When buying a pair of sunglasses, two important questions are generally asked.
The first Do they suit me? On this topic, you can send us your photos, but our opinion will remain (very) subjective.
The second: Do they provide good protection for my eyes against the sun’s UV rays? On this topic, we can share some factual information to help you choose the protection that best suits your needs.
Protecting your eyes from UV rays, a matter of style and tint
To protect yourself effectively from the sun's rays, your glasses must limit the amount of UV rays reaching your eyes. In particular, the retina and the lens.
This protection is primarily achieved by the shape of your glasses. The more your glasses cover your eyes, the better protected they will be.
Protection also comes from the ability of glasses to filter UV rays. Glasses are thus classified into several categories according to the UV protection level which they offer: 1, 2, 3 or 4 (the highest level of filtering).
At Eio, we use Category 3 lenses, which cover most uses. They are not too clear, meaning you won't be dazzled even in bright lighting conditions. Nor are they too dark, allowing for good visibility in slightly dimmer locations.
Why 3 rather than 4? Category 4 is more recommended in conditions of very strong light, such as in high mountains or deserts. Furthermore, it is not recommended for driving.
However, UV.
What are polarised lenses?
Contrary to what one might imagine, polarised lenses do not really offer more protection than non-polarised lenses. It is mainly about’a matter of comfort.
Have you ever been dazzled by rays reflected from a puddle, snow, the water in a swimming pool or the sea? Over time, this can prove quite tiring, even bothersome. This is precisely what polarised lenses are for: to eliminate light rays reflected by certain surfaces to reduce eye strain.
Briefly, rays reflected by these types of surfaces are said to be polarised: they are oriented in one single direction. The polarising film placed at the core blocks the passage of these rays.
Result: we are less dazzled. 
When to wear polarised glasses
Generally speaking, everyone is familiar with polarised lenses. They are widely used in the world of sport, particularly by those who take part in water sports and winter sports, given the numerous reflections created on the surface of water and snow. Polarised lenses filter out around 98% of reflected light.
Brilliant, polarised glasses for everyone?! Yes and no.
A priori yes, for super comfort. In reality no, for practical reasons.
For urban use
Polarised glasses tend to darken the screens of mobile phones and cameras, and/or create a mirroring effect. Not very practical.
For sports use
As we were saying, polarised lenses reduce the glare reflected by certain surfaces to 98%. The problem is that these rays can be crucial: distinguishing fresh snow from an icy patch whilst skiing, spotting the next gust of wind whilst sailing, or seeing an oil slick on the road whilst riding a motorbike or cycling.
Semi-polarised lenses, the miracle solution?
Not far!
As their name suggests, semi-polarised lenses filter out 50% of reflected light.
So they are more comfortable Only non-polarized lenses and more versatile just «classic» polarised lenses.
For those who do not require them, we also offer frames fitted with non-polarised lenses.
As we've seen, semi-polarised and non-polarised lenses offer you equivalent UV protection. You can therefore choose the lenses that best suit how you will use your pair of glasses.
Do you have any other questions about UV protection, polarisation, or choosing lenses for your glasses? We'll be happy to answer them in the comments. 
In summary
• Polarised lenses do not protect and UV protection, but allow visual comfort eliminating distracting reflections.
• They are useful for water sports or in bright light, but can cause problems when driving or using screens.
• Semi-polarised lenses offer a compromise: fewer reflections than non-polarised lenses, more versatile.
