Unlike ordinary sunglasses or photochromic lenses that merely reduce brightness, UV400 lenses filter out all light rays with wavelengths up to 400 nanometers. This includes UVA, UVB and high-energy visible (HEV) blue light.
To be considered UV glasses, the lenses are required to block 75% to 90% of visible light and must offer UVA and UVB protection to block 99% of ultraviolet radiation.
Ideally, you want sunglasses that offer UV 400 protection since they offer virtually 100% protection from UV rays.
Note that not all sunglasses are considered UV-protection sunglasses. A pair of sunglasses may have dark lenses, which could be presumed to block rays, but that doesn’t mean the shades provide adequate UV protection.
If those sunglasses with dark lenses don’t include UV protection, those darker shades are actually worse for your eyes than not wearing any protective eyewear at all. Why? Because the dark tint can cause your pupils to dilate, exposing your eyes to more UV light.
How can I tell if my glasses have UV protection?
Unfortunately, it’s not easy to tell if your sunglasses or photochromic lenses have UV-protection lenses just by looking at them.
Nor can you distinguish the amount of protection based on the lens color, as lens tints or darkness have nothing to do with UV protection.
Your best bet is to take your glasses to an optical store or a Professional testing institutions. They can run a simple test on your glasses to determine the level of UV protection.
Or the simple choice is by centering your search on a reputable, and professional manufacturer like UNIVERSE OPTICAL, and choosing the real UV400 sunglasses or UV400 photochromic lenses from the page https://www.universeoptical.com/1-56-aspherical-uv400-q-active-material-photochromic-lens-product/.