Category numbers on sunglasses refer to their visible light transmission (VLT). This is the percentage of light that the lens lets through. High VLT will means a lighter lens. Low VLT will mean a darker lens. The higher the category number, the higher the VLT. Here is a quick overview of the Categories: Category 0 is usually applied to safety glasses that only provide impact protection. They have a very light tint, or will be clear altogether. Blue light blocking lenses are also in this category. Category 1 glasses are useful in overcast conditions. They provide little value under bright sun. These glasses tend to have a yellow or otherwise pale tint. Category 2 glasses work fine in somewhat sunny conditions. They give decent glare protection. They might be orange, blue, rose, or red. Category 3 are the most common category of lens. These tend to be brown or grey. They have a VLT of 8-18%, meaning they block 82-92% of visible light. They are excellent for glare and great for strong sunlight. Most (though not all) sunglasses fall into this category. Category 4 sunglasses are very dark grey or brown. They are usually used by mountaineers or people in deserts. These types of sunglasses reduce glare by a significant amount. Category 4 sunglasses are actually detrimental to activities such as driving. They can reduce vision to the point of danger.