Lumen maintenance and spectrum maintenance
I was looking at a new and an old bulb today and the old bulb was noticeably dimmer. They were actually reflected off glass because they both just look glaring white bright directly. I took somemeasurements and the old bulb was roughly 15% dimmer.
So far so good, fluorescent bulbs get dimmer as they get older. What is interesting is that it also looked redder. Side by side, the old bulb had a distinct orange cast. Maybe that was just my eyes seeing more red in the dimmer bulb. Or had the spectral output actually changed? Do blue phosphors degrade faster?