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*Update 8/16: If your solar eclipse glasses will not arrive in time for Aug. 21, you have options like Walmart, Best Buy, local libraries and other physical locations that might still have glasses for safe eclipse viewing. See our full story here on what to do. Don't forget, a live stream is perfectly safe if you're not in the totality path. Here's our solar eclipse live video stream roundup.*

*Update 8/4: Reports have recently surface of consumers unknowingly purchasing unsafe or counterfeit solar viewing glasses. If you have already purchased eclipse glasses, there are ways to test whether your glasses are safe to use. If you haven't purchased eclipse viewers yet, be sure to purchase a pair from one of the manufacturers or vendors that have been approved by the American Astronomical Society. If you're looking to buy eclipse glasses online, try to buy directly from one of the approved vendors.* 

If you're one of the tens of millions of Americans who will experience the Great American Total Solar Eclipse on Aug. 21, hopefully you've heard by now that you're going to need eye protection for the big event.

But not all solar viewers are created equal. Using the wrong gear (or using it incorrectly) can burn your retinas, causing irreparable damage to your eyes.

Whether you're looking for a new pair of eclipse glasses or you've already purchased some form of eye protection, here's what you need to know to avoid burning your eyes during the solar eclipse. [Total Solar Eclipse 2017: Path, Viewing Maps and Photo Guide]

You should never look directly at the sun, but there are ways to safely observe an eclipse. <a href="http://www.space.com/15614-sun-observing-safety-tips-infographic.html">See how to safely observe a solar eclipse with this Space.com infographic</a>.
You should never look directly at the sun, but there are ways to safely observe an eclipse. See how to safely observe a solar eclipse with this Space.com infographic.
Credit: Karl Tate, SPACE.com Contributor

When it comes to solar-eclipse glasses and other solar viewers, it's important to ensure that the product you're using is safe and effective at blocking harmful radiation from the sun. Just because something looks like suitable eye protection doesn't mean it's safe to use. Even if a product is advertised as a solar viewer, it's important to look for a label that says ISO, which stands for the International Organization for Standardization.

The ISO is an independent organization that writes safety and quality standards for all kinds of things, including eyewear, health care, food production and more based on a broad consensus of the scientific community. If you find eclipse glasses or other solar viewers that aren't labeled ISO, then they aren't guaranteed to protect your eyes the way they should.

ISO-approved solar-eclipse glasses must meet certain safety requirements:

  • No more than 0.00032 percent of the sun's light may be transmitted through the filters.
  • The filters must be free of any defects, such as scratches, bubbles and dents.
  • Handheld viewers must be large enough to cover both eyes.
  • Labels on the viewers (or packaging) must include the name of the manufacturer, instructions for safe use and warnings of the dangers of improper use.
These eclipse glasses by American Paper Optics are ISO-approved and have instructions and warnings printed on the back.
These eclipse glasses by American Paper Optics are ISO-approved and have instructions and warnings printed on the back.
Credit: EclipseGlasses.com

Most ISO-approved eclipse glasses use solar filters manufactured by AstroSolar and Thousand Oaks Optical, Rick Fienberg, press officer for the American Astronomical Society (AAS), said. However, several different retailers sell eclipse glasses and handheld filters that are approved by the ISO, he noted.

The AAS has published a listof reputable vendors and manufacturers of eclipse glasses that includes Lunt Solar Systems, American Paper Optics and Rainbow Symphony.

Eclipse glasses and sunglasses might look somewhat similar, but they are made of very different materials. You should never look directly at the sun while wearing sunglasses, no matter how tinted your lenses may be.

"Normal sunglasses typically let in between 10 and 20 percent of daylight…but that's still way too bright," Fienberg told Space.com. "The filters that are made for looking at the sun are typically 100,000 times darker."

Sunglasses are typically made of glass, plastic or some kind of polycarbonate material, while solar filters are made of one of two materials: polyester film coated in aluminum, or something called "black polymer," Fienberg explained. Most eclipse glasses and solar viewers use the black polymer, which is a flexible resin infused with carbon particles. Both types of filters will reduce visible light down to safe levels.

Welders planning to observe the solar eclipse may or may not be in luck, as some welding filters will adequately protect your eyes from the sun. But, please, double-check to make sure that the goggles you intend to use are the right kind.

"There is a particular circumstance in which it's safe," Fienberg said. "We don't recommend it because it's too easy to get the wrong kind of welding filter." Only goggles made for electric arc welding can be used to observe the sun, and they must have a shade scale number of 12 or higher. Shade 13 is ideal for solar viewing, but that shade is typically not sold in stores, Fienberg added. 

If you're watching the eclipse from the path of totality, you should absolutely remove your eclipse glasses during totality. "In fact, if you keep your filters on during totality, you won't see anything" because they block out almost all light, Fienberg said.

However, if you're watching the partial solar eclipse from other parts of the U.S., you'll need to keep them on the entire time. If you take them off, not only do you risk burning your eyes, but you also won't be able to see the eclipse. [What You’ll See During the 2017 Total Solar Eclipse]

"A total solar eclipse is truly spectacular and awesome in the true meaning of the word 'awesome,' whereas a partial solar eclipse could pass unnoticed," Fienberg said.

"Even if you do have a solar filter and watch the sun turn into a thin crescent, it's nowhere near as exciting as a total eclipse, because you miss all the really spectacular phenomena that are associated with totality. It doesn't get dark, you don't see the corona, you don't see bright red prominences of gas jetting off from the edge of the sun. It's just not the same at all."

So get to the path of totality if you can. And whatever you do, don't forget to bring the right kind of eye protection!

Email Hanneke Weitering at hweitering@space.com or follow her @hannekescience. Follow us @Spacedotcom, Facebook and Google+. Original article on Space.com.