During the Great North American Total Solar Eclipse on Aug. 21, millions of people will gaze at the sun to see the moon slowly pass in front of it, blocking out the light. But those who aren't careful risk doing some nasty damage to their eyes.
You've probably been told that it isn't safe to stare at the sun and that watching a solar eclipse without proper eye protection can make you go blind. That's because the light from the sun is so intense that it can literally burn your eyeballs — even during a solar eclipse when part of the sun's disk is still visible.
Even the tiniest sliver of a crescent sun peeking out from behind the moon emits enough light to scorch your eyes, Ralph Chou, professor emeritus at the School of Optometry & Vision Science at the University of Waterloo in Canada, told Space.com. "I have seen instances where the patient has eventually shown up with crescents burned into the back of the eye, and you can almost tell exactly when they looked." [How to View a Solar Eclipse Without Damaging Your Eyes ]
How eyeballs get "sunburned"
Sunlight damages the eyes by triggering a series of chemical reactions in the retina, the light-sensitive part at the back of the eye. Retinas contain two types of photoreceptors: rods that help you see in the dark and cones that produce color vision.
When intense solar radiation hits the retinas, it can damage and even destroy those cells, in what doctors call a retinal photochemical injury or solar retinopathy. Whether looking at the sun will cause this type of injury depends on both how long you look without protection and the sun's position in the sky. Overhead, the sun is brighter and more dangerous to look at than when it is close to the horizon during sunrise or sunset.
"You can think of it in the same way as this: Let's suppose you decide to really pig out for dinner, and afterward you're not feeling very well," Chou said. "Well, [it's the] same thing with all the light hitting the light-sensitive receptors at the back of the eye. They get so much of this light energy coming in that they really can't handle [it]."
In severe cases, this type of photochemical damage may also come with thermal injuries, or literal burns, that destroy the rods and cones in the retina. This can happen to people who repeatedly look at the sun without any protection, those who stare at the sun for an extended time, or even those who glance through a telescope or binoculars without solar filters.
Instinct vs. willpower
Not many people look at the sun long enough to be blinded by the light, Chou said, but the risk is certainly exacerbated during a solar eclipse.
Under normal circumstances, it's more difficult to look at the sun long enough to incur damage because of something called an aversion reflex. "We learn early on in life we just shouldn't be looking at something that bright because it is uncomfortable and we can't see anything," Chou said.
"The problem when it comes to looking at a partially eclipsed sun is that you are trying to see something that you know is going on that's different, and willpower is an amazing thing to override an aversion reflex."
To make matters worse, it's possible to look directly at the sun "with a certain degree of comfort" when the sun is partially covered by the moon, Chou said. Even when the sun is almost completely covered, though, the tiny crescent that remains is still bright enough to burn your retinas.
Seeing with eclipse blindness
One thing that makes eclipse blindness particularly dangerous is that a person who looks at the sun long enough to incur damage probably won't notice any of the effects until the next morning, Chou said.
"Let's say you take a look at the sun in the afternoon. The cells get overloaded, and they're actually still able to function for a little while, but overnight while you're asleep … the cells start to lose their function, and then they even start to die depending on exactly how badly they've been affected," he explained.
People who wake up to discover their vision has become impaired may look in the mirror to find their face is a featureless blur, Chou said, or they may try to read the newspaper only to find that there are no words on the page. While peripheral vision is usually spared, the center of vision is affected the worst. That's the part of the retina responsible for seeing in high resolution and in color.
"Most people, they don't see a black spot," Chou said. "For the most part, they have damaged photoreceptors that just aren't capable of doing more than just registering maybe the presence of light but can't really build up enough information for them to be able to see clearly."
An unclear path to recovery
Most patients with eclipse blindness are legally blind when they go to see an eye doctor, Chou said. Unfortunately, the prognosis for these patients is nearly impossible to determine.
"You just sort of end up having to wait it out, and that's the really unfortunate part about it," Chou said. "The typical person who's been injured is going to wait six to 12 months before they know what their ultimate status is going to be."
Statistically, about half of those who are diagnosed with eclipse blindness will recover full vision in six months, he said. The other half either partially recover or are stuck with the problem for the rest of their lives.
And when it comes to treatment, there really aren't any options. "Over the years, certainly ophthalmologists have tried various ways, pharmacological and otherwise, to try and reduce the amount of damage and swelling that is thought to be the main cause for the loss of vision," Chou said. "For the most part, those types of treatments don't seem to be effective."
The only thing doctors can do to help these patients is to treat the case as any other case of visual impairment, Chou said, by teaching the individuals how to get around in the world and function without central vision.
When it's safe to look
The only time it's safe to look at the sun without eclipse glasses or other solar filters is during totality, when the moon is completely blocking out the sun's rays and only the corona is visible. If you're planning on watching any kind solar eclipse, whether it's the total, annular or partial variety, you absolutely must use proper eye protection if you want to spare your eyes. Otherwise, you'll risk long-term or even permanent blindness.
But definitely don't forget to take off your solar eclipse glasses during totality, when the sun is 100 percent covered by the moon. In fact, if you don't remove your solar filters during totality, you won't be able to see anything at all.
While official recommendations by NASA and the American Astronomical society say you shouldn't look directly at the sun when any part of it is showing, experienced eclipse watchers like Chou say it's safe to remove your eclipse glasses during the 2-3 seconds before and after totality to see the so-called diamond ring effect, or "Baily's beads." During this phase of the eclipse, the light of the crescent sun forms points of light on the edge of the disk for just a few seconds.
Other safety tips
Anyone who plans to observe the eclipse with a telescope, binoculars or cameras should practice using the equipment before the eclipse, Chou said. Don't wait until the eclipse starts to figure out how to insert and remove the filters from your lenses. For those using eclipse glasses or handheld viewers, make sure to put the filters in front of your eyes before looking up at the sun, not the other way around, Chou said. And children observing a solar eclipse should always be supervised to ensure they're practicing proper eye safety, he said.
Solar Eclipse Glasses
If you're one of the tens of millions of Americans who will experience the Great North American Total Solar Eclipse on Aug. 21, hopefully, you've heard by now that you're going to need eye protection for the big event.
But not all solar viewers are created equal. Using the wrong gear (or using it incorrectly) can burn your retinas, causing irreparable damage to your eyes.
Whether you're looking for a new pair of eclipse glasses or you've already purchased some form of eye protection, here's what you need to know to avoid burning your eyes during the solar eclipse.
When it comes to solar-eclipse glasses and other solar viewers, it's important to ensure that the product you're using is safe and effective at blocking harmful radiation from the sun. Just because something looks like suitable eye protection doesn't mean it's safe to use. Even if a product is advertised as a solar viewer, it's important to look for a label that says ISO, which stands for the International Organization for Standardization.
The ISO is an independent organization that writes safety and quality standards for all kinds of things, including eyewear, health care, food production and more based on a broad consensus of the scientific community. If you find eclipse glasses or other solar viewers that aren't labeled ISO, then they aren't guaranteed to protect your eyes the way they should.
ISO-approved solar-eclipse glasses must meet certain safety requirements:
No more than 0.00032 percent of the sun's light may be transmitted through the filters.
The filters must be free of any defects, such as scratches, bubbles, and dents.
Handheld viewers must be large enough to cover both eyes.
Labels on the viewers (or packaging) must include the name of the manufacturer, instructions for safe use and warnings of the dangers of improper use.
Where to find ISO-approved eclipse viewers
Most ISO-approved eclipse glasses use solar filters manufactured by AstroSolar and Thousand Oaks Optical, Rick Fienberg, press officer for the American Astronomical Society (AAS), said. However, several different retailers sell eclipse glasses and handheld filters that are approved by the ISO, he noted including sellers such as Amazon.
Sunglasses simply won't do
Eclipse glasses and sunglasses might look somewhat similar, but they are made of very different materials. You should never look directly at the sun while wearing sunglasses, no matter how tinted your lenses may be.
"Normal sunglasses typically let in between 10 and 20 percent of daylight…but that's still way too bright," Fienberg told Space.com. "The filters that are made for looking at the sun are typically 100,000 times darker."
Sunglasses are typically made of glass, plastic or some kind of polycarbonate material, while solar filters are made of one of two materials: polyester film coated in aluminum, or something called "black polymer," Fienberg explained. Most eclipse glasses and solar viewers use the black polymer, which is a flexible resin infused with carbon particles. Both types of filters will reduce visible light down to safe levels.
Welders planning to observe the solar eclipse may or may not be in luck, as some welding filters will adequately protect your eyes from the sun. But, please, double-check to make sure that the goggles you intend to use are the right kind.
"There is a particular circumstance in which it's safe," Fienberg said. "We don't recommend it because it's too easy to get the wrong kind of welding filter." Only goggles made for electric arc welding can be used to observe the sun, and they must have a shade scale number of 12 or higher. Shade 13 is ideal for solar viewing, but that shade is typically not sold in stores, Fienberg added.
Don't forget to look at the eclipse!
If you're watching the eclipse from the path of totality, you should absolutely remove your eclipse glasses during totality. "In fact, if you keep your filters on during totality, you won't see anything" because they block out almost all light, Fienberg said.
However, if you're watching the partial solar eclipse from other parts of the U.S. and Canada, you'll need to keep them on the entire time. If you take them off, not only do you risk burning your eyes, but you also won't be able to see the eclipse.
"A total solar eclipse is truly spectacular and awesome in the true meaning of the word 'awesome,' whereas a partial solar eclipse could pass unnoticed," Fienberg said.
"Even if you do have a solar filter and watch the sun turn into a thin crescent, it's nowhere near as exciting as a total eclipse, because you miss all the really spectacular phenomena that are associated with totality. It doesn't get dark, you don't see the corona, you don't see bright red prominences of gas jetting off from the edge of the sun. It's just not the same at all."
So get to the path of totality if you can. And whatever you do, don't forget to bring the right kind of eye protection!