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.

Michael Karas