The protetion from UV radiation probably is the most important feature of sunglasses. UV radiation begins underneath 380 nanometers outside of the visible light spectrum. Even though the radiation is visible the UV radiation can emblaze the conjunctiva and blear the cornea. Glasses with UV protection prevent the percolation of the short-wave, damaging radiation. This function is very important in the mountains because there often predominates an increased radiation.
The protection of UV radiation by the way is not to mistake with the visor which is achieved with the shade of the glasses. Depending on the shade more or less light is let through. When the sun strongly shines you need a stronger shade so less light is let through than when the sky is overcasted. The visor is classified in five categories demanding on the percental translucence of the glasses. In central-European zones normally category 2 is recommended. Many manufacturers offer optianal glasses with lighter and stronger shades what can be meaningful in the cycling if you are on your way from morning to night. Then the lenses are lighter but still protect the eyes from UV radiation and other dangers. In strong sunlight – in the mountains for example - a stronger visor is necessary.
A hydrophobic covering causes the lotus blossom effect on the glasses. That means the surface tension changes through the covering and makes annoying waterdrops roll of the glasses.
A „free visibility“ is also promised by glasses with an oleophobic covering which makes the glasses resistant against dirt. So dust, dirt and grease spots cannot accumulate on the glasses. The lenses stay clean for a longer time and are easier to clear if it happens that dirt sticks on the glasses – especially useful for the representatives of the off-road fraction.
Another important protective filter is the polarisation filter which offers a reflection protection. A polarisation filter absorbes light reflection on smooth, reflective surfaces and compounds the contrast acidity. The view of the cyclist stays unimpaired and the eyes are preserved from overexertion.
Because you often are on your way with the bicycle at higher speed the glasses should also protect you from draft and wind. Therefor not the covering but the anantomy of the frame and the form of the glasses are responsible. If the frame and the glasses are strongly curved they enclose the eye better and the wind can't easily infiltrate on the sides. Most of the features have a claim to universality and are meaningful on every sun glassess. But the requirement of a mountain biker to good sports glasses are far beyond that. So specific cycling glasses offer features that lighten the decision for cycling with glasses.
There is nothing annoying so much while cycling like sun glasses that don't fit well. If they are too tight a slight pressure behind the ears can turn into an unpleasant headache after several hours of driving. Even more annoying are sun glasses that are too loose and slip in every move. Especially in the off-road where all the concentration of the driver is required the hands should stay on the handlebar and not permanently adjust the glasses. Sun glasses have to be comfortable or best not noticeable at all. The manufacturers have adjusted themselves to that and so they have equipped their models with corresponding features. Most sun glasses are offered with flexible or interchangeable nose pads. There are nose pads out of silicone which are hypoallergenic and long durable or out of special antislip rubber. They look for an individual customization and an optimal fitting. The earpiece endings are mistly added with a wire core and so they are variable. There is a rubber covering in most of the models which additionally grants hold. So the bike glasses can be adapted to the head shape of the wearer. The frame material mostly is polyamide, acetate or grilamide. These plastic materials make the glasses light so no uncomfortable pressure points are created.
Especially in mountain biking we often have changing lighting conditions. While driving from a shady forest up in unwooded heights the danger of being blinded is very high. Then it is good to have sun glasses that act like a chameleon. Photochromic lenses are lenses that automatically adapt to the lighting conditions. In contact with insolation they automatically darken and when there is no insolation anymore they become lighter. They are often called phototropic glasses what means something like „light manoeuvrable“. This feature got invented and patented in the 1960s. Back then the process of darkening and brightening was very slow and only possible when the glasses were mineral. Meanwhile a lot changed in this area and even plastic material glasses have this technology. In contact with sun light the glasses directly react and get darker within a few seconds. If the insolation lasts the glasses slowly get darker in the next 15 minutes until they completely adapted to the lighting coonditions. When there is no insolation this process turns back. This slow adjustment to the new lighting conditions is not being recognized from the driver. Important is the first reaction that makes sure that the driver is not blinded or suddenly feels "blind" in shady sections.
„Unbreakable“ - this is a feature many mountain bikers wish themselves for their bones. At least some sun glasses fulfill this wish thanks to modern glasses and frame technology. Most of the lenses of sports glasses nowadays consists out of polycarbonate a plastic material which is known for its high break resistance. The material is light, scratch-proof and has a long endurance. In the area of break resistance it is constantly being researched and the manufacturers always use new scientific knowledge. Meanwhile many brands have their own patented concept in which additional coverings are used to increase the break resistance. This feature can especially be worthwile for drivers who go to the limit of their driving ability or over it. In some of the newest models of sun glasses the so called NXT technology is used. A bullet-proof and lightweight material that was originally developed as a protective glass for military planes. Consequently the eye is well protected even when the trail fires stones or split on the sun glasses. Even frame and earpiece become more and more break resistant and are partially so flexible that they get back to their original shape after a deformation.
Even if the glasses are break resistant and the eyes are protected from dirt and UV radiation another unpleasant troublemaker can appear while cycling. Because of the fast change in temperature which can be attributed to differences in altitude or changes in the weather pattern the glasses can easily get steamed up. Other causes are body heat, transpiration and the breath of the cyclist. Then the glasses get steamed up from the inside. An annoying phenomenon which only disappears when the moisture on the lenses evaporates. This process speeds up when the lenses are „ventilated“ from the inside. Between head and glasses basically circulates air but some manufacturers offer special ventialtion systems which are sidewise the earpieces, ahead in the nose clips or in the forehead bearing. Others work with an anti-fog_formula which is used as a coating on the glasses. But in the course of time there can be some attrition and once the covering is gone there is no chance to regenerate it except the glasses are changed. There is no 100 percent protection from steaming up but with the offered features the ventilation already got better.