Gradient lenses are those that are darker at the top than they are at the bottom. These lenses are most often used in sunglasses, and they are appropriate to use while driving because they protect the eyes from the sun overhead while allowing better visibility through the bottom.
Lenses can also be double gradient. A double gradient lens refers to one that is dark on both the top and the bottom while the middle of the lens is lighter. This is ideal for an individual who wants sunglasses that shield the eyes from bright glares but are not too dark to see straight through.