I noticed that in my shop light fixtures in the garage, I started with T8 bulbs on HF electronic ballasts that would warm up to be almost painfully bright but after many years, the ends became dark and the tube extremely dim. I can make sense of end of life failure modes in most others, but don’t quite understand what’s going on here. Does the mercury vapor dissipate or do the phosphors somehow wear out? Or is it simply the heater filaments on each end losing emitter coating? What a shame, because the mercury vapor and phosphors are totally intact and getting tossed if it’s just the filaments. I don’t see that failure mode of dimming in neon lighting oddly. They have solid electrodes so that’s what makes me wonder if it’s entirely due to an issue with the filaments.

Thanks guys,

Mike

There is a multitude of failure modes as the tubes age. Each leads to the loss of brightness. First, phosphors age, this is most prominent in cheapo halophosphate tubes, but rare earth phosphors suffer somewhat too, may be about -25% at EOL. That's for well made major brands, off-brand tubes may behave much worse. Next there is cathode sputtering. Also some of the tubes, especially the last 'ecological' ones suffer from  mercury starvation.

You should not forget the ballasts, electrolytic capacitors are getting dry leading to power loss and flicker, also film capacitors sometimes lose their capacitance causing output circuit detune, brightness loss and ignition troubles.

If you think neon tubes are free from these failure modes, you are just plainly wrong. For mercury and phosphor tubes all mentioned failure modes are here. A recent picture here on on L-G just shows this. Especially for 'classic green' ones having just some single thousands of hours phoshphor life before getting dim. Red neon tubes do not generally dim until very badly made, but have a notoriously finite life because of gas capture.

Definitely the most common reason is the phosphorus wear out, even from the best of tubes. There's someone on here with a 14w durotest tube that's been running for over 100k hours and is very dim. I wonder if it's still going today? Haha

I also forgot to mention something as simple and trivial like dust and dirt accumulated on the tubes and reflectors over the time!

Some common reasons that fluorescent bulbs and tube may dim with age are:

<> Phosphors degrading over time and use
<> Mercury getting absorbed into the phosphors
<> And there could be a very slow gas leak by defects.