UV filter, UV radiation and sunglasses
Aside from an aesthetic function, protection against the effects of sharp sunlight and UV radiation is still the main function of sunglasses. The entire electromagnetic spectrum of sunlight is compound of a set of waves continuously changing frequencies.
Visible light represents only an immaterial part (400 - 780 nm). Eyes perceive this wavelenght area as a spectrum of colors from violet through blue, green, yellow, orange to red. Visible light induces phosphene and it is termlessly a necessary means to obtain visual information about the external world. Longer wavelenghts belong to infrared radiation, microwaves, television and radio waves. On the other hand, shorter wavelenghts have ultraviolet, x-ray and gamma radiation.
Solar radiation with a wavelenght of about 780 - 106 nm, which has a nature of infrared radiation is perceived by a human mainly by a surface in the form of heat exposure. This radiation penetrates into the skin and muscles and causes a better blood circulation. Solar radiation with wavelenght less than 400 nm belongs to an ultraviolet radiation, that is very harmful and it is therefore necessary, that anti-sun eyeglass lenses retain this spectrum area. Sunglasses with such a protection are labeled with a reference to UV 400. Generally, plastic eyeglass lenses have better UV absorption than mineral lenses.
According to effects on humans, UV radiation is divided into zones:
- long-wave UVA radiation .......................... 315 - 400 nm
- middle-wave UVB radiation ...................... 280 - 315 nm
- short-wave UVC radiation ......................... 100 - 280 nm
UVB radiation is a radiation, that causes burned skin and can cause visual complications. Although not all of eye specialists agree with that, some surveys show, that exposure of eyes to sunshine, for example on mountains can contribute to a formation of cataract.
Distribution of anti-sun filters
There are four standardized categories of filters:
- Category 0 - colorless or a very light filter (of a visible radiation (SEE) 80 - 100%)
- Category 1 - light filter ( SEE 43 - 80%) Category 2 - medium dark filter (SEE 18 - 43%)
- Category 3 - dark filter (SEE 8 - 18%)
- Category 4 - very dark filter (SEE 3 - 8%)
Categories 3 and 4 are not suitable for driving of motor vehicles.
What to look out when purchasing sunglasses?
To achieve a maximum protection for your eyes, look for sunglasses, that absorb at least 98% of UVA and UVB radiation. UVC radiation is absorbed automatically by higher atmospheric layers.
