Hi I was in work today and the electricians were testing  the emergency lighting system thankfully they all worked however towards the end of the test I noticed one tube in particular looked very dim and all along its length there were dark rings not like sputtering they were the whole length of the tube. To look at they were like the moving dark rings you sometimes get but these were absolutely still kinda weird never seen it before. On a different topic I saw some gaslight lanterns retrofitted with LED panels looked awful there was a post about this but can't find it now also saw some with HPS looked a lot better

As i understand it, the arc in the FL lamp can sometimes make some sound. The sound echoes in the Mercury vapors inside the lamp. The combination of the "new created" sound and its echo from the lamp ends happens to make a standing wave pattern (or a slowly moving sorta standing wave i.e. the wave is moving slowly, but compared to the speed of sound it is pretty much standing)

The standing wave means spots with higher and lower Mercury vapor pressure respectively - just like a normal standing wave of sound in air in an enclosed space. As the light output relates to the Mercury pressure (in the nearest area of the lamp volume), the same standing wave pattern appears in the light..

The emergency drivers are electronic converters from DC (battery) to high frequency AC, between few KHz up to 30 KHz or something in that order, that apparently corresponds to a wave length of a couple inches..

I think the wavelength at 30 Kc/s is several thousand meters I can't work it out exactly but its definitely more than a few inches. The idea its a soundwave caused by the arc makes sense I've seen tubes on buses with moving dark rings in them of course they are also subject to the noise and vibration of the bus itself. The lights in the shop I mentioned run on HF ballasts all the time I saw them started up the other day and all were instant on no flicker just hard on the ones in the staff room light up orange at the ends for about a second then all light fully at exactly the same moment

This is a sound wave that goes at the speed of sound, not electromagnetic wave that goes at the speed of light

The relation between speed of wave, wavelength and frequency is :

c = λv
c - speed of wave advance
λ - wave length
v = frequency

Sound : λ = c/v = 340m/sec / 30000c/sec = 0.011m = 4 inches (if that would be air)

Light : λ = c/v = 3*10^8m/sec / 30000c/sec = 10000m



In the FL lamp the gases density is way below atmosphere air, that would corespond with higher speed of sound so longer wavelengths

Ah OK Ash sorry I missed understood what you said first time  I get it now