Light is energy, and the ultraviolet, or UV, light from the sun has the highest energy, just above the visible light spectrum. This kind of light can damage your eye's cornea and retina. Light intensity is measured in lumens. When direct or reflected light, like sunlight reflecting off of snow, gets to 4,000 lumens or so, our eyes cannot absorb all of the light. We squint because the glare causes discomfort. Extreme intensity of light (more than 10,000 lumens) without eye protection for a prolonged period can cause temporary or permanent damage to your vision.
Sunglass lenses protect our eyes from the different kinds of light. The tint determines the spectrum of light that the lens absorbs. Gray is an all-purpose tint that is effective against glare and brightness. Yellow tints are usually used for snow goggles or glasses because they virtually block out blue light, which affects sharpness and brightness. Green tints also block some blue light and enable sharp vision without distorting color perception as yellow tint does. Brown tints also can distort colors, but with better contrast and clarity and less glare. For green or blue backgrounds, like forests or water, purple and rose tints work best for contrast.
Some sunglasses have a reflective coating that makes them mirrored. This very thin layer on the lens is called a half-silvered surface. The reflective molecules are scattered over the surface, and only about half of the lens actually reflects the light. The mirror coating sometimes is applied so that the lens shades gradually from top to bottom, blocking more light from the top and middle of the lens and allowing more in from the bottom. This gradient in coating improves visibility for driving conditions.
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