The amount of light transmission is measured by VLT percentage (visible light transmission). The lower the VLT percentage the darker the lens tint. Our darkest lens (grey) is 11% VLT, meaning 89% of light entering the lens is blocked. Ideally, a glacier lens blocks more than 89% of light, 6% is ideal. You can always get a pair from us, take them to your local eyewear shop and pop in glacier glacier lenses for specific trips!