How Do Solar Eclipse Glasses Protect Our Eyes?

How Do Solar Eclipse Glasses Work?

Solar eclipse glasses prevent catastrophic eye damage when observing the sun. Here’s how they work

Young black woman wearing eclipse glasses and bright yellow jacket smiles while looking upward

Solar eclipse glasses are a must when looking at a partial solar eclipse.


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This article is part of a special report on the total solar eclipse that will be visible from parts of the U.S., Mexico and Canada on April 8, 2024.

In the U.S., April’s total solar eclipse will travel across a 115-mile-wide band stretching from Texas to Maine. And even those who can’t make it into this “path of totality” can view some of the spectacle because, outside of this zone, the sun will be partially obscured from the vantage point of every state in the contiguous U.S.

To safely view that partial solar eclipse—and the partial eclipse phases that will occur before and after totality during the total solar eclipse—you’ll need some kind of eclipse glasses. These are extra-dark shades that block almost all light, protecting your retinas from damage while you observe the sun.

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This protection is extremely important because looking directly at the sun without protection can permanently damage the eye within a matter of seconds, says Ninel Gregori, a spokesperson for the American Academy of Ophthalmology and an ophthalmologist at the University of Miami. “When looking directly at the sun, you are at risk for solar retinopathy, or retinal burns caused by high-intensity light,” she says.

The retina is the light-sensing layer at the back of the eyeball. When it is stimulated by different wavelengths of light, light-detecting cells called rods and cones release chemical signals, which in turn trigger electric signals that travel along the optic nerve to the brain. Looking at the sun for even a few seconds can send this chemical reaction into overdrive. The overrelease of chemicals from these cells generates free radicals and reactive oxygen species, which are unstable molecules that damage cells. This damage can be long-lasting or even permanent. After a partial eclipse in Hawaii in 1962, military doctors documented eye damage in dozens of people who had looked at the sun without protection. Only half of them had recovered normal vision within six months.

The damage may not be apparent until a few hours after exposure, Gregori says. Symptoms usually include a blur or blind spot at the center of one’s vision and distorted or altered color vision.

Eclipse glasses work by using solar filters to dampen the intensity of the sun’s light way, way down. “They are at least 1,000 times darker than ordinary sunglasses,” says Rick Fienberg, safety lead at the American Astronomical Society (AAS) Solar Eclipse Task Force. No more than 0.00032 percent of the sun’s visible light can get through a proper solar filter, Gregori says.

Solar filters also block infrared and ultraviolet light, Fienberg adds. Neither of these nonvisible forms of radiation cause immediate retinopathy, but they can damage the eye over the long term.

Safe eclipse glasses use either an aluminized polyester or a black polymer made of carbon particles that are suspended in resin, according to a 2023 AAS report. (The aluminized polyester is similar to Mylar but more specialized. Regular Mylar cannot be used to view the sun.) Welding safety glasses with a shade number of 12 to 14 can also be used for safe solar viewing.

Importantly, eclipse glasses shouldn’t be used with binoculars, cameras or telescopes, Fienberg says. The optics of those viewing apparatuses can concentrate the light even beyond what the eclipse glasses can block. Special solar filters designed for binoculars, cameras and telescopes exist and should be used by anyone looking at the eclipse with more than just their eyes.

The standard for a safe solar viewer is known as ISO 12312-2. But just seeing that number stamped on a product is not enough to prove that it actually meets the standard, Fienberg says. Many products on the market don’t have the proper documentation to ensure they’re safe. AAS keeps a list of safe suppliers, and Fienberg recommends sticking to that list. “Unfortunately, there are vendors out there who claim their products are safe but either are claiming it fraudulently or don’t realize the supplier they’re buying from has sent them a bogus test report,” he says.

In the absence of a solar filter, the only other safe way to observe a partial solar eclipse is to stand with the sun behind you and look at the edges of shadows, Fienberg says. A pinhole viewer made of one piece of paper with a small hole punched into it and a second piece of paper as a backdrop for the shadow can show the crescent of the sun as the moon moves over it. The same effect occurs anywhere the eclipsing sun shines through leaves or even interwoven fingers, he says. A kitchen colander can also be an easy DIY option for safely viewing the shadow.

There is no need for eclipse glasses during totality, the brief moment when the moon completely covers the sun’s face in a total solar eclipse, Fienberg says. At that point the only light is from the sun’s corona, or outer atmosphere, and it’s only about as intense as the light from a full moon.

Right now, Fienberg says, the sun is at a point of maximum activity on its surface. That’s a reason to have eclipse glasses on hand even without an upcoming eclipse. Features such as sunspots are sometimes visible to the eye during these periods, and the only safe way to see them is to look through eclipse viewers.

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