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In article ,
(N. Thornton) writes:
Andy Hall wrote in message . ..
Where I do use them I only use electronic ballasts. They are even
worse when run at mains frequency. I am sensitive to flicker from
phosphor devices like tubes and CRTs so find electronic ballasts
essential anyway

Fls running off high frequency ballasts supplied by dc are competely
flicker free. Not all electronic ballasts are flicker free. Filament
bulbs OTOH are not flicker free.

Flicker at twice the line frequency, 100Hz, is not perceptable by
any person. About half the poputation can see 60Hz, and no one can
see above 76Hz. You need to go to things with smaller faster brains
than humans before you can see 100Hz -- insects can.

The main flicker from traditional fluorescent lamps is at twice the
line frequency. It's very bad, most lamps go almost out 100 times a
second. Another issue is that the phosphors making up the various
colour components have different persistance, so the lamp colour
changes 100 times a second[*]. Fortunately you can't see any of this
because it's way above the speed of the fastest human brain (unless
you have an exceptionally small brain but then you've probably got
far more serious things to worry about anyway;-)

The problem from traditional fluorescent lamps is if they don't
give out the same light level when conducting in each direction.
When that happens, you now have a lamp which is changing intensity
50 times a second, and well over half the population can see that.
This could happen with a badly/cheaply made tube where the electrodes
are not well matched. It also happens for a few hours just before a
tube fails completely (assuming regular switch-start control gear --
some other control gear can keep a tube running indefinately in this
state).

The tube ends can also flicker at 50Hz due to the different mechanisms
employed when the electrode switches from an anode to a cathode 50
times a second. Masking the tube ends is a way which has been used
to overcome this prior to electronic control gear.

You are right that electronic ballasts are not flicker free. They
often flicker at twice the line frequency, but as that's way above
what a human can see, it is of no consequence. The issue of unequal
light levels when the tube conducts in each direction is not relevant
now because the electronic control gear reverses the current flow
typically 20,000 times/second, and in any case, above around 5,000
times/second, a different mechanism operates in the discharge and
it becomes continuous anyway, even if the current flow isn't.
[*] Here's an interesting practical to try at home (or maybe out in
the garden;-) Arrange for a mains fluorescent tube to be operating
on the end of around 6' of flex, remote from its control gear.
One of the small 4/6/8W lamps would be ideal, particularly one
of the inspection lamp types where it's suitably encased.
Attach the tube very securely to the flex. Having ensured adiquate
clearence, go outside at night, switch the tube on, and now swirl
it round and round on the flex. You should now see the 100Hz
stroboscopic flashing, and you will also see how the lamp colour
changes 100 times/second, due to the different persistance of the
various phosphors. Obviously, you need to take various safety
precautions, such as an RCD and safety goggles particularly if
you're going to take a class of children outside to observe, and
you might end up having to clear up a broken fluorescent tube....

--
Andrew Gabriel