Dim fluorescent tube

[Huge]

No problem. I'll just killfile you, too.

[Uncle Peter]

Oops, my mistake. I know people from both areas and mixed them up.

[Uncle Peter]

You're an egotistical arse.

[Uncle Peter]

Don't you even have the ability to killfile replies to a particular person? I thought Linux crap was supposed to be highly configurable?

[Tim Streater]

Your k/f must be getting quite long then, it appears to be your standard response to anyone that upsets you.

[Uncle Peter]

Telling him his killfile is long is probably one way of upsetting him. Consider yourself no longer a friend of Huge :-)

[Tim Streater]

Allegedly I'm already *in* his k/f.

[Dave Plowman (News)]

Sadly it's no longer a badge of honour being in Huge's killfile - too many for that.

[Mr Fuxit]

You never really learn to swear until your wife starts learning to drive.

[Uncle Peter]

I thought it was the other way round. Women tend to be overly cautious and get upset when a man "just misses" something.

[Uncle Peter]

Is your surname really Licence? I bet they got confused when you applied for a driving Licence.

[Grimly Curmudgeon]

I'm quite happy with my ex-office ceiling fittings in my workshop. 5 times 4xtube 4ft fittings, giving out lots of lovely lumens, it's like a nova in there.

[Uncle Peter]

I'm partial to 470 (yes four hundred and seventy) LED lamps. Fit into normal BC/ES/etc. Brighter than a 150W tungsten.

[Andrew Mawson]

The REALLY odd thing is that after a couple of days use, the dim tube became indistinguishable from the others, so it was something transitory that was making it dim (only in the middle)

Andrew

[Andrew Gabriel]

After a long period of no use, and particularly if stored in the cold, modern tubes which regulate mercury pressure using an amalgam pellet tend to lose almost all the mercury into the amalgam. It takes a while for it to liberate enough for the tube to run normally, and this can show as dim areas, or even a pink glow when the discharge is running only in the argon base gas.

It should recover within an hour or so use.

It may happen again at end of tube life, when the mercury has been permanently lost into the phosphor and other parts of the tube, and there is no longer enough left in the gas or amalgam to run the tube properly.

[Johny B Good]

That's an interesting development in linear fluorescent tube technology[1]. I take it the amalgam pellet is just to stabilise what whould otherwise have been a small pool of mercury rather than the mercury amalgam vapour used by CFLs. IOW, the tube still uses a 'straight' mercury vapour.

I've witnessed this effect myself with new tubes and, as you say, an hour or so of running settles it down. In some cases, the tube discharge arc 'wriggles' producing ripples in the light output distribution along its length.

That's an effect common to the older tube designs as well.

[1] My reference book, "Lamps and Lighting" (2nd edition) was first published way back in '72. My copy is a '79 reprint so might be a teensy weensy bit out of date with regard to minor tweeks to the latest fluorescent tube technolgy.

[meow2222]

CFLs run hotter than linears, so need amalgam to regulate the mercury vapour pressure. Its not in my 1948 book either.

NT

[Andrew Gabriel]

Yes, the amalgam doesn't vaporise, just liberates and absorbs mercury.

AFAIK, amalgam pressure regulation was first used in some HID lamps. It wasn't required in old T12 tubes because the operating temperature of 40C is only a rise of 15C above ambient (at the ceiling), and they were way over-dosed with mercury back when people didn't worry about that, so they never actually ran out before some other part failed.

This doesn't work with CFLs, where tubes are normally designed to run at 100C and the mercury vapour pressure would be too high. If you simply put less in so the pressure was right when new, the tube would have a very short life (as it is lost into the phosphor, etc with no resovoir to replace it) and would probably start even dimmer than they do today. Sometimes you can see the amalgum rolling around loose in the tube, although I was told by a lighting engineer that it's supposed to be in the colder pinch-off seal area of the tube.

The new T5HE and T5HO tubes also use amalgam pressure regulation. These were designed after people started getting very concerned about the amount of mercury used in fluorescent tubes, and they are dosed with less mercury, by using type of protective coating to stop it getting permanently lost into the phosphor, so less is lost in operation. They have exactly the right amount to last only the expected tube life, so you will see some of these tubes will fail due to running out of mercury if nothing else fails first, which never happened with older fluorescent tubes. Some of this has been copied into CFLs and T8 tubes now, to allow them to use less mercury too, although I don't know if all T8's now have amalgam pellets, but they probably have less mercury. (I don't have an up- to-date reference on their design.)

The average person has 1000 times more mercury in their teeth than there is in a modern CFL.

That happens when conditions in the tube allow a load of electrons to accelerate in the discharge together, and all reach the electron potential necessary to ionise a mercury atom at the same point, which gives you a brighter ring. They will repeat this over and over down the tube if the conditions are just right.

In old tubes, it tends to be the phosphor wearing out which dims the tube.

I don't see it now, but London Underground train flourescent tubes had a fault which I used to see quite regularly - tube glowing bright for about half the length, gradually turning to dull pink in the other half. This is caused by running the tube on DC for a long time, causing the mercury to all migrate to one end. I can't imagine what control gear they used which had such a failure mode occasionally.

Mine is 1st edition, IIRC ;-)

Thorn Lighting used to publish a superb technical handbook, which went into this sort of detail. That sadly stopped in the early

1980's, when they sold their lamp manufacture division to GE.

[Johny B Good]

Thanks for the clarification on that point.

I understood that running out of mercury was one of the possible failure mechanisms but, as you say, it was more likely to fail from filament and phosphor problems.

As I understood it, the mercury amalgam was in the vapour phase to improve the UV emission at the much higher running temperature where the output from a straight mercury vapour would otherwise be down by a significant amount from its peak around the 40 deg mark. The output of the amalgam vapour is down slightly on a straight mercury vapour when each lamp type was up to its optimum temperature.

The slight drop in efficiency and the noticable run up times are simply the (small) price we pay to create a compact fluorescent lamp that can compete with a standard tungsten filament GLS lamp in terms of lumen output.

That's true enough. Thankfully, it's nowhere near as 'active' as a tiny blob of mercury from a broken lamp so the situation isn't quite as bad as it might seem from that statistic (although there are still concerns about the lifelong exposure to the miniscule mercury vapour emission from tooth fillings).

I can only imagine that the DC ballast must have to toggle the polarity on subsequent switch on cycles to prevent this from happening. Either the reversing switch mechanism fails to reverse polarity when it should or else the lamps are very rarely switched off with random on times sufficient to cause an imbalance between the periods of polarity reversals.

Yes, it's a great shame that we're never going to see an updated edition of this book. It really was a comprehensive treatise of the whole subject of not just the lamp technology itself but also the physiology of sight, the production of radiation, and the art of providing appropriate illumination to a whole range of environments and purposes.

I don't know whether a modern, up to date, publication would be available today that could cover the whole subject in a single volume. You might need two or three seperate books to cover the same subject matter these days.

However, the rapid pace of LED lamp development looks set to make a lot of the the lamp technolgy described in the book largely a matter of historical interest so a future publication with the same coverage of the whole subject might yet still fit into a single managable volume.

Specialist lamp usage, e.g oven lamps may still drive a need for some tungsten filament lamp production for another decade or so until the existing ovens are displaced by modern ones designed to use LED lamps optically coupled via a fibre optic bundle or a simple light pipe to keep the lamps suitably isolated from the 300 deg max temperature environment that the tungsten oven lamps were rated for.

That book would still make an excellent present to anyone with a curiousity about how things _actually_ work. The only thing it's missing is the 30 odd years of technological refinements that have gone into LED lamps since the book described the LED lamp technology of the day. A relatively slim volume on LED lamp development should neatly address that one major shortcoming for anyone interested in the subject.