How to look at the eclipse without burning your eyes

Sunglasses are not nearly strong enough. Filters made from food wrappers and other household materials? Also a bad idea. Even masks designed for gas welding are not going to cut it.

For the millions planning to watch the Aug. 21 solar eclipse, ophthalmologists say proper eye protection is essential. Staring at the sun — even when three-quarters of it is blocked by the moon, as it will be in much of Pennsylvania and New Jersey — can damage eyesight.

"People can basically burn their retinas," said Jack Dugan, an attending surgeon at Wills Eye Hospital.

That is true for looking at the sun under normal circumstances. But the risk goes up during an eclipse because the partially blocked sun appears less bright, leading some to think it is OK to stare at it for extended periods of time. It is not. There are no pain sensors in the retina, so the unprotected eclipse viewer may gaze at the sun for minutes without realizing it is harmful. The resulting loss of vision, which can be permanent in extreme cases, does not occur until several hours later.

COMING MONDAY: Derrick Pitts, the Franklin Institute's chief astronomer, will take your solar eclipse questions live on Philly.com's Facebook page at 3 p.m., just after the eclipse's peak in Philadelphia.

A safe approach is to wear a pair of solar viewing glasses. And even then, beware of cheap imitations. Looking through good-quality glasses will seem almost like wearing a blindfold, said Dugan, an astronomy buff who is distributing them to patients at his practice in Voorhees.

"It's black," he said, of his view through the glasses. "I can't see when I wave my hand in front of my face."

The nonprofit American Astronomical Society recommends glasses that have been proven in laboratory testing to block 99.9968 percent of the sun's visible and ultraviolet rays, a safety standard developed by the International Organization for Standardization. Brands that have been certified as meeting that standard typically say so on the label, though some counterfeit brands are labeled with similar claims. To be safe, NASA recommends looking at the society's list of reputable vendors at eclipse.aas.org (click on "Eye Safety").

It's not enough for the glasses to be extremely dark, said Rick Fienberg, the astronomical society's press officer. Proper filters have various coatings made with aluminum or other materials designed to block specific wavelengths of radiation.

Do not wait until the last minute to get glasses, as some online vendors are sold out.

Other safe viewing options include watching a live stream of the event at eclipse2017.nasa.gov, or viewing it indirectly, with a homemade "pinhole" device that projects a silhouette of the eclipse onto a piece of paper (instructions at go.nasa.gov/2qjJnnc). And at least 15 area libraries are holding eclipse-viewing events, along with the Franklin Institute and the Wagner Free Institute of Science. (See a national map at eclipse2017.nasa.gov/libraries.)

The damage that can occur from looking at the sun directly is called solar retinopathy, described in a 2004 study by one of Dugan's Wills Eye colleagues, Sunir J. Garg.

The bluish portion of the sun's spectrum is sometimes cited as the prime culprit, leading people to think that "blue-blocker" sunglasses are protective. Bad idea, as other wavelengths also are harmful, said Garg, a professor of ophthalmology at Wills.

The combination of visible and ultraviolet light, intensely focused on a small spot in the back of the eye, triggers a cascade of injury to the light-sensitive cells called rods and cones, the eye doctor said. That includes inflammation and chemical damage caused by molecules called free radicals.

It also includes the direct impact of the heat.

"The tissue gets cooked," Garg said.

We've all had the experience of looking at the sun for a split-second by accident, with no lasting effects. Does damage start to occur after looking at the sun without protection for 10 seconds? Twenty? And at what point does the damage become permanent? Some scientists have attempted to answer these question with animal studies, but the results are unclear. Best not to try it at all, Garg said.

Patients who develop solar retinopathy will perceive a spot in the middle of their field of vision. The damage may also be perceived as "waviness" or dimness, said Bisant Labib, assistant professor at Salus University's Pennsylvania College of Optometry. In cases where the damage is temporary, recovery occurs after several months.

Special cautions apply for those who want to take pictures of the celestial phenomenon, said Jerry Lodriguss, a longtime astronomy photographer and a contributing editor of Sky & Telescope magazine. The lenses on a camera, telescope, or pair of binoculars magnify the sun's rays, causing damage even faster.

Lodriguss, a former Inquirer photographer who showcases his work and offers eclipse tips at astropix.com, said taking photos of the sun requires the use of a specially designed solar filter.

Though experts are spreading the word about safety, they do not want to detract from the enjoyment of a rare phenomenon. As Dugan is doing for his patients in South Jersey, Wills Eye is giving out safety glasses at its Walnut Street location in Center City and elsewhere, while supplies last. (Check @wills_eye on Twitter for availability.)

Dugan is so excited about the event that he is traveling with family to watch it in South Carolina, from a coastal location within the 70-mile-wide path of "totality" — meaning that for nearly three minutes, the moon will block the sun entirely.

During that brief period of total blockage, it is safe to look at the eclipse without protection, he said. But on either side of totality, when any part of the sun is visible, eye protection is a must, as it will be throughout the entire event here in Philadelphia.

"It's just an amazing event," Dugan said. "You can actually see the moon taking a bite out of the sun as it goes across."