About Fresnel Prism and Q and A on Fresnel Prisms

What are Fresnel Prisms?

Fresnel Prism glasses are used to treat double vision.

What are Prism glasses?

Prism glasses, or prism lenses, are special plastic or glass optics that eye doctors prescribe to treat binocular vision defects, including double vision. They change the direction where light is reflected, directing it to the correct place on the retina. 

Double vision, medically known as diplopia, can be a frustrating and disorienting condition that significantly impacts daily life. When the eyes fail to work together properly, patients may see two images of a single object, making simple tasks like reading, driving, or walking challenging and potentially dangerous. Fortunately, Fresnel prism glasses offer an effective, non-invasive solution for many individuals experiencing this visual disturbance.

Understanding Double Vision

Double vision occurs when the eyes are misaligned and cannot focus on the same point simultaneously. This misalignment can result from various conditions, including:

• Stroke or brain injury
• Muscle weakness or paralysis affecting eye movement
• Thyroid eye disease
• Myasthenia gravis
• Brain tumors
• Multiple sclerosis
• Diabetes-related nerve damage

The condition can be temporary or permanent, and its severity can vary from mild to severely debilitating.

What Are Fresnel Prism Glasses?

Fresnel prism glasses are specialized eyewear that incorporates thin, flexible prism films attached to regular prescription lenses or plano (non-prescription) lenses. Named after French physicist Augustin-Jean Fresnel, these prisms work by bending light rays to redirect images, allowing the brain to merge two separate images into one clear picture.

The Fresnel prism itself is a thin, lightweight film with a series of microscopic ridges that create the prismatic effect. Unlike traditional thick glass prisms, Fresnel prisms are virtually weightless and can be easily applied to existing eyewear.

How Fresnel Prisms Work

When light enters the eye through a Fresnel prism, it is bent or refracted in a specific direction. This redirection compensates for the eye misalignment by moving the image to where the brain expects it to be. Essentially, the prism “tricks” the visual system into thinking the eyes are properly aligned, eliminating the double vision.

The strength of the prism is measured in prism diopters (PD), with higher numbers indicating stronger prismatic power. Eye care professionals carefully calculate the precise amount of prism correction needed for each individual patient.

Benefits of Fresnel Prism Glasses

Immediate Relief

Unlike surgical interventions or other treatments that may take time to show results, Fresnel prism glasses often provide immediate relief from double vision symptoms.

Non-Invasive Treatment

These glasses offer a conservative treatment approach that doesn’t require surgery or medication, making them suitable for patients who may not be candidates for more invasive procedures.

Adjustable and Temporary

Fresnel prisms can be easily adjusted or removed as the patient’s condition changes. This flexibility is particularly valuable for individuals with fluctuating symptoms or those recovering from stroke or injury.

Cost-Effective

Compared to surgical options, Fresnel prism glasses are relatively inexpensive and often covered by insurance plans.

Lightweight and Comfortable

The thin film design adds minimal weight to existing eyewear, ensuring patient comfort during extended wear.

Limitations and Considerations

While Fresnel prism glasses are highly effective for many patients, they do have some limitations:

Reduced Image Quality The microscopic ridges in Fresnel prisms can cause slight image distortion or reduced clarity compared to traditional glass prisms. Some patients may notice a slight “rainbow effect” or decreased contrast.

Cosmetic Concerns The prism film may be visible to others, particularly in bright lighting conditions, which can be a cosmetic consideration for some patients.

Limited Strength Range Fresnel prisms are typically effective for corrections up to about 30 prism diopters. Patients requiring stronger corrections may need traditional glass prisms or surgical intervention.

Temporary Solution

While effective, Fresnel prisms are often considered a temporary or interim solution while patients await more permanent treatments or recovery.

Getting Fitted for Fresnel Prism Glasses

The process of obtaining Fresnel prism glasses begins with a comprehensive eye examination by an ophthalmologist or optometrist specializing in binocular vision disorders. The eye care professional will:

1. Assess the degree of misalignment using specialized tests
2. Determine the appropriate prism strength needed for correction
3. Consider the patient’s lifestyle and visual needs
4. Apply the Fresnel prism film to the appropriate lens
5. Make adjustments as needed for optimal comfort and effectiveness

Q and A on Fresnel Prisms

Follow-up appointments are typically scheduled to monitor progress and make any necessary adjustments

Q1: What is the primary property of prisms that makes them useful in orthoptic practice?

A1: Prisms have the property of deflecting light rays towards the base, which is why they are used in orthoptic practice.

Q2: When were Fresnel prisms first used to manage strabismus, and what are they made of?

A2: Fresnel prisms were first used to manage strabismus after 1960. They are made of thin and transparent plastic sheets with angular grooves on one side.

Q3: Who designed a series of Fresnel press-on prisms in 1970, and what is their power range?

A3: A group of optical scientists from San Rafael, California, designed a series of Fresnel press-on prisms in 1970. These prisms have powers ranging from 0.5 to 30 prism dioptres.

Q4: What is the advantage of Fresnel prisms compared to traditional prisms?

A4: Fresnel prisms are lightweight and easy to handle compared to traditional prisms.

Q5: In what clinical situations are Fresnel prisms commonly used?

A5: Fresnel prisms are commonly used to temporarily relieve diplopia in cases like nerve palsies due to trauma, blowout fractures, decompensated phoria, divergence insufficiency, or convergence insufficiency.

Q6: How do Fresnel prisms shift peripheral images in patients with homonymous hemianopia?

A6: The aim of using Fresnel prisms in patients with homonymous hemianopia is to shift the peripheral image towards the central retinal meridian.

Q7: What is the anatomy of Fresnel prisms, and how do they differ from conventional prisms in terms of weight and magnification?

A7: Fresnel prisms are made from a thin transparent plastic sheet with multiple angular grooves on one side. They are much lighter than conventional prisms of the same power, and they have minimal magnification effects.

Q8: Who originally articulated the optics of Fresnel prisms?

A8: Augustin Fresnel originally articulated the optics of Fresnel prisms.

Q9: What does the angle of refraction of a prism depend on?

A9: The angle of refraction of a prism depends on the angle of the surface and the refractive index of the material of which the prism is made.

Q10: What is the most common use of Fresnel prisms in ophthalmology?

A10: The most common use of Fresnel prisms in ophthalmology is to temporarily relieve diplopia in various clinical conditions.

Q11: What are some of the indications for using Fresnel prisms in clinical practice?

A11: Fresnel prisms are indicated for various clinical conditions, including squints, amblyopia, nerve palsies, early onset strabismus, nystagmus, incomitant strabismus, hemianopsia, bedridden patients, and rehabilitation.

Q12: How do Fresnel prisms promote fusion and reduce deviation in cases of phorias?

A12: Fresnel prisms are used to disrupt fusion, causing the eye to deviate. This disruption promotes fusion and helps reduce the deviation in cases of phorias.

Q13: In what way are Fresnel prisms applied as a treatment modality for amblyopia?

A13: In mild cases of amblyopia, Fresnel prisms can be applied as a weak patch over the dominant eye to promote visual development in the weaker eye.

Q14: How are Fresnel prisms used to manage nystagmus with abnormal head posture?

A14: Fresnel prisms are placed with the base away from the direction of gaze preference in cases of nystagmus. This helps redirect the visual gaze towards the null point of nystagmus, reducing the need for a significant head turn.

Q15: What is the role of Fresnel prisms in patients with nerve palsies, and how does their power change during the recovery period?

A15: Fresnel prisms can be used as a temporary measure in patients with isolated nerve pathologies to avoid diplopia or increase the field of gaze. The power of Fresnel prisms may be changed as muscle function recovers.

Q16: In what clinical situations might Fresnel prisms be applied over part of a lens, and why might different prism powers be needed in various fields of gaze?

A16: Fresnel prisms can be applied over part of a lens to correct specific deviations in specific gaze positions, such as in cases of incomitant strabismus. Different prism powers may be needed in various fields of gaze, and Fresnel prisms can be easily cut and placed over spectacles as needed.

Q17: How can Fresnel prisms increase the visual field for patients with hemianopsia or those who are one-eyed?

A17: Fresnel prisms placed several degrees off the central gaze with the base towards the non-seeing eye can help increase the visual field for patients with hemianopsia or those with only one functioning eye.

Q18: What is the role of Fresnel prisms in bedridden patients, and how do base-down prisms facilitate reading?

A18: Base-down Fresnel prisms can be advised for bedridden patients to read or watch television without the need for head elevation. These prisms change the image’s angle, allowing for comfortable reading.

Q19: How can Fresnel prisms be used in rehabilitation for stroke patients with postural deficits?

A19: Fresnel prisms can be placed to make the patient more aware of their neglected side, helping in postural deficits related to vestibular movements in stroke patients who tend to lean towards one side.

Q20: What are multiplex prisms, and when are they used in patients with limited monocular field expansion?

A20: Multiplex prisms are used in patients with apical scotoma that limits monocular field expansion. Conventional Fresnel prisms cannot achieve field expansion in such cases, so multiplex prisms are used as new devices.