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NASA's eclipse safety page says that eclipse glasses should:

Not be used if they are older than three years, or have scratched or wrinkled lenses

Why shouldn't three year old glasses be used? Is it because the coating somehow degrades with time? Were standards changed three years ago such that the older glasses can no longer be recommended? Is there just a stronger concern for scratches or damage over the years? Will eclipse glasses (or other solar filters) purchased today be unsafe three years from now?

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    $\begingroup$ I don't know definitively, so that's why I'm commenting, but my guess would be that the coating (either aluminum coated polyester or black polymer) would degrade over time (most likely from photodissociation). $\endgroup$ – zephyr Jul 25 '17 at 17:33
  • $\begingroup$ I'm speculating as well (a bit) - I use a solar filter with a similar foil (Baader safety film). For each use, it's recommended ( link ) that the film is checked for damage - so I suspect the OP is correct in highlighting this. $\endgroup$ – MartinV Jul 27 '17 at 21:49
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According to the American Astronomical Society (AAS), the warning that eclipse glasses should not be used if they are more than 3 years old is outdated (provided that they are compliant with the ISO 12312-2 standard).

A page on their web site says the following:

Note: If your eclipse glasses or viewers are compliant with the ISO 12312-2 safety standard, you may look at the uneclipsed or partially eclipsed Sun through them for as long as you wish. Furthermore, if the filters aren't scratched, punctured, or torn, you may reuse them indefinitely. Some glasses/viewers are printed with warnings stating that you shouldn't look through them for more than 3 minutes at a time and that you should discard them if they are more than 3 years old. Such warnings are outdated and do not apply to eclipse viewers compliant with the ISO 12312-2 standard adopted in 2015.

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    $\begingroup$ Adopted in 2015 still cuts out anything over 3 years old. 2015 was only 2 years ago. $\endgroup$ – Wayfaring Stranger Aug 3 '17 at 15:26

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