by Jamie Bartlett 5 min read

What is VLT %?

Yep, sunglasses are good at protecting your eyes.

But what’s this term “visible light transmission” all about?

Looking for a new pair of shades, it’d be good to understand what this phrase means. So, I’ve done a little research to clear things up.

 

Visible light transmission is the amount of visible light that can pass through an optical or sun lens. It can also be called visible light transmittance or VLT%.  This is measured as a percentage which indicates the darkness of a lens within a sunglasses frame. The lower the VLT, the darker the sun lens will be.

 

Sounds good.

But let’s look into how VLT% can influence your purchase decision, the comfort of your eyes and how it categorises your sunglasses into one of four types.

 

Illustration of visible light transmission passing through a sun lens

 

What does light transmission mean?

 

As visible light meets a lens it can either be reflected, absorbed or transmitted.

(See the illustration above.)

These three actions are defined as;

~ Visible light absorptance (VLA)

~ Visible light transmission (VLT)

~ Visible light reflectance (VLR)

 

In the context of Visible light transmission, this action is measured as a percentage ranging from 1% to 99%.

Using a device called a photometer, this measures the intensity of visible light before and after it passes through a lens.

Low VLT% lenses will transmit less light and will be dark to look through.

High VLT% lenses will transmit more light and will be light to look through.

Simple enough.

But how does that affect your purchase decision when it comes to buying new sunglasses?

And what VLT% should you go for?

 

VLT% measures the darkness of a lens. For you, this determines your visual comfort on bright sunny days.

 

Based on the VLT%, the darkness of a sunglasses lens is categorised into 4 types. These categories have varying suitability’s for varying applications.

See the table below.

 

Woman looking towards the sun wearing blue sunglasses frame

 

Sunglasses lens categories

 

Category

VLT%

Sunlight

Driving suitability

0

80% – 100%

Overcast

Moderate

1

43% – 80%

Low

Day only

2

18% – 43%

Medium

Day only

3

8% – 18%

Strong

Day only

4

3% – 8%

Very strong

Never

 

Seen in the table above, VLT% can influence how and when you want to use your sunglasses.

In basic terms, it’s really your personal preference over how comfortable your eyes will be in certain light conditions.

Here’s a run-through.

 

Category 2/3

For daily tasks like driving or taking a stroll in the sun, a category 2 or 3 sun lens will generally work well for you.

Any lighter than a 43% VLT and you’ll probably struggle on those really bright days. You may end up squinting or frowning to compensate which can lead to headaches over a long duration.

Most, if not all general-use sunglasses are fitted with a category 2 or 3 lens.

Some fashion sunglasses may have a lighter tint on their lenses, so it’s worth seeing how they perform outside before committing to buying them.

Just be sure to ask first before you walk outside the shop with them, right?

 

Category 4

On the flipside, lenses with 8% VLT or less can seem overly dark and begin to impede your vision, especially if it’s not a particularly sunny day.

Oh and category 4 sun lenses? They’re illegal for driving your car in the UK.

They’re simply too dark for you to operate a vehicle safely. They can diminish the light from traffic lights and indication from other cars.

If you want a dark lens, be sure to stick to a category 3 at the very most.

 

Some context

Category 4 sun lenses are generally used in extreme-exposure situations.

For example, Mountaineering sunglasses can be fitted with 5% VLT lenses and side-shields to minimise the risk of snow blindness.

So, unless your planning to watch the next solar eclipse, it’s worth checking the VLT% of your next pair of sunglasses if they seem a little too dark.

This way, you can keep yourself on the right side of the law (and the road.)

 

What about your sun lenses?

At Banton Frameworks, our lenses are a category 3 with 14% VLT.

This puts them on the darker end of category 3 which makes them perfect for most daily tasks on bright sunny days.

Oh yea, and they’re also polarised... which a fantastic way to block reflected glare.  

What are polarised sun lenses?

 

 

Group of women all wearing sunglasses watching a solar eclipse

 

Does VLT% affect UV protection?

 

To control the VLT of a lens, a darkening tint must be applied in form of a coating or a film.

For sunglasses, tint coatings are applied by dipping the lens in a coloured liquid solution inside a heated tint-bath.

 

Despite this dark coating, the reduction of visible light transmission does not protect you from ultraviolet light.

 

This is because visible light is not the same as ultraviolet light (UV.)

Sure, they’re both on the electromagnetic spectrum. But UV inhabits an invisible range of frequency which is why we can’t see it.

It’s invisible.

 

Illustration of the visible light spectrum

 

UV ranges from 10 to 400 nano metres which is sectioned into UVA, UVB and UVC. These are the most damaging frequencies of light which are the biggest threat to your skin and eyes.

Because of this, UV needs a different type of filtration to visible light in order to prevent you from permanently damaging your eyes.

 

Even if a sun lens is really dark, this doesn’t improve it's UV protection.

 

Yep.

UV protection has nothing to do with VLT%.

Instead, you need to check the UV rating of sunglasses to ensure they’re suitably protective. Under European law, sunglasses must be physically CE marked and come with a UV rating of 99-100%.

Most often, you’ll see a label that states UV40 or UV400 which means that the lenses are capable of blocking ultraviolet light up to 400 nano metres.

If you see this rating, you’re good to go.

 

At Banton Frameworks, all our polarised sunglasses are CE marked, 100% UV protective and have a VLT of 14%.

 

Shop polarised sunglasses

 

 

Rear view of a man wearing sunglasses looking towards a sunlit window

 

The back of your sun lenses…

 

Ok, so this might a little bit contradictory.

But letting light to pass through a sun lens can actually be a good thing, as long it’s going in the right direction…

Let me explain.

Sunglasses without side shields or with a low base curve have a habit of letting light in from behind or above the frame.

Which describes our sunglasses perfectly. Because our frames are designed for daily use, not mountaineering or competitive road cycling.

As such, this light can create annoying and distracting reflections as the light is reflected backwards into your eye. Optically, this occurrence is known as bounce-back which can get pretty annoying.

To prevent this, an anti-reflective coating can allow “intrusive” sunlight to pass through from behind your lens. Meanwhile, the front of your lens reduces the amount of light passing through.

 

Sort of light an exclusive nightclub, it’s harder for light be allowed in than it is to be allowed out.

 

Which brings you to the good news.

At Banton Frameworks we provide anti-reflection on the rear-side of all of our polarised lenses at no extra cost.

This reduces the dreaded bounce back effect, leaving you to see things clearly, smoothly and most of all, safely.

  • 100% UV protection
  • 14% VLT
  • Polarised

 

 

 

polarised tortoise shell sunglasses

Jamie Bartlett
Jamie Bartlett

Co-founder of Banton Frameworks.



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