The brightness or intensity of light is measured in lumens. For example, when you are indoors, most artificial light is around 400 to 600 lumens. If you go outside on a sunny day, the brightness ranges from about 1,000 lumens in the shade to more than 6,000 lumens. When the brightness of the direct or reflected light gets to about 4,000 lumens, our eyes begin to have difficulty absorbing the light- to reduce the discomfort we squint. Once you get to around 10,000 lumens, your eyes are so bombarded that they begin to completely block out the light. Prolonged exposure to light of such intensity can cause damage resulting in temporary or even permanent blindness.
Sunglasses work by tinting the lenses – this reduces the amount of light hitting your eyes. Different colour lenses block out different colour frequencies, and can have slightly different effects, which can have different effects, such as improving contrast.
You also want your sunglasses to have a special coasting to filter out ultraviolet radiation, which can cause problems with your eyes. This is the same stuff as you are trying to protect against when you wear suncream.
There are a lot more special features sunglasses can have, but these are the most important, and what you should look for when buying your sunglasses