Light travels as electromagnetic waves, similar to the waves on the sea, with peaks and troughs. We specify the different colours of light by the wavelength of radiation they emit. (The wavelength is the distance between two adjacent peaks or troughs of that wave). As light has a very small wavelength, these measurements are extremely small and are measured in units called "nanometres" (nm), where a nanometre is one billionth of a metre!
Think of the colours of a rainbow, with red on the outer part of the curve and blue/violet on the inside. Red has a wavelength of 710nm, green is 500nm, and blue/violet is 400nm. So the wavelength is getting smaller as we pass from the red end of the visible spectrum through to the blue end. Ultra-violet lies beyond the blue/violet band, between wavelengths 400nm to 100nm, but the human eye cannot usually see light with a wavelength smaller than 400nm, making ultra-violet light invisible to us - so we call it ultraviolet radiation.
Although UV radiation is invisible to humans, many animals and insects can detect UV light, which they use in helping to find prey, etc. For example, kestrels can detect the UV light given off by the urine trails that field voles make as they move around, so from high up in the sky, when these birds of prey find many criss-crossing urine trails on the ground, this indicates a good place to hunt for the voles.
One possible reason why we cannot detect UV light is that if the crystalline lens in the human eye has evolved to only focus on a limited range of colours, this helps to give us a sharper image of what we see, whereas if we could focus over a larger range of wavelengths this could lead to chromatic aberration, a distortion of vision that would reduce image clarity.
Ultra-violet radiation can be divided into three main bands or groups, depending on wavelength :-
1. UVC radiation - 100 to 290nm
UVC rays are absorbed by the all-important ozone layer that surrounds the Earth as part of the outer atmosphere, thus preventing this radiation from reaching the ground.
2. UVB radiation - 290 to 320nm
UVB radiation is the most dangerous, as this is what can damage the eyes and cause sunburn to the skin. UVB is the cause of snow-blindness, where the radiation induces a photochemical reaction in the cornea and overlying conjunctiva after a few hours exposure, producing swelling of the cornea (oedema) and inflammation of the surrounding tissue. The swollen, oedematous cornea becomes cloudy - hence the blurred vision, and also very painful. There is photophobia (dislike of any exposure to light) and spasm of the eyelids. It usually lasts for a couple of days, and is only alleviated by using local anaesthetic drops, amethocaine 1%, to reduce the pain, and adrenalin drops 0.01% to relieve the congestion.
UVB radiation does not pass through glass, so a simple glass lens would prevent it from reaching the eye. Polycarbonate, a type of plastic sometimes used for spectacle and sunglass lenses, also cuts it out completely.
Some believe that regular exposure to UVB radiation can, over a period of time, contribute to cataract formation at an earlier age than normal, and may also cause the development of pterygia, which are growths of tissue that encroach over the cornea from the sides.
UVB radiation varies with the time of day, being most intense between 10am and 2pm, when the sun is at its brightest. It is stronger at high altitudes, and more intense near the equator, due to the more direct nature of the sun's rays in this region.
3. UVA radiation - 320 to 400nm
In contrast to UVB, which varies in intensity with the time of day, UVA radiation is more constant with less variation. Also it is not blocked by glass, and so can still pass through to reach the eye. However, the crystalline lens of the human eye absorbs most of the UVA radiation, so hardly any UVA is allowed to pass through to the back of the eye. There is no known evidence that UVA causes harm to the eye, although scientific opinion is still divided and under debate.
UVA radiation was originally thought to have only a minor effect on the skin, but recent studies have shown that whilst UVB causes damage to the surface layers of the skin as sunburn, the UVA penetrates into the deeper layers of the skin, causing damage further down.
UV radiation not only reaches us directly from the sun, but a considerable portion is reflected from various surfaces around us, to reach our eyes and skin indirectly. Fresh new snow can reflect the most, with up to 80% reflected UV rays. On holiday on the beach, sand can reflect 15% UV light, along with concrete buildings and walls etc. A foaming, frothy sea reflects 25%, whereas still water and normal ground reflects around 10% UV radiation.
Choosing your Sunglass tint
Ideally, you should go for a tint that absorbs at least 98% of both UVA and UVB radiation. If you see a label "UV400", this means that all radiation up to 400nm (thus all UV radiation) is absorbed or blocked by the lens.
Dark grey, grey/green, or brown are the three main colours to choose from, and is a personal preference. I, myself, prefer grey or grey/green, because it seems to keep the colours true to life, but many people like brown because it can enhance the contrast and so help to make things appear sharper. Amber coloured tints are used to block out blue light, which again enhances contrast, so making the image sharper. Amber tints can therefore be used for skiing, sailing, flying, target shooting etc.
Lenses made from polycarbonate absorb most UV radiation, and wrap around designs are good for preventing light from entering the eye from the sides.
Children and UV Light
As UV damage tends to build up over many years of exposure, such as going on holiday to warm bright climates or playing outside on a sunny day, children and young adults are at particular risk, and you should always consider some sort of eyewear protection for them even though they seem to cope with bright light better than us adults (when playing at the seaside, for example). Wearing a peaked cap is not really enough protection, as a lot of the UV radiation is reflected off the ground (see earlier “reflected light") and straight into the eyes, especially on a sandy beach. Make sure the sunglasses have a full UV protection.
Advice on which tint to choose for sunglasses
Different coloured tints are available to enhance vision and improve visual comfort for many activities. Sunglasses are very important as they reduce glare and protect against harmful ultraviolet radiation. The main tints available can broadly be divided into neutral grey, polarising, yellow-brown, green, red and photochromic.
1. Neutral Grey
This filters out all wavelengths of light by an equal amount, and so colours appear more natural looking than with tints of other colours. Grey tints are good for activities where subtle colour differences are important, such as golf, mountaineering and skiing.
2. Polarised tints
Polarised tints are good for reducing glare from reflected light bouncing off surfaces such as water or wet roads, making them a good choice for fishing, water-sports, driving or cycling (on wet surfaces). However, polarised lenses might also reduce important details in skiing or golf.
3. Yellow / Amber tints
Yellow or amber tints help to increase contrast by blocking out the blue end of the visible spectrum, resulting in a reduction of blue light scatter. They enhance differences in contour and make things appear brighter in low light levels. This makes yellow or amber tints good for shooting, snow sports, driving or cycling.
4. Green tints
Green tints help to enhance the background, which makes them a particularly good choice for golf, tennis and some forms of shooting, where the object needs to show up against the background.
5. Red tints
Red tints enhance objects at the red end of the spectrum, and can be used in clay-pigeon shooting where the target is orange, in skiing to give contrast to the changes in reflected light, or in motor sports to reduce the reflected glare from the road.
6. Mirror tints
Mirror tints limit glare, increase absorption and reduce infra-red, thus reducing the build-up of heat. Mirror tints are good for snow sports, water sports, cycling and running.
7. Anti-reflection coatings
Anti-reflective coatings minimise lens reflections, especially from the back surface, and are recommended for racket sports, fishing, shooting and archery.
8. Photochromic tints
In photochromic tints the darkness of the tint changes according to the light levels, most commonly between around 20% to 80% transmission. Generally, two photochromic colours are available, brown or grey. Photochromic lenses are good for sports played in variable light levels, such as golf or tennis.
The following list gives a summary of suggested tints for different sports and activities:-
- Cycling - polarising, yellow/amber
- Golf - green, neutral grey, yellow/amber, brown
- Shooting - reddish-brown, orange, yellow to brown, green
- Skiing - brown, red/orange, neutral grey
- Tennis - green
- Fishing - polarised
- Motor sports - polarising, yellow, red
- Water sports - polarising brown or grey
- Running - green, neutral grey
- Football, and fast moving ball sports - amber
I hope this guide has been of some use to you, but please remember that it is only a guide, so feel free to take it “lightly"(excuse the pun - I couldn't resist) as the colour of tint you choose for your sunglasses is very personal so if you prefer a certain colour of tint that falls outside of these guidelines - don't worry. As long as it blocks out all the ultraviolet light (UV 400) then that's all that matters. Enjoy your sunglasses!
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