This article aims to help you make an informed choice to choose the correct lenses for your application and explain how light can sometimes make sunglasses an essential requirement.
How Much Should I Pay?
On the face of it, sunglasses can appear simple; two pieces of colored plastic or glass (lenses) fixed within a metal or plastic frame that sits on your nose and above your ears – what more can there be?
However, many things can be done to those lenses giving differing results and so determining when and how they are used. As you will soon see, there is quite a difference in what your hard-earned pennies will buy about sunglasses. In a nutshell, generally sunglasses should:
Protect from bright, intense light.
Provide adequate protection from ultraviolet rays in sunlight
Protect from glare Eliminate certain frequencies of light.
In the case of motorcycle sunglasses, provide adequate protection to the eye from airborne particles, ie dust, grains of sand, fag ash was thrown out of a car window, etc., and, more seriously, stones thrown up from the vehicle in front of you and into your face.
When too much light hits the eyes the natural reaction is for the iris to close to reduce the amount of light entering. When the iris has reached its limit of closure, the next natural thing that happens is that we start to squint. If these two reactions are still inadequate to reduce the light entering the eye, damage to the retina can occur both temporarily and permanently. Skiers can often become ‘snow blind’ as a result of the sun reflecting off the snow. Good sunglasses prevent light from entering the eyes to avoid damage.
Light reflected from surfaces such as a flat road or smooth water generally is horizontally polarized. This means that, instead of light being scattered in all directions in more usual ways, reflected the light generally travels in a more horizontally orientated direction. This creates an annoying and sometimes dangerous intensity of light that we experience as glare. Good sunglasses can eliminate this kind of glare using polarization and UV rays are also blocked.
Some frequencies of light can enhance vision and others can blur your vision – the right color of your sunglasses allows them to work better in certain situations.
Poor quality sunglasses often forego many of the benefits of more expensive ones. A lack of UV protection increases your exposure to harmful UV rays by allowing them through to an iris that is open wider and so increasing the damage to the retina.
Little or no impact resistance from a lens can be catastrophic should the projectile pierce the lens and enter the eye causing blindness and/or disfigurement.
Buying the right sunglasses for the conditions in which you intend to use them offers maximum performance and protection and prices can range from £30 to several hundred £’s for the major top brands.
Counterfeiting – A Growth Industry…
… and also something that you should bear in mind when buying sunglasses that are designed for your protection and health. I probably don’t need to go into too much detail here as I’m sure you’re aware that counterfeiting is rife everywhere. If you find a well known branded pair of sunglasses being sold at a fraction of their normal retail price, you can be fairly sure that they’re dodgy. They may look the same and they may feel the same, but they’re cheap for a reason and that reason, in the case of a product designed for protection, may be detrimental to your safety. Inexpensive sunglasses may not have the impact resistance you need and claim to have, or those lovely looking tinted lenses may not have the necessary UV protection; it’s a case of buyer beware in these circumstances.
Different Types of Light – It’s Almost Rocket Science!
Good quality sunglasses are extremely good at ‘light management’. They change the way light behaves and tailor it to the right conditions for your eyes for a particular situation.
Visible light waves range from 390 to 750 nanometres in height, but just above this range and commonly found in sunlight, is Ultraviolet (or UV) light. It is a high energy range of light that can damage both your retina and your cornea (the window of your eye) in high doses or long exposure.
UVA rays are the most penetrable rays and are responsible for accelerating the aging process of the skin of mere mortals. UVB rays penetrate less deep into the skin and are responsible for causing freckles and age spots – both types of ray are a good idea to keep out of your eyes!
Glare
The brightness or intensity of light is measured in lumens and A lumen is a unit of measurement that is used to express how much illumination a source of light provides. You may need to Google it properly for an in-depth definition but, suffice to say, when the intensity of light goes over about 3,500 lumens, our eyes find them difficult to absorb and the excess is seen to us as bright flashes – this is glare. An example of when glare can occur is when looking down onto water with the reflection of the sun or when sunlight bounces off snow. Prolonged exposure to light of intensity around the 10,000 lumens mark can cause damage resulting in temporary or even permanent blindness.
Good quality polarized lenses increase detail, color, and contrast; from a safety point of view, it is possible to identify a diesel spillage in the road hopefully in time to avoid it.
Lens Technology
Sunglasses use a whole host of technologies to eliminate the problems created by light; tinting, polarisation, scratch resistance, UV protection, light reactivity (Photochromatic lenses)
Different color tints on lenses determine which parts of the light spectrum are absorbed:
Grey tints are the ‘good all-rounders’; reducing brightness with the least color loss and good glare protection. An excellent choice for general, riding and driving use.
Brown tints are also a good general choice with slightly better glare reduction removing higher frequency colors such as blue and UV.
Tinting can be achieved by coating the surface of the lens with light-absorbing molecules by immersion. The tint is absorbed into the PolyCarbonate and the longer it’s left in, the darker it becomes.
Polarisation.
Light waves from the sun vibrate and extend outward in various directions. Applying a filter to the lens that only allows light traveling in one direction to pass through polarises the light either horizontally or vertically ie. all the light would have the same orientation. If you were to have two lenses in front of each other so that you can see through them both then, as you rotate one lens 90º, the filters would eventually cancel out all the light preventing any from getting through.
Photochromatic Sunglasses Lenses
Sunglasses that darken when exposed to the sun are called photochromatic, or sometimes photochromic lenses, and rely on a specific chemical reaction to UV radiation.
They are coated with special molecules that react to UV light by changing their size, shape, and color when the light hits them, thus making the lens appear darker and so filtering out the harmful rays – cool or what! If you go back indoors, the lack of UV light forces them to revert back to their original form. One instance where photochromic may not react is when driving in a car and the windscreen has a filter applied that blocks out the UV rays. If UV rays are removed then the lenses won’t react.