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How stuff works: The ultimate sunglasses

It is that time of year in the United States — the peak of summer. The sun is at its strongest, and when you go outside on a bright day the urge to squint is instantaneous. With so many photons raining down from the sun, people definitely like to be wearing sunglasses.

But it brings up a question: which sunglasses should you wear? Is a $5 pair bought at a discount store just as good as a $150 pair bought at a specialty retailer? It turns out that the expensive pair of glasses really does have some advantages, which we can explore by looking at the ultimate pair of sunglasses.

You can then decide whether these advantages are worth $150 to you.

Any pair of sunglasses starts with the lenses. And there are real differences in lens quality. Fortunately, you can actually see the difference by looking through the lenses. A high end pair of sunglasses will use optical quality glass or plastic for the lenses. A $5 pair might use any old piece of plastic. The way to tell if the cheap glasses are going to cause a problem is to look through the lenses at straight lines (e.g. on the ceiling). Then move your head. If you see the lines distorting, waving or curving as move, you know that the lenses are bad.

The next thing to look for in a pair of sunglasses is the color and density of the tinting. This is the reason why you buy sunglasses, and it is especially important in certain environments because the tinting can prevent eye damage.

You may have heard of snow blindness. So much light reflects off the snow into your eyes that it actually causes damage to the retina. The same can happen on white sand beaches and near water. At a good sunglass retailer, you can see different levels of tinting from 70 percent to 97 percent.

Also important is UV protection. You need to look for glasses that block 100 percent of both UVA and UVB light. Without UV protection, your eyes will be damaged by UV light.

Polarization is another technique for blocking light, and it is especially effective on glare. Glare occurs when there are very bright reflections in the scene. The reflections can be bouncing off of water, lass, polished surfaces (e.g. cars) etc. Polarized lenses can eliminate a great deal of glare by selectively filtering it out.

Another thing to look for in the ultimate pair of sunglasses is scratch resistance. Plastic lenses are especially susceptible to scratching. But a film of diamond-like carbon on the lenses can almost completely eliminate scratches so your glasses will last for many years.

And then there are anti-reflective coatings, similar to those used on high quality camera lenses. When used on the inside of the lenses, anti-reflective coatings eliminate back-reflections in very bright environments.

Now you can see what the lenses look like in the ultimate pair of sunglasses. They have optical-quality materials that create no distortion, the right tinting density, UV protection, polarization, scratch-resistance and an anti-reflective coating.

Do you need mirrored glasses? This is strictly a matter of taste, because there is no inherent advantage to mirrored lenses. The mirror coating is simply another tinting technique. One problem with many mirrored lenses is that they scratch easily.

Now that you have the perfect lenses, you need great frames. Most importantly, frames need to be the right size for your face. If they are too large or too small, they will slip off more easily. Spring-loaded temples can give your glasses a tighter fit and may also help protect them against damage. If you are spending a lot of money for glasses, you might look for some kind of frame warranty or damage guarantee.

The frame will also control the size and shape of the lenses. For example, wrap-around frames can provide more protection for your peripheral vision. Try on many different styles and see what you like best.

Do you need every one of these features? No. But at the very least you should make sure that your next pair of sunglasses has good optical quality, complete UV protection and the right tinting density for your environment.

Looking for more? For extra info on this or the scoop on other fascinating topics, go to HowStuffWorks.com. Contact Marshall Brain, founder of HowStuffWorks, at marshall.brain@howstuffworks.com.

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