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JoannA - HAAAAA!!! Finally. I get to, for the first time, contribute at least SOME knowledge to this forum instead of just opinions. (lolol - really - this is a VERY big moment for me.)
I am Director of Quality for a company that makes personal safety equipment, inlcuding welding lenses. I have a very expensive piece of test equipment in our lab - a spectrophotometer - that we use to scan welding lenses to make sure that they are blocking all of the radiation that is harmful to the human eye. It scans the radiation spectrum from UV through visible and into the near IR ranges.
I wondered if slabs would prevent fading too. Instead of finding out on one of my cards, I used a PSA slab that had once contained an E93 Donovan PSA 1. I break out PSA 1 cards automatically, and usually save the two halves.
I had one of my Techs scan the Donovan slab, being careful to shoot it through the clear middle instead of the textured borders.
It does not block UV. I don't remember - the actual scan and transmittance curve is in my desk drawer at work - but it allows transmittance of a substantial portion of the UV rays. I swear I will scan the scan (hee) and post it in the near future!
Finally. Knowledge. Trivial, yes, but knowledge nonetheless.
Joann