Ultraviolet Light and Tints – A Guide to Choosing Your Sunglasses

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 Blue light 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.

Reflected Light

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.

Leave a Reply

Your email address will not be published. Required fields are marked *