Hi.

I've been wondering, what causes faint flashing at the end of fluorescent lamp. It's not the kind you experience when cathode is going bad. More like a slower pacing inconsistent flashing that seems to happen around the cathode, most notably seeing behind or side of it. Not all the lamps are doing it but many lamps that I have in use does but usually just one end. And it's not happening always. Sonetimes it gets faster paced, sometimes just couple random ones.

Ih, yeah and it's happening 230v magnetic preheat gear and couple of different type of fixture

Its caused by the 50Hz cycles, try the same tube on a HF ballast, and you will see nothing.

There is one other effect:

As the cathode slowly get consumed over the life, the hot spot travels along the filament. Normally it slowly progresses as the soat gets consumed, but sometimes the arc root is "experimenting" and swirling around in the order to look for the next sweet spot to sit. As the arc moves, it may cause some slight flickering in the tube end. This flickering is then way slower than the mains.
And this effect is present in the same ways regardless of the ballast type, so it mnay appear even on a HF electronic, even with DC supply and so on.
It usually takes few hours till few 10's of hours till the arc root settles to the new position.
It happens at random during the lamp life, so it does not mean anything about the lamp future life.

It quite frequently happens even with HID. The arc root is "dancing" around the electrode tip; as the arc is there quite thin, even the small electrode tip diameter allows the movement to be significant enough to cause some flicker, mainly on the edges of the lanterns beam pattern.
And again, it suddenly starts and after some time it ends.

Just to add to Medved's post, I have noticed that a lot of times lamps that are near the end of their life will form a hotspot that travels along the cathode. Unless that's what he was trying to say in the first place? I've never seen it on brand new tubes, only ones that are near the end.

Well I guess it is what medved described. I have one 18w osram that does it daily (the lamp runs 12h per day everyday) and it has done it almost three years now. One t12 lamp in my hallway does it some days but since it's thicker it's hard to notice. Some other daily used lamps does it too, one of them being pl-s 11w.

why does it appears to happen always at the same end of the lamp?

At first I suspected it was inflicted by some interence coming from a power grid because I had not noticed it happening this frequent at my previous apartment.

Just to add to Medved's post, I have noticed that a lot of times lamps that are near the end of their life will form a hotspot that travels along the cathode. Unless that's what he was trying to say in the first place? I've never seen it on brand new tubes, only ones that are near the end.

With really worn out lamps the hot spot increases it's temperature (the worse emission causes it to regain it back by increasing the temperature) till it becomes so bright its incandescent glow is visible even with the lamp lit.
In good tubes the hot spot temperature is way lower, so it remains dark, so practically invisible within the lighted lamp. But if you study the arc itself (on a germicidal bulb; behind a filter glass or with other UV protection means), you may notice it tend to concentrate into one spot on the electrode. And that is the hot spot, or better to say "arc root".

Well I guess it is what medved described. I have one 18w osram that does it daily (the lamp runs 12h per day everyday) and it has done it almost three years now. One t12 lamp in my hallway does it some days but since it's thicker it's hard to notice. Some other daily used lamps does it too, one of them being pl-s 11w.

why does it appears to happen always at the same end of the lamp?

At first I suspected it was inflicted by some interence coming from a power grid because I had not noticed it happening this frequent at my previous apartment.

On the mains frequency ballast the arc cathode root works in that way only 8.3 or 10ms (60 or 50Hz), then the other half cycle it cools down and only the next half cycle it has the next chance to become the root again. But during that time it cools down and so another spot may become the preferred root. When there are irregularities in the mains (e.g. some high current nearly saturating load and long distance to the substation; it may be asymmetric, mainly when permanent magnets are involved), the arc current may vary in shape and so alter the root spot condition just before the zero cross, so altering it's chances to become root the next cycle.

Personally I've seen such dancing to appear only sometimes (one day it was flickering, the other not; many times it starts to flicker without any apparent reason and after some time stops again, really random) and it didn't appear to depend on the lamp age or wear...