VLT stands for "Visible Light Transmission" and refers to the amount of light that reaches your eyes through the lens of sunglasses or goggles. It is measured as a percentage, with a higher percentage indicating that more light can pass through the lens, and a lower percentage indicating that the lens is more opaque and allows only a small amount of light to penetrate. The VLT of standard sunglasses is typically between 15% and 25%, while high-altitude mountaineers may require sunglasses with a VLT of 5% to 10% due to more intense light at higher altitudes. Here are some general guidelines for choosing sunglasses based on VLT percentages:

• 0-19% VLT: Ideal for bright, sunny conditions.
• 20-40% VLT: Good for all-purpose use.
• 40+% VLT: Best for overcast and low-light conditions.
• 80-90+% VLT: Virtually clear lenses for very dim and night conditions.

Its important to note that VLT is affected by the color and thickness of the lenses, the material theyre made of, and the coatings they have on them.