What is the truth about UV400 ?

UV400 lenses block 99% or more of ultraviolet radiation across the full UVA, UVB, and UVC spectrum (wavelengths below 400nm). 

This is the baseline standard recommended by the American Academy of Ophthalmology for any quality sunglass.

A common misconception is that darker lenses mean better UV protection. In reality, dark tints only reduce visible light. Without proper UV400 blocking, dilated pupils behind dark lenses can actually allow more harmful UV rays to enter the eyes.