How and Why You Should Protect Your Eyes
Both total solar eclipses AND partial solar eclipses require specialized eye protection. I've already written about eye safety, but it never hurts to provide more precaution.
More people will see a partial solar eclipse as it's rare to be on the path of totality. However, it is never safe to look at a partial eclipse, or the partial phases of a total solar eclipse, without the proper equipment and techniques.
In fact, technically, the only time that the Sun can be viewed safely with the naked eye is during a total eclipse, when the Moon completely covers the disk of the Sun. That said, even those in the path of totality should also use eye protection during the hour-long partial eclipse that precedes and follows totality.
Even when 99% of the Sun's surface (the photosphere) is obscured during the partial phases of a solar eclipse, the remaining crescent Sun is still intense enough to cause a retinal burn, even though illumination levels are comparable to twilight. Failure to use proper observing methods may result in permanent eye damage or severe visual loss. Most individuals who sustain eclipse-related eye injuries are children and young adults.
Never risk your eyesight. As my new book Zapped makes clear, solar radiation includes fascinating components like ultraviolet waves. Even normal lifetime exposure to solar UV ages the eye's outer layers and is responsible for cataracts. But right now we are concerned about the immediate damage produced by looking directly at the sun without adequate eye protection. Pull up a chair: there's a lot of misinformation out there.
The sun sets during a the partial solar eclipse of May 2012. Photo Credit: Evan Zucker
What Does Direct Sunlight Do to Your Eyes?
Even high-intensity visible light can produce retinal burns resulting in temporary or even a permanent loss of visual function. I personally know such a person, who forwarded me his eye doctor's report. This photochemical injury occurs mainly when the retina is exposed to the blue and green components of sunlight. It's why people who routinely wear sunglasses in everyday life are wisely protecting their eyes.
But sunglasses are totally inadequate shielding for looking directly AT the Sun. There, the greatest danger comes from deep reds and invisible near-infrared radiation, which is absorbed by the dark pigment below the retina, where it heats up the tissue so much that it can literally cook and destroy retinal cells, leaving a permanently blind area. This thermal damage causes no pain, since there are no pain receptors in the retina. In fact, you won't experience the visual damage until at least several hours later.
This is why staring at the sun is so bad. The heat in the retina builds up. That's why—although everyone has taken quick glimpses at the Sun during their lives without any harm—staring at the sun is dangerous. And that's just what a solar eclipse makes people do.
How to Watch a Solar Eclipse Safely
So, you need a filter. The cheap commercial eclipse filters made of mylar plastic are fine. But I've always preferred welding goggle filters because of their superior optical quality. My eclipse tour groups have safely used them for decades. But if you go to a welding supply store, you must buy shade 12, 13, or 14—and nothing else. In this eclipse, the high-up sun will be a bit brighter than average, so 14 will be the most comfortable, although a shade 12 is definitely still safe, web misinformation notwithstanding.
Beware of fake, knock-off eclipse glasses; they may not be strong enough to fully protect your eyes. You can test your glasses by simply looking through them. If you're able to see light from lamps, headlights, or even reflections of the sun, your glasses are not strong enough! You should only be able to see the sun itself through your glasses. Here are reputable dealers: https://eclipse.aas.org/resources/solar-filters
Photo Credit: Mark Margolis/Rainbow Symphony
Eclipse Glasses Alternatives
If you've not bought any welding goggles or eclipse glasses and are desperate on eclipse day, all may not be lost. You can ask your Welding supply store to create a 12, 13, or 14 shade by cobbling together a few lighter-shade filters and stacking them. Or you can create a safe filter by looking through two layers of exposed and developed black and white film. It's what many astronomers used to use, and has been well tested. Those are the inky black strips at the very end of negatives. But don't try to look through negatives with images on them. And don't use color film. And don't use black and white film unless it's been exposed and developed.
Even then, use a doubled up layer. Two thicknesses. We're talking about old fashioned Kodak products like Plus-X and Tri-X, which contain the silver granules that block harmful light. Some newer films don't have the silver, so only use the older negative strips—the ones you've had lying around since forever.
Our grandparents used to hold a piece of glass over a candle flame and let it get black from soot. Was that dangerous? Were they endangering their eyesight? Surprise: Modern testing shows that such smoked glass only transmits 0.0032 percent of the Sun's visible and invisible radiation. So it WAS adequate for solar viewing.
Nonetheless, to slightly paraphrase our parents, "Please don't smoke glass." You should avoid it because you never know how deep the soot layer is or whether some of it has smudged off. And definitely avoid such dangerous web suggestions as looking through a CD disk. The Sun's image may seem safely dim, but too much infrared (heat) may be arriving on your retina.
How Do I Know My Solar Filter is Safe?
If you're ever in doubt about your filter—and when it comes to safeguarding your eyesight, paranoia is a good thing—just take quick glimpses at the eclipsed sun. Limit each glance through your filter to less than a second to prevent heat from building up inside your eyes.
Again, avoid knock-off eclipse glasses at all costs! You should not be able to see anything other than the sun through your glasses. If you can see lights, or even reflections of the sun, then your glasses are not strong enough to fully protect your eyes and you should use a different pair. Here are reputable dealers: https://eclipse.aas.org/resources/solar-filters
Alternatively, punch a pinhole in paper or cardboard and project the sun's image onto the ground. Don't mess with your eyesight. Obtain filters now and be safe on August 21!