How solar eclipse glasses work

Most of North and Central America as well as the Caribbean will be able to see a celestial show on April 8 as the moon passes between the Earth and the sun. While individuals in the path of totality could view a total solar eclipse, with the moon totally obscuring the sun, many others will see a partial eclipse—the sun looking like someone took a bite out of it. Hundreds of thousands of people all around the continent will don eclipse glasses and fix their eyes directly at the sun as the event takes place. 

But what makes those eclipse glasses-usually basic cardboard frames with two thick sheets in place of lenses-so effective?

Made for viewing the sun, the dark film of eclipse glasses is a kind of solar filter. While certain solar filters are made to make one particular wavelength of light visible, neutral density filters are the classification for eclipse glasses. They thereby dim light over all wavelengths. Combining two elements—a sparkly layer on the front of the film and a dense dark layer on the rear— Eclipse glasses filter sunlight enough to make it safe for human eyes. Usually comprising aluminum or chromium, the glossy coating reflects light away from the observer. Black polymer, a plastic darkened with carbon particles that absorbs all light wavelengths, makes up the thick dark layer. (This is why carbon appears black to us; it is really good at absorbing light.) The end effect is a filter so strong that your surrounds seem to be completely dark.

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"Eclipse glasses let in thousands of times less light [than normal sunglasses]," notes NASA Goddard Space Flight Center research heliophysicist Dr. Ashley Greeley.

Everybody staring at the sun should make sure their eclipse glasses follow ISO 12312-2, the safety criterion. Solar viewers have to let in no more than 0.0032% of visible light in order to satisfy this criterion and follow restrictions on the quantity of UV and infrared light they emit.

Simply wearing several pairs of regular sunglasses instead of appropriate eclipse glasses carries one risk: while it may seem dark enough to view the sun without much discomfort, UV and infrared rays may still be damaging your eyes without you feeling, or even realizing, that damage is being done.

J. Patrick Haas, senior instrument engineer in the Heliophysics Science Division of NASA Goddard Space Flight Center, notes that in some respects ultraviolet and infrared are more harmful than visible light since we do not react to them. Regular sunglasses are not safe to use that manner, even several at once as they just aren't made or standardized for direct sun viewing.

During the brief period of totality, when the moon is totally obscuring the sun, the one and only time eclipse viewers should remove their glasses. But when observing the partial eclipse—from outside the path of totality or just before or after totality—you must keep those glasses on when viewing the sun.

According to Greeley and Haas, keep your eclipse glasses on even beyond April 8 since they are for viewing the sun anytime, not only for eclipses. Under eclipse glasses, one may view sunspots on the surface of the sun even without a telescope. Sunspots are more common during the expected soon, or perhaps already started period of solar maximum in our sun. "The sun is highly active right now," Haas notes. "It's more lively than I have ever worked in.”

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