One is to add hardness, because resin lenses are soft and easy to hang flowers, and a hard film should be added outside to prevent scratches. The quality of this film also determines the wear resistance of resin lenses;

Another function is to increase the light transmittance, so it is also called antireflection film. The light transmittance of ordinary optical lenses is about 9 1%, and about 9% of the light will be reflected by the lens surface. The antireflection film is a very, very thin coating with a thickness similar to the wavelength of light. More light can pass through the lens, and the light transmittance can reach more than 95%, even more than 99%. If you want to know its principle, you can look at optics.

In addition to these two outer coatings, it also has the functions of anti-reflection, anti-glare, waterproof and dustproof.

When you see green reflection, it is generally called green film, which has many kinds, such as blue, purple, yellow-green, red, gold and so on. Some optical shops will say that blue anti-radiation and purple anti-ultraviolet are easy to sell and expensive, but they are all nonsense. Lens radiation protection is fake, even if thousands of famous brands have no radiation protection function, only famous brand lenses are so nonsense, and ultraviolet protection is even more outrageous. Ultraviolet rays are invisible, so you can't see them.