VLT stands for “Visible Light Transmission”. This is how much light passes through the lenses and reaches your eyes. It’s measured by the thickness of the lenses. The different VLT percentages work well for certain conditions. A VLT of 0-19% is good for sunny and bright conditions. VLT 20-40% is good for everyday use. 40-80% BLT works best for overcast or low light conditions. 80-90% VLT is good for dark or night time conditions.