Eclipse Glasses

In North America, we’re on the precipice of a total solar eclipse. On 8 April 2024, a swath of the continent, from Mexico, through the heart of the United States, and a small part of Canada, will experience one of the natural world’s great expositions. In some places, for up to four minutes, day will become night. According to many who witness this rare phenomenon, the experience sports no equal.

During totality, the moon’s disk completely blots the sun’s disk, leaving only the faint corona visible. This period is the only time a human can safely look at our star without protective gear. Needless to say, the sun is extraordinarily powerful, and the human eye cannot look at it without risking serious, permanent damage. Millions of years of evolution have not even come close to allowing us to peek at the sun’s radiation. The sun is so powerful that even partial eclipses in which the moon obfuscates 99.9% of the disk do not make it safe to look to the skies. As the popular astronomical adage goes, there’s no such thing as 99% totality; totality is binary.

Thankfully, we can harness the power of chemistry and physics to gaze at the sun before and after totality. Seeing the crescent of the moon creep toward a full circle and then recede is interesting in its own right. One can safely watch this process with a pair of eclipse glasses.

How do they work?

First, let’s think about normal sunglasses. The science behind dimming what we see via eyewear is more complex than one might imagine, but the basics are easy to distill. Normal sunglasses employ two methods to ease the harshness for our eyes. The first is tinting, which is a fairly straightforward concept. As we learned with the blackest black and the whitest white, a lot of the color we perceive in objects depends on what colors of visible light are absorbed or reflected by those objects. Tinting on sunglasses works on the same principle. Lenses absorb certain wavelengths of the visible light spectrum, which keeps those wavelengths from reaching our eyes. Thus, a percentage of the visible light from the sun does not hit our retinas. The sun does not emit only radiation in the visible spectrum, however. One of the big differences between artificial light and sunlight is ultraviolet radiation. That portion of the electromagnetic spectrum gives us sunburn, and our eyes are not immune. It’s not good for the sun to bombard our skin with UV light and the same holds for our retinas. So, just cutting out a portion of the visible spectrum isn’t necessarily great for our eyes. To combat UV radiation, engineers have developed coatings to apply to lenses that block those rays. Think of it like sunscreen for glasses. The magic, of course, happens in the ability to make this coating transparent to the human eye, while still effective against dangerous UV radiation.

Over the decades, scientists have become quite proficient at blocking UV with sunglasses, often 99% or higher. So, why can’t we use normal sunglasses to view a partial solar eclipse?

The answer is not complex: the sun is simply too mighty even for the strongest of everyday glasses.

To the rescue come eclipse glasses!

These specialty devices harness the power of metals to combat the sun. The “lens” of eclipse glasses is comprised of mylar, a polyester film that is simultaneously robust and cheap. On top of this film, manufacturers layer tiny amounts of metal, usually aluminum, chromium, or silver. These elements absorb nearly all of the bad types of radiation. Further, makers slather enough on the mylar to keep nearly all the visible light from reaching the eye. Essentially, eclipse glasses let in just enough light to make out the celestial forms. They are so effective that one could not efficiently wear them in everyday life, but they will do a fantastic job on eclipse day.

The effectiveness of eclipse glasses - photo by Jason Howell

Since viewing the sun is so dangerous, the eclipse viewers that one procures is not a decision to make lightly. Scientists have developed testing criteria to deem glasses safe. The International Organization for Standardization currently recognizes a certification level called ISO 12312-2 necessary to look at the sun. Unfortunately, not all companies selling glasses can test for this level. Many companies look to jump on eclipse mania to make a quick dollar, eschewing safety standards. Some note their glasses meet ISO 12312-2, even if they haven’t been properly evaluated. 

To combat this issue, the American Astronomical Society maintains a list of companies that manufacture glasses in certain nations that also subject their products to the proper testing. One should only purchase glasses from a vendor on this list.

As an eclipse approaches, one might be tempted to make a last-minute purchase from someone not on the list. Please, do not gamble with your eyes. It might sound funny to burn a crescent-shaped blotch into your retina, but, when it doesn’t go away, you’ll wish you had spent a few extra dollars.

Many of the companies on the list have created whimsical, fashionable glasses. At TMAC HQ, we snagged a set of glasses that feature cats shooting lasers from their eyes. For the 2024 eclipse and beyond, a list of approved AAS vendors is linked below. You don’t need to get cat lasers, but why wouldn’t you? No matter your stylistic choice, make sure you and your loved ones view the eclipse safely!

Cat laser eclipse glasses from ICSTARS Astronomy

Further Reading and Exploration

Total Solar Eclipse Safety – NASA

How Do Solar Eclipse Glasses Work? – Scientific American

The science behind solar eclipse glasses – University of Rochester


What to Know About Solar Eclipse Glasses – WebMD

Suppliers of Safe Solar Viewers & Filters – American Astronomical Society

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