Once upon a time choosing which sunglasses lens to buy was pretty straightforward but the sheer range and variety of options available today mean taking a long hard look before making that crucial choice.
Sunglass lens technology has come a long way in recent years and new materials are coming onto the market all the time with some significant advances in lens coatings.
This is the most straightforward choice and cosmetics will play a big part in what lens shade you choose. It is merely a matter of selecting the colour and the depth of tone that suits you best.
But there are some other considerations to take into account here. Much depends on the conditions in which you expect to wear the sunglasses. OK, bright sunlight is the obvious candidate here, but this is not always the case.
Individual colours can enhance vision in certain conditions and the choice may not be dependent only on cosmetics. A light blue tint, for example, may look good outdoors but it not okay when glare is a factor, such as when sailing and water is reflected upwards off the surface.
A blue-tinted sunglasses lens can even increase glare in the big outdoors so choose this tint mainly as a fashion item for streetwear and in the city. For general outdoor holiday wear, brown tinted sunglasses often make the best choice.
Brown filters out the blue glare from water and sky and can also help to increase contrast, so brown coloured lenses are suitable for general outdoor use and brown is a popular choice for standard holiday sunglasses.
Grey is an excellent alternative to brown, retaining decent colour differential and reducing outdoor glare. For this reason, a neutral grey tone is probably the most popular choice, not just for the big outdoors but also for night driving too.
Many also prefer a green tinted sunglasses lens. Green tint lenses tend to be darker and may be favoured by those with sensitive eyes. Despite blocking out plenty of glare, they can give the highest contrast and the right colour balance.
Yellow and orange tints are the most popular choice for those who are into snow sports such as skiing and snowboarding. Yellow tint sunglasses will soften the harsh white-blue light of pure snow while at the same time increase colour contrast and depth perception.
Pink and rose-tinted sunglasses can help relieve eyestrain and are more often used by people at computer screens a lot. Colour perception may not be as accurate as other tints.
Many sunglasses carry a graduated tint in any of the above colours. With graduated tones, the colour is strongest at the top of the lens and gets progressively less tinted until it is quite clear.
Graduated tint lenses are suitable for reading, or when carrying out some task that requires clarity when looking downwards but plenty of shade when the eyes are raised.
This type of sunglasses lens has a special coating that reacts to sunlight. Often referred to as transition lenses they become automatically darker as you move into the bright light and lighten when you head out of the sun.
The reaction time of photochromic lenses can vary but the transition usually gets underway in less than 60 seconds. They continue to get darker, or lighter, for several minutes until reaching the optimum tint for the lighting conditions.
The coating is usually activated by ultraviolet light, which is, in fact, invisible, so the darkening and lightening effect happens even on cloudy days and depends only on the amount of UV light that hits the lenses.
Photochromic lenses usually retain a small degree of tint, even in dark conditions. They can be very useful for those moving about a lot in varying light conditions such as indoors and outdoors but they are not much use for driving as headlights pass much too quickly for them to react. Even in daylight, the car windscreen may itself block UV light so the photochromic lenses may be nowhere near as efficient when behind the wheel.
Popular with drivers and sailors, polarised lenses are particularly useful in eliminating glare from large surfaces such as the sea and sky but with little or no loss of definition. Unlike standard tint lenses that reduce light at any angle, polarised lenses will cut out light waves travelling in the horizontal plane, but let vertical light waves pass through.
Polarisation only reduces glare but also improves both depth and colour perception. Vertically aligned light is much more critical visually than the horizontal rays, the central component of glare.
The glare of the sun on the sea and other flat surfaces can be highly polarised and, in some conditions almost entirely horizontally polarised. Likewise, most reflected light is horizontal and, with this filtered out by polarised sunglasses images appear clearer.
Polarised sunglasses are especially suitable for driving as the light from bright reflections and the road surface tends to be horizontally polarised. Polarising lenses give higher definition vision for driving, removing the effect of dazzle and reducing eyestrain.
Take care however when choosing polarised lenses. Cheap polarised lenses tend to cut out less light, some as little as 10%. High quality polarised lenses will filter out more than 90% of the horizontal light.