific formula. This formula involves certain measurements of your eye, such as ‘phoria’ which is a condition when the direction of the eye is off when your eyes are relaxed, and ‘compensating fusional vergence’, which is the ability of your eyes to stay aligned.
For example, let’s say a patient exhibits a type of ‘phoria’ known as ‘exophoria’ with a measurement of 9D and needs a 6D base out prism to blur. Using the formula, the power of the prism required would be 2/3(9) – 1/3(6). Therefore, this patient would need a Fresnel prism with four D dioptres base in prism to stop them from seeing double imagery, a common issue in people with exophoria.
Preparing for Fresnel Prisms
Preparing lightweight Fresnel prisms, which are lenses that help correct vision, involves placing them in a desired direction on a carrier lens, which is the main part of your glasses. The Fresnel prism is cut to about the size of the carrier lens. Then, final adjustments are made to ensure the Fresnel lens matches the carrier lens in terms of size and shape.
The key thing to do is align the optical center of the Fresnel lens with the carrier lens. The flatter side of the Fresnel prism is placed toward the inside surface of the glasses. These prisms come in a range of strengths so they can address multiple vision problems, and can vary from 0.5 prism diopters (a unit that measures the power of a lens) to 30 prism diopters.
The flexible prism sheet is super thin, less than 1 millimeter thick. It’s cut to match the shape of the glasses lens and is then attached to it. It is important to ensure that the fit of the prism is slightly smaller than the spectacle lens, and should be about 1 mm from the edge of the glasses. Also, make sure that the prism doesn’t overlap the edge of the glasses, as this can trap air bubbles and cause issues between the prism and the glasses lens.
The prism is then attached to the glasses lens with water. However, if any air bubbles get trapped, it can cause the prism to take in the air and detach from the glasses lens. There is a special kind of Fresnel prism, called the hemianopic Fresnel prism, used for treating patients with hemianopic visual defects. These defects are typically seen in one half of the visual field. A 30-diopter Fresnel plastic prism is used, which is then attached to the glasses.
How is Fresnel Prisms performed
Fresnel prisms are a type of lens that’s used to change the way light enters your eye. They can be attached to your eyeglasses in two different ways, depending on whether the lens of your glasses is in or out of the frame.
If the lens is out of the frame, the Fresnel prism is trimmed with scissors to fit the size of the lens. Next, it’s fitted along the edge of the lens with a razor blade, then put back into your glasses frame.
If your lens is still in the frame, we mark the inside rim of your glasses frame on the Fresnel prism. It’s then trimmed to match this mark with scissors and placed on top of your glasses lens.
To attach the Fresnel prism sheet onto your glasses lens, both the prism and your glasses need to be washed with a mild detergent to get rid of any grime. After that, both are immersed in lukewarm water. This helps to clean off any small air bubbles. The prism is then placed onto the inside of your glasses lens while it’s still in the water. Adjustments are made by moving the prism in place and pressing it dry. It’s important to make sure the edges of the prism don’t overlap the glasses frames. If you notice any dust or air bubbles trapped, the prism is removed and reapplied.
Possible Complications of Fresnel Prisms
Prisms, which are used in eyewear, can sometimes cause issues over time. They might change color as they get older, become difficult to clean, are hard to put back if removed, or even cause minor cosmetic imperfections that alter the aesthetic look of the eyewear. On top of this, something known as chromatic aberrations can occur, which is where colors appear distorted or blurred.
Additionally, using prisms can sometimes reduce your vision if they are stronger than ten prism dioptres. Prism dioptres are a measure of how much the prism bends light. Specifically, Fresnel prisms, which are a type of lightweight prism used in eyewear, can sometimes reduce our visual ability more dramatically than regular, conventional prisms. For example, a conventional prism of 30 prism dioptres could make someone’s perfect vision of 20/20 go down to 20/30, which is still fairly good. However, a Fresnel prism might reduce someone’s perfect vision all the way from 20/20 to 20/100, which is a more significant change. So, while prisms can be helpful, they can also cause some issues and potential reductions in visual quality.
What Else Should I Know About Fresnel Prisms?
Fresnel prisms are a type of optical device used to help treat a condition called diplopia, which is when you see two images of a single object, often referred to as double vision. This double vision can be caused by conditions that affect your nerves (fourth and sixth nerve palsies), issues related to thyroid disease in the eye (thyroid-related orbitopathy), or problems with your eyes converging, or turning inward, to focus (convergence insufficiency).
While not everyone prefers Fresnel prisms, certain patients find them more comfortable and beneficial as compared to traditional prisms. Doctors also sometimes use these prisms on patients before they undergo surgery to measure how much their vision deviates, which helps in planning the surgery more accurately. After the surgery, these prisms can help prevent the recurrence of double vision.
A study conducted by Flanders and his team on 141 patients using Fresnel prisms reported encouraging results. They found that the vast majority (90%) of patients received the prisms over their non-dominant eye. The outcome was successful for 80% of these patients, with the prisms effectively reducing their double vision. Nonetheless, there were a few (6%) who had to stop using the prisms due to reasons like decreased vision, persistent double vision, or visual distortions.