They are not LEDs...
What are they, then?As xlbee says, they are a vacuum fluorescent display, not unlike a tube. They have filaments, and they do slowly degrade.
Unfortunately, no one seems to have come up with a way to rejuvenate them.
Emission may usually be restored by raising filament voltage. Thirty-three percent voltage boost can rectify moderate fade, and 66% boost severe fade. This can make the filaments visible in use, though the usual green-blue VFD filter helps reduce any such red or orange light from the filament.
Well, this may be off thread-
But how did the term/name Fluroscan come to be?
It's weird- some seem to last forever, and some get noticeably dimmer in just a few years, or get mottled-looking with some portions of the glow being brighter than others.
I would expect that raising filament voltage would work, but that's a difficult proposition in most cases- one would have to add a different transformer and connect it to the filament drive circuits.