You really need to do a side by side comparison to decide if it is still 90% of original brightness. Compare a new one with the one that's been in use for 15 years. It may help to also have some measuring equipment as well due to the fact the visual perception of brightness has nothing to do with a linear function. Add to that the fact that eye and the head behind them will change a lot in 15 years (also a non-linear function) thus making the "perceived as 90%" statement very questionable.

the observation was while the company still made the same unit.

while I (do not recall that I) did not do a "side by side" I did - on 100s of occasions - look at an old one in the repair shop after working most of a day with a brand new one in the lab.

that, of course can not be considered an absolute 'measurement' but I did not percieve ANY lack of brightness in the old ones.

so, while there is no way in hades I can prove anything - I now work elsewhere -, I looked at 1000s VFDs - old and new - and thus disagree with Pers statement "VFD quite quickly lose intensity."

Erik