Eclipse glasses: How are they made?
With the total solar eclipse passing just south of Chicago, all of us at Microtrace were surprised to find that the eclipse could be seen through even a decent layer of clouds – with glasses. While discussing the eclipse over lunch, we started to wonder about the construction of a pair of disposable eclipse glasses and how they were made. A quick search on the internet showed that there isn’t a whole lot of information. These glasses (filters) are designed to transmit no more than 0.00032% of the sun’s light according to the ISO 12312-2 standard which governs their design. This is a stronger filter of light than most welding goggles. Ultimately, the filter acts as an exceptional neutral density filter that is opaque to all but the strongest sources of light.
Since we spend a lot of time looking at the finest details of all sorts of everyday objects, we thought we’d take a look at these goggles to see how they were produced and what makes them opaque. Depending on how often you think about these things, what we found may not be a surprise, but nonetheless, we thought it was interesting enough to share.
First, here’s a pair of eclipse glasses that were given to one of our scientists as a gift. They were purchased for $1 at a megastore. They are noted to comply with the ISO 12312-2:2015 standard (Eye and face protection — Sunglasses and related eyewear — Part 2: Filters for direct observation of the sun).
We cut a cross section of one of the filters to take a look at its construction. From the transmitted light image of a thin section (above, left), we can see that the eclipse filter consists of two layers. The majority of the filter thickness (about 2 mil / 55 micrometers) consists of an oriented (center) polymer that has been darkened with carbon black. The main tinting agent; however, is a thin, reflective, metallic layer that can be observed in the far right reflected light image above. This metallic layer is only on the order of 2 micrometers thick (for reference, a typical head hair is about 40 times thicker).
An elemental analysis of this thin section using an energy dispersive x-ray spectrometer on a scanning electron microscope shows that the thin, metallic layer is composed largely of nickel, with lesser amounts of chromium and iron. Together, this thin metal film is claimed to be sufficient to block out a safe amount of the sun’s intensity.
While higher resolution images could be prepared to obtain more specific measurements of film thickness, more detailed cross sections could be used to accurately quantify the composition of the film, and spectroscopic analyses could be conducted to determine if this film is successful at filtering out the 99.999 percent of sunlight across the relevant portion of the visible and ultra-violet spectrum, this analysis addresses some basic questions about how the glasses work and how they were made.
With these approaches, it’s possible not only to understand how the filter is constructed, but also to scientifically test the efficacy of eclipse glasses. Surely the reports of fraudulent glasses (even going so far as to incorporate notations implying compliance to ISO standards) will be the origin of some lawsuits and probably (though hopefully not) some damaged eyes.
Finally, if you get this far and you are done with your eclipse glasses, please send them to us. If we get enough, we will analyze them and put together a talk about them for Inter/Micro 2018. If you are willing to send them to us (Microtrace LLC, 790 Fletcher Drive, Suite 106, Elgin, IL 60123), we will send you a Microtrace patch and double-eraser pencil as a thank you!
Please note, the above information on this page was assembled only for information purposes and is not intended to (nor should it) be used for any legal purpose.
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