What Causes the Strange Effect on this Exit Sign?

[fawk3s]

So today I noticed something pretty weird on an exit sign above a door in my school, looking somewhat like this:
http://www.examiner.com/images/blog/wysiwyg/image/exit_sign.jpg

The effect is pretty difficult to describe by words, since I didnt expect to see anything like that on it. But it looked somewhat like the effect you would get in a dim pool room in the ceiling when the light in the pool would be on - a nice play of light by the water waves.
And this effect was in the sign, which presumably has a fluorescent light behind it. Ofcourse the effect is only on the sign and there is no water whatsoever nearby, so please let's rule this out. What could cause this effect?

I have no good guess besides maybe that the fluorescent bulb might have a filament or something inside it, which makes contacts with the glass shield and acts somewhat like a plasma lamp would? I know, crazy theory, plus I haven't heard of any of these kinds of bulbs, but its the best one I've got so far.

Any ideas?

Thanks in advance,
fawk3s

 

[JaredJames]

Light bulbs and tubes flicker, I've had some that do it and produce the effect you're talking about to varying degrees.

You can actually buy bulbs specifically designed to reproduce it.

Flourescent tubes don't have filaments.

 

[fawk3s]

Light bulbs and tubes flicker, I've had some that do it and produce the effect you're talking about to varying degrees.

You can actually buy bulbs specifically designed to reproduce it.

Flourescent tubes don't have filaments.

I know they dont, but I figured maybe it was a plasmalamp-like bulb in there, which wouldn't make much sense anyways so never mind.
But what is the effect actually caused by? It can't be just flickering.

Thanks in advance,
fawk3s

 

[JaredJames]

But what is the effect actually caused by? It can't be just flickering.

Why not?

I have an old ish bulb where the filament is particularly secure and any vibration causes a flickering effect.

 

[fawk3s]

But it doesn't look anything like flickering nor vibration, these are slowish, looking somewhat ordered waves. I can't really imagine how a vibration could cause that, especially as they are slow-looking waves.

 

[rollcast]

Its prob caused by the flicker of the bulb, all bulbs do this as ac power has an off phase in its wave, added to the after image caused by your eyes delayed reaction to the flicker

 

[alxm]

Yeah, fluorescent lights can do that. They flicker at twice the mains frequency, so it's not normally visible. But old ones can start flickering more or cause the patterns you saw. I think it's because you get some resonance with the ballast circuit that's not supposed to be there, normally. Same thing can cause them to make noise.Why is the guy on the sign wearing a cape?

 

[gb7nash]

This exit sign is hilarious. That guy's bolting for the door. It should be the bathroom sign instead.

 

[DrGreg]

Why is the guy on the sign wearing a cape?

It's an arm, not a cape!

Emergency exit signs like this are commonplace throughout Europe and many other parts of the world. www.google.co.uk/images?hl=en&q="emergency+exit+sign"