If you’ve ever ventured, squinting, out into the glorious sunshine on a summer’s day in Britain, then the chances are the first item you’ll reach for is a pair of sunglasses. These devices not only help us look the part – they also help to protect our eyes against ultraviolet light, and thereby guard against long-term macular degeneration. A high-quality pair of sunglasses will block more than 99% of incoming UV light.
But how exactly do they do this? Let’s take a look at some of the components that go into a decent pair of sunglasses.
High-quality sunglasses come coated with several layers of protection, each of which performs a slightly different job. In some pairs, some of these layers will be absent – eliminating glare is something, for example, that only sports-oriented sunglasses will need to do.
The topmost layer consists of a reflective coating, which does pretty much what you’d suppose it to do: reflect light away from your eyes. It forms the first line of defence, and helps the glasses to look cool, in the process.
Just beneath the reflective layer you’ll find a rugged polymer film. This is there to prevent the lenses from being scratched, and is what ensures that all the layers beneath it aren’t damaged irreversibly from the moment you sling your sunglasses into your backpack.
This layer helps to diffuse glare, and makes it possible to see even when light is bouncing off snow and water. They work in much the same way that the 3D glasses you find at the cinema do. Light waves vibrate upwards and downwards, and from side to side. A polarised lens helps to filter out all light that isn’t on a specific plane (the vertical plane, usually). It thereby helps to reduce the ‘jamming’ effect of light entering the lens from different angles, which we perceive as glare.
To understand glare, it’s important to think about the intensity of light, which is measured in lumens. If you’re looking at an indoor lightbulb, the chances are that it’ll be just a few hundred lumens. The sun, on the other hand, can be several thousand lumens.
Our eyes can comfortably see up to 3,500 lumens. When it gets slightly north of that, you won’t be able to see the light in question – you’ll instead see a flash of white known as glare. When the light reaches ten-thousand lumens, your eyes will be totally unable to absorb the light.
Reflected light can make this exposure all the worse. If you’re standing on top of a ski slope on a clear day, your unprotected eyes will be exposed to more than ten-thousand lumens. This can lead to temporary ‘snow-blindness’. As such, it’s important that skiers and snowboarders invest in a rugged pair of glasses or goggles.
The lens itself is the most vital component of the sunglasses. It protects against UV light thanks to an infusion of dyes and pigments, which also help to influence how dark or light it is. Bear in mind that the darkness of a lens is not necessarily indicative of its ability to protect your eyes – in fact, some darker lenses might have the opposite effect, as they force your pupil to open wider and allow more light to pass into your eye.
It’s not just the outside of the lens that comes coated with layers of protection; the interior, too, comes with an anti-reflective coating that’ll ensure internal reflections and back-glare is kept to a minimum.
What is Ultraviolet light?
It’s worth considering just what all of this technology is protecting us against. Ultraviolet light is the stuff that’s so high-frequency that we can’t see it – with each wavelength being shorter than four-hundred nanometres. Ultraviolet light is further divided into two categories – there’s UVA, which is between 400 and 320nm, and UVB, which is between 320 and 290nm. Of these, the former are by far the more common – though the latter is more dangerous, being the sort that causes sunburn, skin cancer and all the ocular problems we’ve already touched upon.
Sunglasses don’t just look the part – they help protect your eyes, too. A pair of Tom Ford sunglasses, or Dior sunglasses, for example, will look fantastic, and they’ll probably last longer than an inexpensive equivalent. It’s worth checking the front of your glasses for the letters ‘CE’; these guarantee that the pair in question have been subjected to, and have passed the UV-protection requirements.