Basic of Prism - Chadwick Optical, Inc.

Prism can feel intimidating, but it’s really the foundation of everything we do. After all, optics is all about bending light, and that’s exactly what a prism does.

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This page will cover the basics of prism. If you’re ready to go deeper, we have troubleshooting prism guides for when you want to get nerdy.

At its simplest, a prism bends (or redirects) light. Light goes in, splits and exits at a different angle. That’s a prism at work.

When it comes to eyeglasses, we focus on refracting prisms. These bend light by refraction, not reflection.

Here’s where things get tricky (and where people’s brains start to melt): Prism and lens power are linked.

Every lens with power has a focal length. For example, a +4 diopter lens has a focal length of 25 centimeters. Light rays enter, bend, and focus at that point. (A -4 diopter lens has a negative focal length, but let's not worry about that right now)

If you want to measure the amount of prism at a specific point in the lens, you use Prentice’s Rule:

distance (cm) × lens power (diopters) = Prism (in prism diopters)

If you move away from the lens center, you automatically create prism. The further off-center you go, the more the light bends.

Introduction to The Tomlin Guide to Fresnel Prisms

Welcome to The Tomlin Guide to Fresnel Prisms, a concise reference for eye health professionals and patients seeking to understand and manage diplopia, or double vision, caused by thyroid eye disease (TED).

This guide aims to serves as an essential resource for practical insights into the use of prisms in treating TED-related vision challenges. It is written by the esteemed Liz Tomlin, Head Orthoptist and Eye Team Lead for the Guy’s and St Thomas’ NHS Foundation Trust where she has worked in eye health for more than 25 years. Liz was also the first orthoptist to become a Master of The Worshipful Company of Spectacle Makers, in the Company’s 400 year history.

The Eye Health Professionals’ Guide to Fresnel Prisms

1. Dealing with diplopia, or double vision

We know many TED patients experience diplopia, or double vision. Diplopia can be both vertical and horizontal. TED patients find this is often worse in the morning, when the eye muscles have been engorged from lying down, so sleeping a little more upright, using an extra pillow or two, can help.

2. What is a prism and how does it help?

Prisms are pyramids of glass that can be used to bend light round to a new position of the eye so that the brain can use both images and restore single vision to the patient. The downside is that prisms can often be bulky and heavy in glasses. For a high prescription, you may need to order them from a specialist manufacturer as not all suppliers will provide prisms above a power of 7.5 dioptres.

• Fresnel prisms: Augustin Fresnel was a Frenchman who developed rings of lenses that were high powered but thin and could focus beams of light for use in lighthouses. This technique came to be known as a Fresnel lens. He then went on to develop Fresnel prisms. The Fresnel prism is made up of small multiple prisms which, when made in plastic, are light and flexible and relatively cheap. These plastic membranes range from 0.5-40 prism dioptres.
• Benefits: Compared with incorporating a prism straight away into spectacles, a Fresnel prism has the huge advantage that it can be tried and adjusted, over a period of a few weeks, to test how it corrects the patient’s vision in their usual activities and how the patient feels as a result.  A fresnel prism is easily adjusted, especially if the angle of squint is changing (either getting better or worse) and it’s light, both in weight and price, compared with incorporating a full prism into one or more pairs of spectacles!
• Drawbacks : The downside of a Fresnel prism are that the lines degrade the vision, but this is often ignored after 48 hours. The lines also can get a little dirty and dusty so need regular cleaning. 

3. Prism fitting tips for eye health professionals 

Collaborative Approach

Include the patient, dispensing optician, and orthoptist in the decision-making process.

The role of the orthoptist

Orthoptists perform specific assessments to ensure proper prism strength and placement:

• Squint measurement: Assess eye alignment in various gazes for both near and distance vision.
• Prism fusion range: Evaluate the range over which a patient can maintain single vision with the prism.

This information allows the orthoptist to calculate the lowest diopter needed to align images for the brain, typically placing the prism inside the non-dominant eye lens to avoid dizziness or nausea.

Usually having one third of the fusion in reserve will make the patient’s vision comfortable.

4. Guide to fitting a prism

Basic principles

Single prism first: Always try and fit just one prism, even if the patient has vertical and horizontal problems. Try and fit for the biggest deviation and see if the smaller one is compensated e.g. a newly acquired vertical squint corrected with a prism means that a longstanding exophoria can be compensated by the patient!

Use pythagoras for complex cases: If you have a vertical and horizontal squint that needs correction use Pythagoras to calculate the prism needed at 45 degrees!

Orientation for different conditions

• Convergent squints (Esotropias): Need a base OUT prism so the lines need to be vertical.
• Divergent squints (Exotropias): Need to have base IN prisms so the lines are also vertical.
• Vertical misalignments:
  • Hypertropia (eye up) requires a base DOWN prism with horizontal lines.
  • Hypotropia (eye down) requires a base UP prism with horizontal lines.
• For a combination of vertical and horizontal squints the Fresnels can be at an angle.

Placement and cutting tips

Smooth/rough side: The lines of the Fresnel prism indicate the base. If you feel the prism, there is a smooth side which goes against the spectacle lens. The rough side goes outwards.

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On the rough side of the prism feel it across the lines it is rough one way and smoother the other way. The rough way is when you feel along indicates this is the base of the prism!  The base is also marked on full prisms.

Lines: The lines must be accurately placed to ensure the power of the prism is correct.

Fitting : The prism should be fitted with the apex pointing the way the same way as the eye.

• Hold the prism in place and cut so that the prism is the same size and shape at the lens inside the frame. This gets easier with practice, take your time and use all your Blue Peter crafting skills.
• Use small scissors – embroidery or small, straight nail scissors are perfect.
• If it’s too big it won’t stick on. If it’s too small it will be visible and look unsightly as the edge draws attention to it.
• When you’re happy with the shape and size check that the patient can still see single vision through it. Then clean the spectacles and the prism. Over a sink, wet the inside of the lens then push the smooth side of the Fresnel prism down onto the water.
• With a tissue or paper towel remove excess water, push out the air bubbles and dry.

It’s important to put the prism on the inside of the lens of the NON dominant eye. Otherwise the patient may not be able tolerate the blur the Fresnel gives and may experience dizziness and/or nausea.

5. Incorporating a Fresnel prism into spectacles

Incorporating a prism into spectacles means the lens will look normal and lose the lines. It will be easier to clean and clearer to see through.

A Fresnel prism is just placed on one lens, but when incorporating into spectacles it makes for a better balance of lenses to split. So a Right Eye 8 base out will become RE 4 base out LE 4 base out. A Left Eye 4 base up will become RE 2 base down and 2 base up 

NB. Contact lenses can only have up to 4 Diopters BASE DOWN so the prism will have to be in one eye. This means is it only suitable to correct small vertical deviations.

A spectacle lens with a prism incorporated will be thicker at the edge so listen carefully to dispensing optician colleagues and advise the patient to take advice from their dispensing optician on the right frame to make the glasses and the person look their best! If the patient already wears a high prescription the prism may be incorporated by de-centering the lens.

6. Final collaboration tip

I recommend orthoptists to put their contact details on any prism prescription so the optometrist, who will keep the prescription up to date, and the dispensing optician who advises the patient on their spectacles and/or contact lenses can easily contact the orthoptist if needed, for further discussion.

The Patients’ Guide to Fresnel prisms

If you need a Fresnel prism, the orthoptist or ophthalmic practitioner will fit if for you. Usually a Fresnel prism is placed on the inside of your glasses unless the prescription is very high. If it is very high, the practitioner might say that the outside of the glasses has to be chosen.

The Fresnel prism will be placed on the lens in front of the non-dominant eye and the smallest value of correction possible will be tried first to minimise the blur created.

Care of your Fresnel prism

Regular cleaning is really important. Even when they are clean a Fresnel prism can degrade your vision a little, and if the prism is grubby – a lot! 

1. Reference photo: You may want to take a picture of the way your prism sits on the lens, as fitted by your practitioner, before you start cleaning, especially if you have round lenses.

2. Cleaning: Your prism can be cleaned in the same way as you normally clean your glasses ie with a tiny amount of washing up liquid, rinsing in water and then drying with a soft, dry cloth. You may use a soft toothbrush to gently remove dust, makeup and grease from the ridges of the prism.

3. Removal: Removal of the prism is achieved by carefully picking it off with a finger nail.

Liz Tomlin. Master of the Worshipful Company of Spectacle Makers

My name is Liz Tomlin and I am Head Orthoptist at Guy’s and St Thomas’ Hospital NHS Trust. In that role, I see many patients with a wide range of thyroid eye disease (TED) symptoms and help them on their journey. 

In I also gained the honour of being Master of the Worshipful Company of Spectacle Makers, a City of London livery company founded in and dedicated to better vision for all. This enables me to network with a whole community of lens manufacturers and suppliers, dispensing opticians and optometrists and discuss with them the range of solutions for the challenges experienced by TED patients.

I recently issued a series of posts on LinkedIn about the use of prisms, originally used as tips for the St Thomas’ TED Patients’ Group. This short document brings those posts together in one place as a reference guide for eye health professionals and for patients. This article has been prepared by me and Helen Perkins the Clerk of the Worshipful Company of Spectaclemakers.

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