UPDATE August 4: This article was originally posted July 25, 2017. A lot has changed since then. All of the links below are to safe sources of solar eclipse glasses, viewers, and filters from reliable companies, but many of them have now sold out. As reliable dealers sell out of their stock of safe products, Amazon and other third party online sites have been flooded with unsafe, look alike, counterfeit products from fly-by-night companies. Read this important article for details: Unsafe Counterfeit Solar Eclipse Glasses Are Flooding Amazon and Other Online Markets
Here’s the short answer. Don’t use any kind of eclipse glasses or eclipse viewers unless you can find evidence that they are ISO 12312-2 certified. If they aren’t, don’t use them.
The long answer is here, complements of the American Astronomical Society.
Most solar filters that are designed for photography are photographically safe, but not visually safe. That means they are not safe to use to view the sun. Even on a camera with a solar filter on the front of the lens, you can’t safely look the the viewfinder of the camera. You have to use the camera in live view mode. So if you have a solar filter that is not ISO 12313-2 certified, don’t use it to look at the sun. The big exception is solar filters from Thousand Oaks Optical which are ISO 12313-2 certified and safe to use to look at the sun.
If you are looking for eclipse glasses, goggles, or binoculars, be sure they are ISO 12313-2 certified. I recommend Eclipsmart glasses, goggles, and binoculars from Celestron who has been in the telescope and astronomy business for decades. Their glasses are made by American Paper Products who use safe materials made by Thousand Oaks Optical. EclipseView filters by Meade Instruments (another fine telescope company) are made by by Rainbow Symphony which is another company who uses materials from Thousand Oaks Optical.
I also recommend a 90mm solar filter from ICSTARS for viewing and photography. Their solar filter is also ISO 12313-2 certified. They use SolarLite film which is from, you guessed it, Thousand Oaks Optical.
The same filter comes in a smaller pair (50-69mm outside diameter) for binoculars.
Whatever brand you choose to buy, look for “ISO 12312-2” or “ISO 12312-2:2015” to be sure your eyes are safe. The material used to make these products is somewhat fragile. Every time you get ready to use one of these products, make sure the material has not been damaged before you look through them at the sun.
Stay safe out there. The articles below will point you in the right direction.
The Great American Eclipse Series
How To Tell If Your Eclipse Glasses or Handheld Solar Viewers Are Safe – the American Astronomical Society
Make or Buy Your Own Inexpensive Solar Filter to Watch and Photograph the Eclipse – It is not too late. Do this today.