You cannot work in optical store without encountering requests for sunglasses on a daily basis. Hence, it is important to know about all the different ways that lenses can be tinted.
How Do Sunglasses Work?
First, let’s consider what sunglasses actually do. You might say they:
- Make everything darker
- They protect your eyes from the sun
- They reduce glare
- They make everything a different color
- They make you look cool
They are all correct answers. In this lesson, you will learn how sunglasses do all of these things as well as why the word ‘sunglasses’ is actually an umbrella term for many different types of tinted lenses.
How Sunglasses Make Everything Darker – Absorption & Transmittance
To understand how sunglasses make the work darker, we must understand the concepts of absorption and transmittance.
Absorption is a measure of how much a material stops light from passing through.
Transmittance is a measure of how much a material allows light to pass through.
A clear lens (non-sunglasses) does not darken vision at all. That is because they allow all the light that enters the lens to pass through. None of the light is absorbed, all of the light is transmitted. Therefore, a clear lens has an absorption rate of 0% and a transmittance rate of 100%.
Absorption rate: 0%
Transmittance rate: 100%
An occluder is a tool used by optometrists to temporarily block one’s vision. An occluder does not let any light pass through it. All of the light hitting the occluder is absorbed, none of the light is transmitted. Therefore, an occluder has an absorption rate of 100% and a transmittance rate of 0%.
Absorption rate: 0%
Transmittance rate: 100%
Sunglasses darken the vision, but still allow you to see through the lenses. This means that some light is absorbed by the lenses and some light is transmitted. Typically, sunglasses have an absorption rate of ~80% and a transmittance rate of ~20%.
Absorption rate: 80%
Transmittance rate: 20%
If a customer tries on sunglasses and feels that they are too dark or not dark enough, sun-glass lenses can be ordered with the exact absorption and transmittance rates specified by a customer.
How Absorption is Achieved
In order to achieve the desired absorption rate, lenses under go a tinting process. Tinting lenses is a fairly simple process or submerging them into a chemical dye. The longer they remain in this dye solution, the darker the lenses will be when they emerge.
Here is a video of the process taking place.
Full Tint Vs Gradient Tint
Note that in this this video, the lenses were not completely submerged in the dye, but instead dipped about half way in. This is in order to create a gradient tint.
Gradient tints have the darkest tint at the top of the lens, and they gradually get lighter towards the bottom. Gradient tints are useful for blocking out the sunlight from above while not diminishing the overall brightness of vision too much. Additionally, they offer a unique look that many find desirable in their sunglasses.
Sun-glass lenses need not just be grey or brown. They can be tinted almost any color imaginable. The video above showed a blue gradient tint. The following video shows a red full-lens tint.
Here is a set of different colored full and gradient lenses readily available on the market. Can you can differentiate the full tints from the gradient tints?
Here is another image showing a collection of ready-made sunglasses. Can you can differentiate the full tints from the gradient tints in this one?
How Sunglasses Protect Your Eyes
Perhaps the top reason for wearing sunglasses is the fact that they protect your eyes from the harmful effects of UV light constantly being emanated from the Sun. There are 2 types of UV light which any standard pair of sunglasses must protect against. These are called UVA and UVB rays. Sunglasses must block at least 99% of these UVA and UVB in order to be labelled with UV400, which means they are safe to use.
It is important to understand that tinting lenses in itself does not provide any protection from UV light. In addition to tinting, lenses must be treated with special coatings or layers that block the UV light. In fact, if tinted lenses are worn without the UV blocking treatments, it is even more harmful to they eyes than not wearing sunglasses at all.
Also keep in mind that certain lens indexes like 1.6, 1.67, 1.74 provide 100% UV protection even without any special treatment.
Why Tinted Glasses That Don’t Block UV Light Are Dangerous
You might think that if sunglasses don’t block out UV light, then it’s no more dangerous then simply not wearing sunglasses at all, right? No, Wrong!
- We know that UV light from the sun entering the eyes causes damage.
- Without sunglasses, when in bright sunlight, the pupils constrict which restricts the amount of UV light entering the eyes.
- With sunglasses, because they make everything darker, the pupils dilate, allowing more UV light to enter the eyes.
- Therefore, when sunglasses are worn, if they don’t block UV light, the eyes will be exposed to more UV light than if no sunglasses were worn at all.
What Are Photochromic Lenses?
Photochromic Lenses are lenses that remain clear in the absence of UV light, but darken in the presence of UV light. They are more commonly known by the industry leading company Transitions®.
When offering customers Transitions® lenses, it is important to realize that there are many different versions and variations. If the store where you work offers Transitions®, I would highly recommend becoming familiar with their portfolio of options.
Mirror coatings can be applied to sunglasses to reflect light off of the lenses and to give them a unique look.
And last but not least, we come to polarized lenses. Polarized lenses offer a unique advantage over regularly tinted sunglasses. They not only reduce brightness and block 100% of UV, but they also filter out glare.
Regular sunglasses block a certain percentage of light from passing through them, darkening the overall vision. Polarized sunglasses darken the vision by selectively blocking out the light that causes glare thus making vision better and more comfortable.
Polarized sunglasses are extremely useful during activities where glare can be distracting and bothersome such as driving, biking, fishing, boating, skiing, etc.
Here is a type of promotional image often used to advertise the effect of polarized sunglasses.
Unfortunately, these are always made in Photoshop and exaggerated. It gets the main point across but can make it seem too good to be true.
Here’s a more realistic look.
I recommend comparing regular sunglasses and polarized sunglasses back to back in an outdoor environment to get a feel for the differences yourself. That way you can better explain it your customers.
We have covered many different types of sunglasses in this article, all of which are important to be familiar with. So let’s recap.
- Full tint
- Gradient tint
- Colored tints
- Mirror coatings
Make sure you are able to explain the difference between all of these, and become familiar with which ones are offered at the store/office where you work.