What are eclipse glasses, and how do they keep your eyes safe during an eclipse?

B. Ralph Chou is an expert on how to watch eclipses safely. Not only did the professor emeritus of optometry and vision science at the University of Waterloo write the safety standards for eclipse glasses, he’s also an eclipse chaser. Chou has seen 30 solar eclipses in his lifetime, and counting. “The only places in the world that I have not touched yet are India and Antarctica,” he says. “My passport’s got all sorts of interesting stamps.”

Chou’s next eclipse will be the April 8 Great North American Eclipse, during which much of Mexico, the US, and Canada will experience at least a partial eclipse. Around 31 million people live in the path of totality, the area where the sun will be completely blocked by the moon. Many more are expected to travel to the area of totality (Chou is going to Texas).

If you’re one of these millions planning on watching the upcoming eclipse, you probably know you need eclipse glasses to protect your eyes. But what are these cardboard-and-plastic contraptions that you’re slapping on your face? And how do they work to protect your eyes from potentially damaging solar rays?

Why do you need eclipse glasses?

Most of the time, people don’t stare at the sun—because it hurts. But when an eclipse is happening, people have a reason to gaze skyward. Even though the moon covers up all or most of the sun and the sky gets dark during an eclipse, the sun is just as likely to hurt your eyes, Chou says. “The fact is that there’s still an unoccluded part of the sun’s disk that is every bit as shiny as on any other day.” These small bits of sun shining through are more than enough to permanently injure your eyes.

This damage occurs when too much light hits your eyes, kicking off a photochemical oxidative reaction within the cell membranes, Chou says. The full reaction starts with a photoinduced isomerization, which leads to a radical cascade inside your eyeballs and ends with multiple destructive compounds chewing up important chemicals you need for seeing. If this reaction goes on long enough, it can cause cell death and harm your retinas. This is bad.

Eclipse glasses are dark enough to keep a dangerous amount of light from getting into your eyes and potentially wreaking this radical havoc. The strongest sunglasses absorb or block up to about 99% of light, but to look at the sun safely, you need glasses that block or absorb 99.9997% of light, Chou says. “It’s about 1,000 times darker than the darkest lens that you can get at your sunglasses store,” he says. “It’s like wearing a blindfold. You do not walk around with these things on.”

Many of the eclipse glasses on the market today block the light with dark polymer film made by embedding carbon black powder inside a resin matrix, Chou says. Manufacturers can control the film’s darkness by adding more carbon to the mix, he says.

For extra protection, these types of polymers are often then backed with a semitransparent surface coating of aluminum. The carbon black lenses absorb the light, while the aluminum backing reflects the light. This combination works very well, Chou says.

How to look safely at an eclipse

For eclipse glasses to be safe, they need to meet the international safety standard ISO 12312-2 (sometimes written as ISO 12312-2:2015) from the International Organization for Standardization (ISO). The American Astronomical Society keeps a list of trusted suppliers, and Chou suggests shopping there instead of sifting through what’s available on Amazon.

But there are more ways to make sure you have safe eclipse glasses, Chou says. One is to take a close look at the label. “In addition to the warning label and the instructions on how to use the device, there should be identification of who made the product,” he says. This should include the name and address or location of the manufacturer and who did the compliance test of the glasses. “So if you have all of those four pieces of information—warnings, instructions, manufacturer, and testing labs—as well as the statement of compliance with the ISO standard, then you’ve got a product that is more likely to be safe,” Chou says.

If you don’t have eclipse glasses, Chou suggests watching the eclipse through your phone. You could fry your camera, but that’s better than damaging your eyes, Chou says. “Even though the image on the phone is extremely bright, that’s perfectly safe to look at because the phone is only capable of producing so much light in that image,” he says. It’s going to be glaringly bright, he says, but it’s not dangerous to your eyes. To spare both phone and eyeballs, you can put your eclipse glasses over the phone’s lens.

Chou is a fan of solar eclipse–viewing apps for smartphones. Some of the several apps he has on his phone include Totality by Big Kid Science, My Eclipse, Solar Snap, and the Great American Eclipse app. These can give you information such as the closest spot to view the totality, what percentage of the sun will be covered at your location, and when you can start watching for the partial eclipse. “The tools have changed a lot. They’ve also made the whole idea of observing an eclipse a lot more accessible,” he says.

Watching the eclipse through a pinhole viewer is another option. This simple contraption made with paper, aluminum foil, and cardboard acts like a teeny camera lens. Although it may not be the most satisfying way to watch a total eclipse, it is pretty easy and accessible. Chou watched his first eclipse with a pinhole viewer in 1963.

“It’s amazing how it doesn’t matter what culture you’re from, what your background is—you get the same reaction,” he says. “It hits every one of us.” But make sure you’re wearing your eclipse glasses, Chou says. “That way you can experience and enjoy the eclipse and still have your vision the next day.”