Nowadays you can buy a cheap pair of sunglasses just about anywhere – from supermarkets and petrol stations to corner shops and online fashion retailers, but such ubiquity may prompt doubts as to the protection they can offer. While their lenses usually meet certain minimum UV protection requirements, their quality in other respects, such as visual clarity, is dismal.

Opticians, however, are governed by national health regulations, meaning that they have to meet higher standards such as the CE and UKCA marks. Professional opticians are therefore much better equipped to help customers find sunglasses that not only meet their practical demands, but which are also comfortable and well suited to their facial features.

Rules for protection

European law classifies sunglass lenses as “personal protective equipment”, outlining five levels of protection. Category 0 lenses allow 80-100% of light through, while those in category 4 only let 3-8% through, meaning they are not suitable for driving. Category 3 is the most commonly used, as they are appropriate for most situations, including driving.

So, do darker lenses protect you better? The short answer is: not necessarily. The degree of light absorption depends on how and where they will be used: category 4 lenses are designed for extremely bright areas like high mountains or deserts, but may actually reduce your visibility in other situations.

However, all sunglasses that meet the established standards will protect your eyes from UV radiation.

Quality of vision

Wearing sunglasses can give the feeling that your vision is impeded. This is because a tinted lens selectively filters light: it lets in one type of radiation and limits another. Three concepts can help us understand how this works, and why it matters.

First up is visual acuity. This is the term used to measure how “sharply” we see, and it is tested by asking patients to read increasingly small letters that appear on a screen or wall chart, with an ideal result around 100%. However, this test is performed with black letters on a well lit white background, so it does not take into account how visual quality may vary in other lighting conditions.

Contrast sensitivity is defined as the ability to differentiate objects from the surrounding background. It is, for instance, more difficult to distinguish black letters on grey backgrounds than on white backgrounds.

Lastly, we have refractive errors, which affect both visual acuity and contrast sensitivity. These are what cause astigmatisms and make us long or short sighted. Wearing non-prescription sunglasses when suffering from any of these conditions can make vision even worse than it would be without them.

Colour matters

With all this in mind, we should also address the common question of lens colour. While colour does not affect protection, it can affect contrast and the feeling of brightness, because each colour filters out a different wavelength of light.

Grey lenses filter out all wavelengths of light, while tints such as brown or green have a different effect on light absorption. This means that visual perception tends to be brighter through a brown lens, very dark through grey ones, and more natural through green ones.

Plastic or glass lenses?

Colour isn’t the only thing that affects quality of vision: we also have to pay attention to what the lenses are made of. While they are most commonly made of specialised plastics – known as organic lenses – you can also find glass ones, known as mineral lenses.

These two materials are not the same. Organic lenses are lighter and stronger, while mineral lenses are more scratch resistant and are less prone to colour distortion. This means that mineral lenses offer greater quality of vision.

What about polarised lenses?

Polarised lenses block light that reaches the eye at a certain angle after reflecting off a surface such as a road or water, thus reducing glare. They are particularly suited to driving or spending time around water.

However, they can be unsuitable in certain situations. For example, they block light from screens, making them appear darker or even completely black, meaning devices often have to be rotated to improve visibility. They are also not ideal for certain winter sports: by eliminating glare they make it very easy to miss icy patches on the ground.

Other features

Lastly, there are certain design features that we cannot overlook when choosing a pair of sunglasses.

Gradient lenses are darker at the top, and graduate down to a lighter tint at the bottom – they usually absorb around 85% of light in their darkest area, and 10% in the lightest. These lenses are stylish, but that does not affect their protection against UV rays. That said, they might be better suited to cloudy conditions, and on a bright summer day they may not be the best option.

Reflective lenses have a layer on their outer surface that reflects light and enhances protection. Unfortunately, this treatment is easily damaged or scratched.

Lenses with anti reflective coating on the inside of the lens provide greater clarity of vision by eliminating light reflected from behind the wearer. This treatment is usually applied to large sunglasses, or glasses with optically very high quality lenses.

In the event of any doubt, the best possible advice is to speak to your optician or optometrist to find the sunglasses that best meet your needs.

Jacobo García Queiruga, Profesor Interino en el Área de Optometría (OD, MSc, PhD), Universidade de Santiago de Compostela and Verónica Noya Padín, Investigadora predoctoral - Área de Optometría, Universidade de Santiago de Compostela

Image credits: Shutterstock 

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