Hello and welcome to the first lesson of The Lost Contacts’ Introduction To Optical Dispensing course! Before tackling this course, please keep in mind that it is meant to build on the knowledge laid out in The Lost Contacts’ The Optics of Vision course. Please make sure you are familiar with the topics in that course before taking this one.
Today’s topic will be something known as a lens index. But let’s make sure we’re familiar with a few things first.
What is a Lens?
Basically, a lens is a clear (transparent) & curved material used for the purpose of refracting light.
Please review The Optics of Vision – Lesson 2: Introduction to Lenses for more information about what lenses are.
What is Refraction?
Refraction is what happens to light when it passes from one medium to another.
A medium in this context is anything that light can travel through. For example, the first medium for light produced by the sun is the vacuum of outer space. Eventually it reaches Earth and then switches medium from space to Earth’s atmosphere (air). Then it switches medium again when it enters your eyes.
Other common examples of media that light can travel through are: water, glass & clear plastics.
Whenever light passes from one medium to another, the trajectory of the light changes. This change in trajectory is referred to as refraction. Lenses (usually plastic or glass) are a different medium than the air around them. So as light passes through lenses, it is refracted.
Generally speaking, the denser the medium is, the more it will change the trajectory of the light. Every medium has a different index of refraction, which is a number describes how much it changes the path of light.
The Index of Refraction
In this lesson, we will learn about one of the most important properties of a lens; the index of refraction.
Nowadays, lenses for glasses are made from plastic – not glass. But there isn’t just one plastic that’s used for all lenses. We have a lot of different plastics at our disposable when making a pair of glasses.
The reason for having so many types of plastics to make lenses is to take advantage of their different indices of refraction.
Let me explain.
As mentioned before, different media with different indices of refraction will alter the trajectory of light passing through them by varying amounts.
- Plastic lenses that have a lower index of refraction will not bend light as much and therefore the focal point will be further away.
- Plastic lenses that have a higher index of refraction will bend light more and therefore the focal point will be closer.
You may recall from The Optics of Vision – Lesson 3: Lens Power, that the curvature of lenses also affects where the focal point will be.
- Plastic lenses that have a lower curvature will result in the focal point being further away.
- Plastic lenses that have a higher curvature will result in the focal point being close.
This means that we now have 2 tools with which to adjust the position of the focal point; the curvature of lens and the index of refraction of the lens.
In the every day operations of an optical store, the index of refraction is simply referred to as ‘the index’.
Why Is The Index Important?
As you may already intuitively know, the more curved a lens is, the thicker it will be. That means that people with prescriptions for strong lenses will have thick lenses. But when it comes to lenses that we wear on our heads all day long, we don’t want those lenses to be too thick.
The solution is to increase the index. Doing this allows us to reduce the curvature (which reduces the thickness) without affecting the focal point of the lens.
Here are two lenses of the same power side by side. Notice that one of them is significantly thicker than the other. This is because they are made from two different lens indices.
Different Lens Indices
- Also known as CR-39.
- 1.50 CR-39 is the ideal lens index for low prescriptions. It is the least expensive lens material, but also the thickest.
- Recommended for prescriptions between +2.00 and -3.00
- 1.50 CR-39 lenses block some but not all UV light.
Keep in mind that 1.5 is not very impact resistant and should not be sold to people who are prone to breaking their glasses/lenses.
Be careful when mounting 1.50 CR-39 lenses into certain frames as they are prone to chipping.
- Also known as Trivex.
- 1.53 Trivex is a light-weight and highly impact resistant lens material. It is more expensive than 1.5 CR-39, but also a little thinner.
- 1.53 Trivex blocks 100% of UV light.
- Also known as polycarbonate.
- 1.59 polycarbonate is also light-weight and highly impact resistant. It is slightly thinner than 1.53 Trivex but also slightly heavier.
- 1.59 polycarbonate blocks 100% of UV light.
1.53 Trivex and 1.59 polycarbonate have very similar uses. They are generally used for children’s glasses & safety glasses.
- 1.61 index lenses and above are considered to be ‘high index’ lenses. 1.61 index offers considerably thinner lenses than 1.50.
- Recommended for prescriptions +2.50 to +4.50 and -3.00 to -5.00.
- 1.61 index lenses block 100% of UV light.
- 1.67 index is similar to 1.61 index but used for even strong prescriptions.
- Recommended for prescriptions +4.50 or higher and -5.50 or higher.
- 1.67 index lenses block 100% of UV light.
- 1.74 is the thinnest and also most expensive lens material offered by most optical stores. The benefits of 1.74 are marginal as it does not make lenses considerably thinner than 1.67, but it does make them considerably more expensive.
- It can be used instead of 1.67 if customers insist on the thinnest lenses possible.
- 1.74 index lenses block 100% of UV light.
Here is a diagram showing the relative thicknesses of different lens indices.
Clear Lenses Can Block 100% of UV light?
Yes, all lens indices except for 1.50 CR-39 block 100% of UV light and yet they are not tinted. This is something that most people aren’t aware of as we usually think of lenses that block UV light to be sunglasses (tinted lenses).
Despite the fact that clear lenses can block 100% of UV light, that does not mean that sunglasses aren’t necessary. Sunglasses are generally larger and hence offer more protection then regular glasses as well as increased visual comfort from reducing the overall brightness.
Remember to Look at the Cylinder
Recommended power rangers for different lens indices typically just show the ‘sphere’ power. But what if your customer has ‘sphere’ and ‘cylinder’ in his/her prescription?
If this is your customer’s prescription:
You see a bunch of -2.75 everywhere so you might think that 1.50 index would work fine for this prescription. But this would be incorrect.
What a prescription with cylinder is really saying is that half of the lens contains the ‘sphere’ power only, and the other half has the ‘sphere’ + ‘cylinder’ power.
This means that the strongest portion of the lenses in this example prescription above would be -5.50 (-2.75 + -2.75).
Making these lenses with 1.50 index would result in half the lenses being very thick. Therefore, 1.67 index would be a more appropriate.
Remember to Look at the Add
The ‘add’ power is similar to the ‘cylinder’ power in that it get’s added to the ‘sphere’ power when making reading glasses.
If this is your customer’s prescription:
If you only look at the sphere or the add you might think that 1.50 index would be appropriate for this prescription. But that would be incorrect.
An ‘add’ is always added to the ‘sphere’ when making reading glasses.
This means that the strength of the reading glasses in this example would be +4.00 (+2.00 + +2.00).
Making these reading glasses with 1.50 index would result in glasses that are thick and heavy. Therefore, 1.61 index would be more appropriate.
The Index of Refraction of Other Materials
If you’re interested in learning what the index of refraction of other materials like air, water, glass, ice, diamonds, & more, click here.
When making glasses, we select the most appropriate lens index for the every prescription in order to maximize the satisfaction of the customer. Knowing the different lens indices and when to use them will make you a much better eye-wear consultant and optical associate. Please take the time to memorize the different indices and be sure to keep a reference chart for each index’s prescription range somewhere in your store.
Next in this lesson series we will learn about something called the base curve.