The biggest difference between cheap and expensive sunglasses is the material of the lenses. Cheap sunglasses use ordinary plastic lenses with very poor clarity and shape. Expensive sunglasses use crystal lenses with very good clarity and shape.

The outer packaging of regular anti-UV sunglasses must be marked with ultraviolet protection level (UV) in accordance with national standards, with UVA (filtering wavelength between 320-420 nanometers), UVB (filtering wavelength between 275-320 nanometers) and There are three types of UV400 (filtering wavelength below 400 nanometers).

All three have UV protection. Usually the UV index of sunglasses is between 96% and 98%. Generally speaking, a UV index of 100% is unlikely.

Sunglasses are the same size as frames and are designed and produced according to standard specifications. The size parameters of the sunglasses are marked on the temples, nose bridge or label for reference. For example: 53-16-130, which means the frame size is 53mm, the nose bridge size is 16mm, and the temple size is 130mm.

The principle of sunglasses

Sunglasses can block uncomfortable bright light and protect your eyes from ultraviolet damage. All this is possible thanks to metal powder filters that "select" light as it hits it. Colored glasses can selectively absorb some of the wavelength bands that make up the sun's rays because they use very fine metal powders (iron, copper, nickel, etc.).

In fact, when light hits the lens, it is reduced based on the so-called "destructive interference" process. That is, when certain wavelengths of light (in this case, UV-A, UV-B, and sometimes infrared) pass through the lens, they cancel each other out on the inside of the lens, toward the eye.