Everyone knows you shouldn’t look directly at the Sun. The rule also applies during a solar eclipse (except during totality). NASA and the media have done a great job explaining that it’s important to use special eclipse glasses to view the eclipse, but you may be wondering whether eclipse glasses are that important. Can you wear sunglasses? Welding glass? Can you just view the sun through a dark neutral density filter? Here are the answers, along with personal experience, which largely falls into the do as I say, not as I do category.
Why Sunglasses Aren’t Safe
Sunglasses are intended to protect eyes from scattered light, not looking directly at the Sun. During an eclipse, the sky seems darker, so your pupils open to accept more light. Sunglasses dim light even further, so your pupils are wide open. While sunglasses may block a lot of ultraviolet light (the range that causes cataracts and immediate retina damage), they don’t block all of it. With dilated pupils, your eyes are set to take it all in. UV light is not, however, the only concern with the Sun. Viewing our star, eclipsed or not, through sunglasses still exposes the eyes to the rest of the spectrum. Keep in mind, visible light is the big killer in an atomic blast, not esoteric radiation. Infrared light, too, can damage eyes. Basically, you wouldn’t look at the Sun with sunglasses normally; you don’t gain any protection during an eclipse.
What the Sun Does to Your Eyes
If you look directly at the Sun before totality (when the Moon completely covers the Sun), it’s highly unlikely you’ll immediately go blind. You could glimpse the Sun, look away, and think everything is fine. However, any amount of exposure can lead to blurry vision or to a condition called solar retinopathy. Basically, this means solar radiation damages the retina. The damage is permanent, irreversible, and cumulative.
And now, for full disclosure: I’ve glimpsed the Sun during an eclipse, just before totality. I promise you, it doesn’t look any different from the Sun on any other day. To see the “bite” the Moon takes out of the Sun, you need eclipse glasses or a solar filter. Anything less simply won’t filter enough light to make a difference to your eyes.
Eclipse Glasses, Solar Filters, and Welding Glass
NASA and the American Astronomical Society (AAS) maintain a list of reputable eclipse glasses and solar filters that permit safe viewing. Safe filters meet the ISO 12312-2 standard that brings solar brightness down, blocks ultraviolet, and blocks infrared. If you get glasses that aren’t on this list, they may or may not be safe. Be aware the market is flooded with eclipse glasses that bear the ISO logo, but are unsafe fakes!
Some green welding glass approximates the ISO standard, but some glass does not. Solar filters for telescopes or binoculars may be intended to filter specific regions of the spectrum, allowing dangerous light to pass. Neutral density filters knock down all visible light, but may not filter UV or infrared effectively or cut out enough light for safe viewing. Eclipse glasses are the safest option.
If you have old eclipse glasses from a previous eclipse, they may be okay for incidental viewing, but the AAS recommends using them for no longer than 3 minutes of viewing at a time. Eclipse glasses older than three years were made before an international standard was in place.
If you wear regular glasses or contacts, wear eclipse glasses over them.
How to Test Your Eclipse Glasses
Since eclipse glasses could be fakes or could be good, yet not on the list, you should test them before the eclipse.
Hold the eclipse glasses up to a bright light. If you can see it at all, the glasses don’t filter enough light. Do not use them to view the Sun!
If you can’t see a bright light, test the glasses to view the Sun. Good eclipse glasses should only show the Sun, at about the brightness of the full moon. The Sun shouldn’t be surrounded by any haze, be out of focus, or be too bright to view comfortably.
What About Totality?
When the Moon completely covers the face of the Sun, the eclipse is total. During a total eclipse, the brightness of the Sun is only about the same as that of the full Moon. At this point (and only at totality) it’s safe to view the Sun directly or take pictures without a filter.
Keep in mind, if you are taking pictures with magnification, you may still need a filter. DSLR users in particular should take note. During an eclipse, never view the magnified Sun directly, even at totality.