UV 400 both UVA and UVB radiation. Blocking light rays as small as 400 nanometers. This means that anywhere from 99 to 100 percent of the sun's harmful radiation is kept away from the eyes.
Visible Light Transmission (VLT). This measures how much light your sunglass lens lets through. VLT of 1% means very dark lenses and a VLT of 100% means very light lenses.