UV 400 is a form of ultraviolet radiation protection associated with sunglasses. UV 400 protection is considered to provide close to 100 percent protection against harmful ultraviolet rays.
UV 400 protection is supposed to provide protection against 99 to 100 percent of UVA and UVB ultraviolet radiation. It is called UV 400 because it blocks all rays of light up to a wavelength of 400 nanometers. According to Crush Eyes, it is considered the highest form of protection that lenses can provide. UV 400 has a higher level of protection than the European standard of protection, known as EN 1836:2005. This European standard allows for up to 5 percent of 380-nanometer rays to be transmitted through sunglasses.