First, do not be misled by faulty UV claims. When you decide to buy a pair of sunglasses, you have to make sure that there is an available equipment to check the lenses' UV protection capabilities in order to make sure that UV labels are accurate. Or you can ask your optometrist to help you purchase one.
Second, make sure that the lenses' tint is uniform, not darker in one area than another. As to the gradient lenses, the tint should lighten gradually from top to bottom. Take driving sunglasses as an example. The recommended color tint is gray, which can offer proper traffic light recognition.
Third, make sure that lenses are distortion-free. So you have to hold the sunglasses at arm's length, look through them at a straight line in the distance, and slowly move the lens across the line. If the straight edge distorts, sways, curves or moves, the lenses have imperfections.
Fourth, To be sure that the lenses block enough light. To do this, you have to try them on in front of a mirror. If you can see your eyes easily through the lenses, they probably are not dark enough. However, this test does not apply to photochromic lenses.
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