Americans are making their last dashes for glasses and viewers to watch the rare total solar eclipse that will glide across the continental US on Monday. Meanwhile, eye doctors are trying to clear away any orbiting debris that's obscuring vision safety information—and spotlight the dangers of unsafe viewing.

Everyone knows that watching an eclipse—or staring into the Sun in general—can damage eyes. But in a series of articles published Friday in JAMA and JAMA Ophthalmology, a group of ophthalmologists explains in detail how sunlight damages the retina, plus dispels some misconceptions about viewing techniques for the rare event. They also provide a case study of what happens when you go into an eclipse event eyeballs-out.

David Calkins and Paul Sternberg of The Vanderbilt Eye Institute in Nashville, Tennessee, (which will experience a total eclipse) wrote one of the pieces in JAMA Ophthalmology. In it, they point out that many people have the misconception that an eclipse allows safe viewing of the Sun—that the lunar disk will cover everything but the Sun’s beautiful corona. This is true for those lucky ones that are along the path of the total eclipse, albeit only briefly. For those in the totality path, the Sun’s core will be blotted out for no more than two minutes and 41 seconds. “However, for most people, at least some portion of the Sun’s core will be visible during the event,” Calkins and Sternberg note.

If total eclipse viewers take off their protection, it’s important to have it back in place before the moon begins sliding aside. Otherwise, you risk damage. Everyone else should be cautious the whole time.

Core sunlight is intense, at about 1,350 Watts per square meter irradiance, they note. With the refractive nature of our eyes, this leads to a hefty dose for the teeny 1.5mm fovea—the central pit of the retina. This pit is packed with photoreceptor cone cells responsible for color vision and is what gives us our sharp central vision.

Sunlight damages the eye in two distinct ways

Exposing your naked eye-ball to sunlight causes solar retinopathy (aka photic retinopathy or solar retinitis), which involves two types of damage to the retina, particularly the fovea: burns and photochemical toxicity.

The former may be the most easily understood. A big chunk of sunlight is near-infrared radiation (700 to 1,500 nanometers), which can cause heat and thus burns, Calkins and Sternberg note. Because our eyes lack pain receptors, we can scorch the fovea without even noticing as we gaze at the Sun.

But, the eye experts write, a "more pressing concern" is photochemical toxicity. The large wallop of visible light from the Sun gets soaked up by photoactive materials in the eye that are just itching to form free radicals and reactive oxygen species. These include heme proteins, melanosomes, lipofuscin, and other chemicals. Once generated, free radicals and reactive oxygen species can attack many types of molecules and break down membranes, leading to tissue damage and cell death. Once retina tissue is destroyed, it cannot regenerate.

The authors say that a study of 45 people who damaged their eyes during a 1999 eclipse in the UK supports the idea that photochemical toxicity is a frequent cause of vision damage in solar retinopathy. In those cases, most of the damage was not permanent. Only four of the 45 reported lingering symptoms—discomfort and vision problems—after seven months.

But, of course, irreversible damage is more likely with longer and larger exposures. A case report published by Ta C. Chang and Kara Cavuoto of the University of Miami’s Bascom Palmer Eye Institute detailed lasting injuries in a 12-year-old girl. She suffered severe vision damage after staring at the full Sun for about one full minute. The eye experts presented images of her damaged retina, one of which was obtained by optical coherence tomography (OCT), which is kind of like an ultrasound for the eye that uses light to create cross-sectional tissue images. The images show bright pitted spots (see white arrows) in her fovea where photoreceptor segments have been wiped out by Sun damage. On follow-up exams, her vision hadn’t gotten better.

Here’s how to avoid such damage

In JAMA, Neil Bressler, of Johns Hopkins University School of Medicine, and colleagues, lay out how to safely view the eclipse.

First, what not to do: Don’t view the eclipse with your naked eye or unfiltered telescopes, binoculars, sunglasses (yeah, even if they’re really dark), camera lenses, or other optics devices. Don’t use anything that focuses light, even if you’re wearing eclipse glasses. NASA notes that the concentrated solar rays can damage the filter and enter your eyes.

To safely view the eclipse, turn to:

No. 14 Welders Glasses, available at welding supply stores.

Pinhole projectors. These allow you to see the eclipse by projecting sunlight through a pinhole onto a viewable surface (Note: it is not just looking through a pinhole). Here’s more information on these.

Aluminized Mylar filters/commercial glasses that do not have any damage or scratches. The American Astronomical Society says these should be verified by an accredited testing laboratory to meet the ISO 12312-2 international safety standard. Watch out, there are scams out there. The AAS has a handy list of reputable versions on their website.

If you do fear that your eyes have been damaged by sunlight, promptly see an eye doctor, who will be able to diagnose you with a clinical evaluation and diagnostic tests, such as OCT.

JAMA, 2017. DOI: 10.1001/jama.2017.9495 (About DOIs).

JAMA Ophthalmology, 2017. DOI: 10.1001/jamaophthalmol.2017.2936