Solar Eclipse Glasses: Design Considerations
Solar Eclipses
This weekend, on October 14th, millions of people throughout the U.S. will try to catch a glimpse of the sun being eclipsed, at least partially, by the moon. Partial lunar eclipses are rare but at least two happen every year somewhere on the earth. This weekend’s partial lunar eclipse is unique because it is also an annular eclipse. In other words, all solar eclipses start off (and end) as partial solar eclipses, where a portion of the sun’s projected disk is blocked by the moon from a viewpoint on earth, but some eclipses become annular eclipses while others become total eclipses.
Annular solar eclipses occur when the moon is aligned in such a way that it leaves a ring of the surface of the sun still visible while the moon is transiting in front. This often happens when the orbit of the moon, at the time of the eclipse, (which is always considered a “new moon,”) is farther from the earth, since the moon’s orbit is not exactly circular. The term “annulus” refers to the ring of the sun’s disk that is still visible, and often called the ring of fire.
A total solar eclipse will occur if the moon’s orbit or distance from the earth is small enough such that the moon will entirely cover the sun’s disk, as viewed from someone on earth. It is interesting to think that there is always a spot somewhere in our solar system where a total eclipse is happening - wish I was there. The next total solar eclipse occurs on April 8th, 2024.
Go See an Eclipse!
If you haven’t yet made plans to see a solar eclipse, please do, especially if you live close and can readily access the path of the eclipse without significant cost. Eclipses, especially total solar eclipses, can be unforgettable experiences. I often think about the one I saw in 2017. It was breathtakingly beautiful.
Once you’ve made plans to see one, don’t forget about buying or using solar eclipse glasses since looking directly at the sun without eye protection can do severe damage to the most impressive organ of the body. You can take them off and look at the eclipse ONLY during the time considered the total solar eclipse. At all other times (i.e. even before and after a total solar eclipse) where the sun is still partially visible you should keep your solar eclipse glasses on. For an annular eclipse, never look at it without filters or eye protection.
Design Features of Solar Eclipse Glasses
The context and design requirements of these eclipse glasses is a unique challenge for engineers. First of all, these glasses are often only used once and perhaps for just a few minutes. Most people will discard them after the main eclipse event (although some might hold onto them for two eclipses if they can work it into their schedule). This means that the millions of people who are buying them online don’t want to pay more than a few dollars. (More expensive but longer lasting and more stylistic glasses are also available on the market.) Other clubs and organizations give eclipse glasses away for free, and will seek a cost effective option for their marketing or philanthropic actions that is mostly for education and community outreach purposes. (My local library was handing out free glasses last weekend.)
On the other hand, since millions of people are looking at the sun, the performance and quality of these glasses needs to be high, kept high, without degrading over time. The manufacturing of these glasses needs to have little or even zero variation and the reliability needs to be extremely high for any, and all, users. This ideal of safety is not always easy when trying to keep the costs down, but no one wants to be damaging eyes or blinding people by the millions.
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Eclipse glasses must meet the standard ISO 12312-2:2015 to be considered safe. A lot of people know about a suggestion, policy, or law (or two) that they disagree with and think may be a government overreach, but I have yet to meet someone who doesn’t like this standard for eclipse glasses. The ISO, or the International Organization for Standardization, has a number of best practices that are established for many activities across hundreds of domains to keep people safe and healthy. But they also establish standards that will save money, time, or both. Other standards will protect the environment, investments, property, and even one’s quality of life in many countries and in various socio-economic classes. Some governments will adopt these standards as law, and therefore not keeping them becomes a criminal offense. However, others remain as important guidelines to follow or even exceed by choice in our own companies, projects, or homes. It’s not a crime to look at the sun without eclipse glasses, and no government will prosecute you if you do, but I highly suggest you follow this ISO 12312-2:2015 standard.
Furthermore, although you may eventually run into a standard that will cause you some extra effort from time to time, we all forget about the hundreds of other standards that benefit us throughout the vast majority of our lives. We tend to focus on the one or two standards, or even codes, that gives us some discomfort and requires extra work or cost, but we take for granted all the others that enabled progress, reduced cost and time, and increased our freedom. You are reading this very article because of computer, IT, and networking standards.
This type of freedom, along with techanological advancements, result in unprecedented progress and experiences for human-kind. Solar eclipses have been occurring for billions of years, but only recently can we all enjoy the freedom to view one without fear of injury or pain.
Although even the affordable cardboard frames are impressive in themselves and the result of many generations of progress, the key technological feature of these eclipse glasses is the paper-thin filter which reduces the amount of light that passes through at the different harmful wavelengths. Ideally, we want the glasses’ filter to lower the passage, or transmittance, of the visible light so that as a viewer we can enjoy the phenomenon without oversaturating our sensory receptors in our retina. Most people do not enjoy it when others put a flashlight directly into their eye, and, similarly, these glasses should reduce the luminous intensity, and most do, by reducing transmittance by more than 99.99%. It may be you will want to reduce this even more as uncomfortable afterimages can still occur for people with very sensitive eyes after looking through eclipse glasses. Thus, there is a range of acceptable values at which eclipse glasses can adopt and still satisfy the standard: maximum transmittance of 0.0032% and a minimum transmittance of 0.000061%.
What may be even more important is removing the damaging UVA and UVB radiation from entering your eye. The same reduction for visible wavelengths is suggested for UV, meaning that transmittance should be at a maximum of 0.0032% and a minimum of 0.000061% compared to the incident radiation, which corresponds to a range of welder’s glass, of shade 12 (maximum) to shade 15 (minimum). Additionally, in the context of eclipse viewing, a higher value for the transmittance of infrared radiation (IR) is acceptable in the ISO standard, which is set at 3% compared to the incident IR.
Reducing the amount of light and radiation through a filter is important but making sure that the filter will satisfy this requirement across the entire surface and at any look direction is also necessary. Eclipse glasses will not be acceptable if only certain angles and certain spots or sections make the filters safe. The whole surface must be uniformly designed within the range of acceptability.
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Lastly, the materials themselves should be safe, affordable, and disposable. Currently, the market offers filters made from a variety of materials with some of the most popular being black polymer or silver mylar. Different materials will have certain properties, and impact attributes such as the color of the sun from inside the glasses, but a design engineer will need to think about more than just the experience of the end user. Design analysis should be made about the toxicity of the films, the likelihood of tearing, correct warning labels, storage and degradation, different head sizes and eye distances, and the environmental impact of millions of these glasses if disposed of the very next day into landfills. Even the probability of wind gusts that are strong enough to remove the glasses while looking at the sun should enter into the calculations!
Conclusion
We live in an era where we have the technology to view amazing celestial events. Never before could the masses enjoy these remarkable alignments of the sun and moon. The cost-effective eclipse glasses that you may use this weekend, or next year, to enjoy are a culmination of thousands of hours of research in optics, manufacturing, and resource extraction and processing. Before you throw them away, or store them for the next eclipse, take a moment and appreciate the effort that was put in so that you can see the beautiful moment of the moon passing in front of the sun. A design as simple and straightforward as some polymer films in a cardboard frame has more than meets the eye!
