Polarised

Polarised lenses are used in sunglasses to reduce glare from reflective surfaces such as the surface of a lake or the bonnet of a car. They accomplish this through a process called polarisation, much like the way a venetian blind controls sunlight through a window.

Sunlight from the sun is scattered throughout the atmosphere and will either be absorbed or reflected horizontally, diagonally or vertically. Sunlight bouncing off horizontal surfaces, however, will strike the viewer's eyes at similar angles which appears as strong glare. Polarised lenses have a laminated surface containing invisible vertical stripes which only allow vertically polarised light to enter the wearer's eyes. Glare is substantially reduced because the horizontally reflected lightwaves cannot bypass the polarised layer.

Fishermen and drivers seem to derive the most benefit from polarised lenses because they deal with mostly horizontal surfaces. Fishermen wearing polarised sunglasses are often able see below the surface of the water. Drivers vision in improved by less glare from bonnets and roads as well as improving the visibility of other drivers behind windscreens.

There are some limitations to the use of polarised lenses. Due to the vertical polarising filter reducing the amount of light entering the eye they cannot be produced with clear lenses. Some drivers discover that polarized lenses can cause distortions in liquid crystal displays or onboard clocks and other instrument displays may be unreadable. Some wearers who use polarised lenses in snowy areas may find that they lose some contrast between snow and shadows.

Photochromic

Photochromic lenses are lenses that change their colour according to light conditions. When exposed to sunlight or UV light they darken to eliminate brightness.

These lenses generally have a slight tint in them (around 15% absorption) when there is a reduced amount of light and change to a dark tint (around 80% absorption) in very sunny conditions. Many people wear these lenses and find them convenient, as they automatically darken in the sunshine and can avoid the need for you to carry prescription sunglasses. Modern photochromic materials react and fade quickly, though not instantly and suit many prescriptions.

They can also be a little temperature dependant, often working a little better in cooler conditions. When driving you may find photochromic lenses do not become as dark as they do outside. Car windscreens (and windows) filter and absorb most of the short wavelength light (near UV) that would normally trigger the darkening process.

Concern is often raised about whether it is safe to use photochromic lenses for activities in low light conditions where vision is critical - such as driving at night.
Several factors should be considered:

Any tint will reduce the amount of light that is entering your eye. The darker the tint the more the light will be reduced. You should not wear a dark tint for night driving.

We generally see better in the light than in the dark so the more light that enters your eye the better (providing it is not dazzling you).

As some light is lost from reflection from the lens surfaces, the performance of all lenses is enhanced if they are coated with an anti-reflection coating to reduce the reflections from the front and back surfaces of the lens.

The amount of light transmitted by modern photochromic lenses in their clear state with a multi anti reflection coating, is not much less than that transmitted by a clear, non photochromic lens without an anti reflection coating.

The performance of some of the older photochromic materials degrades over time If you notice that your photochromic lenses do not go as clear as they used to you they should be replaced.

 

Drivewear

Drivewear is the first lens designed specifically to meet the unique visual demands of driving

These lenses combine both photochromic and polarised technologies. Activated by Transitions the first photochromic lens to darken behind the windscreen of a car allowing the lenses to change colour depending on the current driving conditions and further enchanced by a polarised filter to block blinding glare.

 

Tinted

Tints are available in a huge variety of colours depending on their function and to some extent whats fashionable. Generally the darker the tint the more sunlight they will will filter out. It is very important to note that a darker tint does not necessary filter out more harmfull UV light. A tint or filter that blocks all the harmfull UV light can be nearly colourless. When a tinted lens is worn your pupils automatically dilate to allow more light into you eyes and if not protected can allow more harmfull light into them. It is therefore essential that any tinted lens contains the correct UV filters to the relevant standards to block out all the harmfull light.