Short Answer
We used to say that you should look for evidence that they comply with the ISO 12312-2 international safety standard for filters for direct viewing of the Sun. But now the marketplace is being flooded by counterfeit eclipse glasses that are labeled as if they're ISO-compliant when in fact they are not. So now we suggest that you make sure you get (or got) your eclipse viewers from one of the suppliers listed on our Reputable Vendors of Solar Filters & Viewers page.
Long Answer
The #1 rule for observing a solar eclipse, or for looking directly at the Sun at any other time, is safety first.
As noted elsewhere on this site, with one exception, it is never safe to look directly at the Sun without a special-purpose safe solar filter. That exception is during totality, when the Moon completely blocks the dazzlingly bright face of the Sun. On August 21, 2017, this will happen only within the roughly 70-mile-wide path of the Moon's dark inner shadow from Oregon to South Carolina — and only for a minute or two. Before and after totality, and at all times outside the path of totality, you must use a special-purpose safe solar filter when looking directly at the Sun.
"Special-purpose" means designed exclusively for looking directly at the everyday Sun. Filters for direct viewing of the Sun are typically sold in the form of wearable “eclipse glasses” or "eclipse shades" or as solar viewing cards that you hold in your hand. What makes them special is that they reduce sunlight to safe levels so that you don't injure your eyes. Our daytime star shines about a half million times brighter than the full Moon in visible light and emits potentially harmful ultraviolet (UV) and infrared (IR) radiation too. Looking directly at the Sun through anything that isn't specially made to deal with all that visible light and invisible radiation is a recipe for serious eye injury, perhaps even blindness. Note that special-purpose solar filters are many thousands of times darker than ordinary sunglasses!
What to Look For
How do you know if your eclipse glasses or handheld solar viewers are truly safe? You need to know that they meet the ISO 12312-2 (sometimes written as ISO 12312-2:2015) international safety standard. Filters that are ISO 12312-2 compliant not only reduce visible sunlight to safe and comfortable levels but also block solar UV and IR radiation.
Unfortunately, you can't check whether a filter meets the ISO standard yourself — doing so requires a specialized and expensive piece of laboratory equipment called a spectrophotometer that shines intense UV, visible, and IR light through the filter and measures how much gets through at each wavelength. Solar filter manufacturers send their products to specialized labs that are accredited to perform the tests necessary to verify compliance with the ISO 12312-2 safety specifications. Once they have the paperwork that documents their products as ISO-compliant, they can legitimately use the ISO logo on their products and packaging.
Even more unfortunately, unscrupulous vendors can grab the ISO logo off the internet and put it on their products and packaging even if their eclipse glasses or viewers haven't been properly tested. This means that just seeing the ISO logo or a label claiming ISO 12312-2 certification isn't good enough. You need to know that the product comes from a reputable manufacturer or one of their authorized dealers.
The AAS Solar Eclipse Task Force has been working diligently to compile a list of such vendors, now posted on our Reputable Vendors of Solar Filters & Viewers page. We've checked manufacturers' ISO paperwork to make sure it's complete and that it comes from a recognized, accredited testing facility, and we've personally examined manufacturers' products. We've asked manufacturers to identify their authorized resellers, and we've asked dealers to identify the source of the products they're selling. Only when everything checks out do we add a vendor to our listing.
If we don't list a supplier, that doesn't mean their products are unsafe — only that we have no knowledge of them or that we haven't convinced ourselves they are safe.
How can you tell if your solar viewer is not safe? You shouldn't be able to see anything through a safe solar filter except the Sun itself or something comparably bright, such as the Sun reflected in a mirror, a sunglint off shiny metal, the hot filament of an unfrosted incandescent light bulb, a bright halogen light bulb, a bright-white LED bulb (including the flashlight on your smartphone), a bare compact fluorescent (CFL) bulb, or an arc-welding torch. All such sources (except perhaps the welding torch) should appear quite dim through a solar viewer. If you can see shaded lamps or other common household light fixtures (not bare bulbs) of more ordinary brightness through your eclipse glasses or handheld viewer, and you're not sure the product came from a reputable vendor, it’s no good. Safe solar filters produce a view of the Sun that is comfortably bright (like the full Moon), in focus, and surrounded by dark sky. If you glance at the Sun through your solar filter and find it uncomfortably bright, out of focus, and/or surrounded by a bright haze, it’s no good. You should contact the seller and demand a refund or credit for return of the product, then obtain a replacement from one of the sources listed on our reputable-vendors page.
Remember that a genuinely safe solar viewer does more than reduce the Sun's visible light to a comfortable brightness level. It also blocks potentially harmful UV and IR radiation. The only way to tell whether your viewer does that is to be certain that it meets the ISO 12312-2 safety standard, and the only way to know that is to be certain that it came from a reputable vendor.
What if you received eclipse glasses or a handheld solar viewer from a relative, friend, neighbor, or acquaintance? If that person is an amateur or professional astronomer — and astronomers have been handing out eclipse viewers like Halloween candy lately — they're almost certainly ISO-compliant, because astronomers usually get their solar filters from sources they know and trust (in other words, from the ones listed on our reputable-vendors page). Ditto for professional astronomical organizations (including college and university physics and astronomy departments) and amateur-astronomy clubs.
If you bought or were given eclipse viewers at a science museum or planetarium, or at an astronomy trade show, again you're almost certainly in possession of ISO-compliant filters. As long as you can trace your filters to a reputable vendor or other reliable source, and as long as they have the ISO logo and a statement attesting to their ISO 12312-2 compliance, you should have nothing to worry about. What you absolutely should not do is search for eclipse glasses on the internet and buy whatever pops up in the ads or search results. Check our list of reputable vendors and buy from one of them.
In addition to making sure your eclipse shades or handheld viewers come from a reputable source, make sure they're in good condition:
- If the filters are torn, scratched, or punctured, discard them.
- If the filters are coming loose from their cardboard or plastic frames, discard them.
Note: If your eclipse glasses or viewers are compliant with the ISO 12312-2 safety standard adopted in 2015, you may look at the uneclipsed or partially eclipsed Sun through them for as long as you wish. Furthermore, if the filters aren't scratched, punctured, torn, or coming loose from their frames, you may reuse them indefinitely. Some glasses/viewers — even new ones — are printed with warnings stating that you shouldn't look through them for more than 3 minutes at a time and that you should discard them if they are more than 3 years old. Such warnings do not apply to eclipse viewers certified to meet the ISO 12312-2:2015 standard and may be ignored. Then again, why would you stare at the Sun for more than 3 minutes? A partial eclipse progresses so slowly that there's no point in looking at it for more than a few seconds every few minutes.
What to Avoid
Ordinary sunglasses (or multiple pairs of sunglasses), neutral density or polarizing filters (such as those made for camera lenses), smoked glass, photographic or X-ray film (unexposed, exposed, or developed), "space blankets," potato-chip bags, DVDs, and any other materials you may have heard about for solar viewing are not safe. In some cases these homemade filters may seem like they dim the Sun to a comfortable level, but that doesn't mean they do so across the whole electromagnetic spectrum. While you're enjoying a "comfortable" view of the "dim" Sun, solar infrared radiation could be cooking your retinas. And you wouldn't know till later, because your retinas don't have pain receptors. Only after the eclipse, when you notice blind spots or other vision problems, would you realize you'd made a catastrophic mistake.
What about welding filters? The only ones that are safe for direct viewing of the Sun with your eyes are those of Shade 12 or higher. These are much darker than the filters used for most kinds of welding. If you have an old welder's helmet around the house and are thinking of using it to view the Sun, make sure you know the filter's shade number. If it's less than 12 (and it probably is), don't even think about using it to look at the Sun. Many people find the Sun too bright even in a Shade 12 filter, and some find the Sun too dim in a Shade 14 filter — but Shade 13 filters are uncommon and can be hard to find. In any case, welding filters generally give a sickly green image of the Sun, whereas special-purpose solar viewers give a white, yellow, or orange image, which is much more pleasing and natural. If you really want to get a welding filter, we recommend that you buy it from a welding supply company; we've heard reports of people ordering "Shade 14" welding goggles from random online stores and receiving much lighter filters than they were promised. Our Reputable Vendors of Solar Filters & Viewers page doesn't list any suppliers of welder's filters, only suppliers of filters made exclusively for viewing the Sun.
Note that on this page we're talking only about filters made for direct viewing of the uneclipsed or partially eclipsed Sun with your eyes. Eclipse glasses and handheld viewers should never be used when looking through telescopes, binoculars, camera lenses, or other optical devices. For tips on using solar filters with optics, see Solar Filters for Optics: Telescopes, Binoculars & Cameras. To find telescopes and binoculars specially made for observing the Sun, see our Telescopes & Binoculars page (look for the Special-Purpose Solar Binoculars & Telescopes section).