In preparation for Monday’s total solar eclipse - the first visible from the mainland United States for over 30 years - millions of Americans have bought heavily-shaded glasses to let them safely view the event. But unscrupulous dealers have taken advantage of the huge demand by selling fake glasses, and using them could lead to permanent eye damage.

So before the big day, it's probably worth making sure the glasses you'll be using are real. Here's how to find out.

Find the Source

The American Astronomical Society has a list of companies that either make eclipse viewers that have been specifically reviewed by the AAS, and retailers who only sell genuine glasses. Some of the manufacturers sell components that might be marketed under other brands, and the AAS doesn’t claim the list is comprehensive, so there may be other genuine brands and sellers out there. But using a brand or vendor on the list should be a safe bet.

For last-second online shoppers, there's also a list of Amazon sellers who will send you certified specs.

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Check the Code

The safety standards for eclipse viewing glasses are set by the International Organization for Standardization, or the ISO. All compliant glasses should have the ISO safety code "ISO 12312-2" printed somewhere on them.

Unfortunately, the AAS has received reports of fake glasses that also display the code, so it's not quite enough to make sure you're safe.

Test the Glasses

Legitimate eclipse-viewing lenses are designed to completely block out all but the very brightest light – that is, the sun, the brightest thing in the solar system. You can test this by putting on your eclipse glasses and staring at a bright light bulb or LED flashlight. If the glasses let through any light other than the sun's - even a little bit - they may not be safe.

Finally, don’t think you can substitute proper eclipse shades with something from around the house. Even many welding helmets, designed to let workers look at incredibly bright welding arcs, won’t cut it.

See original article on Fortune.com

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