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lighting fluorescent lamps, with a TC
I think that when we talk about lighting up a fluorescent bulb with a TC
there are a couple of aspects we should think about:
1) The brightness... When I wave a bulb near my coil, it glows fairly
brightly, but nowhere near as bright as when running it from the mains with
a regular ballast. In a darkened room it looks pretty bright, but without
actually measuring it, I'd say it was easily 100 times dimmer (6 or 7 stops
photographically speaking). Next time I think about it, I'll get a spot
meter out and measure it. Your eye has an awfully good "AGC" circuit, so
absolute brightness is very hard to determine by eye.
2) The mechanism which is lighting it up. Several things to think about
here:
a)I note that holding the tube in my hand lights it brighter than if I
clamp it in a nonconductive stand. Obviously, I am acting as some sort of
antenna to couple more power into the tube. I haven't tried a metal stand,
grounded or not. A good experiment to try.
b) Does the tube temperature make a difference? The brightness of a regular
fixture changes noticeably over temperature, and is much harder to start
(with the usual rapid start ballast) when cold. Is this because the mercury
is less vaporized, or is it because the electrodes are cooler. Do
fluorescent lights run with hot electrodes (heated by the power flowing
through the tube after start, and through the starter/ballast when cold)?
The big rapid start tubes have only one wire on each end, and start the
ionizing process from the end (you have to have a ground plane next to the
tube for it to start... If you pull the tube out of the fixture (extending
the wires) they won't start), which might heat the end first, etc. In any
case, the temperature dependency is going to greatly affect the excitation
characteristics from a TC.
Another thing that just occurred to me is that I have noticed that the
phosphor in some tubes definitely has a different time constant. In my
garage, when it is cool (say 10 C) , and the lights have been on for
several hours, and I turn them off, some tubes glow faintly for several
minutes, other go dark instantly. Cheap tubes (the <$1 variety) glow,
expensive ones don't (the $2 variety).
And, is the TC exciting the Hg vapor, or is it exciting the phosphor
directly?
Do they use a fill gas in fluorescent tubes along with the mercury, perhaps
to aid in starting (Argon) and to raise the pressure so that electrode
sputtering isn't as much an issue.