Why do fluorescent tubes operated at very low power emit wierd colours.

[Ian Stirling]

I was playing with some failed CFL lamps, in CC mode.
I notice they glow very orange at low powers, almost neon-like, though
not as pure orange.
Is there some other emission rather than mercury driving this, or does
the phosphor somehow "know" what the average photon flux is.

 

[Victor Roberts]

I was playing with some failed CFL lamps, in CC mode.
I notice they glow very orange at low powers, almost neon-like, though
not as pure orange.
Is there some other emission rather than mercury driving this, or does
the phosphor somehow "know" what the average photon flux is.

The phosphor does not "know" what the average photon flux is, and does
not care.

Not all the light comes from the phosphor. The mercury emits a few
blue and green lines directly, so the phosphor has a slightly yellow
tint in order that the final light output is white. I'm not sure what
you are seeing, but if the Hg visible lines are depressed then you
would be seeing mostly phosphor emission. The other possibility is
that these lamps use neon as the buffer gas and a very low currents
you are exciting the neon instead of the Hg. However, this is rather
unlikely, since neon is rarely used as the buffer gas, is never used
as the ONLY buffer gas as far as I know, and its energy levels are far
above Hg. If you have a spectroscope you can see if the orange glow is
made up of multiple narrow lines, which would be neon, or broad bands,
which would be the phosphor.

 

[Ian Stirling]

The phosphor does not "know" what the average photon flux is, and does
not care.

Thought it was vanishingly unlikely.

Not all the light comes from the phosphor. The mercury emits a few
blue and green lines directly, so the phosphor has a slightly yellow
tint in order that the final light output is white. I'm not sure what
you are seeing, but if the Hg visible lines are depressed then you
would be seeing mostly phosphor emission. The other possibility is

It's certainly not yellow-white.

that these lamps use neon as the buffer gas and a very low currents
you are exciting the neon instead of the Hg. However, this is rather
unlikely, since neon is rarely used as the buffer gas, is never used
as the ONLY buffer gas as far as I know, and its energy levels are far

It's a deep orange, only slightly desaturated from what I'd expect from
a neon bulb.

The T8 tube I played with at similar currents (though DC) just gave
the expected reduction in brightness. Very striking at only a few
microamps in a otherwise dark room.

above Hg. If you have a spectroscope you can see if the orange glow is
made up of multiple narrow lines, which would be neon, or broad bands,
which would be the phosphor.

Something to try.
I don't own a spectroscope, but I do have several compact disks (looks
at cupboard piled full) that could at least give me an idea if the
frequency is right for neon (compare to a bulb).
I have doubts as to the ability to measure bandwidth.

Could the fact that at the time I was feeding the bulbs very spiky HF AC
(bulb was being used as a test load) cause any preferential emission
from the neon?

The bulbs in question were CF, using simply an inductor and a starter
all sealed up in one buzzing jar.
Take quite a while to come up to operating brightness too.

 

[Victor Roberts]

Thought it was vanishingly unlikely.

It's certainly not yellow-white.



It's a deep orange, only slightly desaturated from what I'd expect from
a neon bulb.

The color of neon is quite distinct from the other rare gasses. Almost
everything else is blue or weak pink. The only other gas the comes
close is sodium, and I don't know how you would get enough sodium in
your CFL.

The T8 tube I played with at similar currents (though DC) just gave
the expected reduction in brightness. Very striking at only a few
microamps in a otherwise dark room.


Something to try.
I don't own a spectroscope, but I do have several compact disks (looks
at cupboard piled full) that could at least give me an idea if the
frequency is right for neon (compare to a bulb).
I have doubts as to the ability to measure bandwidth.

The difference between neon emission and phosphor emission is quite
striking. While I have never used a CD as a spectrometer, it may have
the resolution to see the difference.

Could the fact that at the time I was feeding the bulbs very spiky HF AC
(bulb was being used as a test load) cause any preferential emission
from the neon?

Yes. But I still wonder why anyone would use neon.

 

[Don Klipstein]

Yes. But I still wonder why anyone would use neon.

I wonder also, but I have seen CF lamps that appeared to me to have
neon, or probably something like 99.5% neon .5% argon, instead of just
argon.

Example: The Philips 18 watt "Earth Light", apparently now called the
SL/O after a change in the texture of the outer bulb. I am not sure about
current SL/O units, but 18 watt "Earth Lights" that I purchased in the
early and mid 1990's glow a distinctly neon-like orangish red when started
at very cold temperatures (roughly -15 degrees C or colder). The color
became more normal as the lamp warmed up.

- Don Klipstein ([email protected])

 

[Victor Roberts]

I wonder also, but I have seen CF lamps that appeared to me to have
neon, or probably something like 99.5% neon .5% argon, instead of just
argon.

Example: The Philips 18 watt "Earth Light", apparently now called the
SL/O after a change in the texture of the outer bulb. I am not sure about
current SL/O units, but 18 watt "Earth Lights" that I purchased in the
early and mid 1990's glow a distinctly neon-like orangish red when started
at very cold temperatures (roughly -15 degrees C or colder). The color
became more normal as the lamp warmed up.

- Don Klipstein ([email protected])

Neon is sometimes used to raise the discharge voltage, which reduces
end losses and increases lamp efficacy. However, due to its lighter
molecular weight, neon does not moderate the velocity of the mercury
ions arriving at the electrode as much as the heavier rare gases,
leading to shorter electrode life. Perhaps Philips has determined a
neon pressure that provides both higher lamp efficacy and acceptable
electrode life. I have an SLS 15 in my desk lamp. Perhaps I will stick
it in the freezer and see if the buffer gas is indeed neon.

BTW Don -- I am getting quite a bit of SPAM with your return e-mail
address that I know is not really coming from you. I just wanted you
to know that someone has hijacked your return address for their own
twisted purposes.

 

[Ian Stirling]

Neon is sometimes used to raise the discharge voltage, which reduces
end losses and increases lamp efficacy. However, due to its lighter
molecular weight, neon does not moderate the velocity of the mercury
ions arriving at the electrode as much as the heavier rare gases,
leading to shorter electrode life. Perhaps Philips has determined a

Stupid question: does ion velocity depend on tube length?
Or is it essentially set by the mean-free path (microns?)

 

[Victor Roberts]

Stupid question: does ion velocity depend on tube length?
No.

Or is it essentially set by the mean-free path (microns?)

Yes - and electric field.

 

[TKM]

Neon is sometimes used to raise the discharge voltage, which reduces
end losses and increases lamp efficacy. However, due to its lighter
molecular weight, neon does not moderate the velocity of the mercury
ions arriving at the electrode as much as the heavier rare gases,
leading to shorter electrode life. Perhaps Philips has determined a
neon pressure that provides both higher lamp efficacy and acceptable
electrode life. I have an SLS 15 in my desk lamp. Perhaps I will stick
it in the freezer and see if the buffer gas is indeed neon.

Vic Roberts
http://www.RobertsResearchInc.com

This sounds like something recently reported by a colleague who described a
42 watt triple-tube CFL operating at low output with an orange color and
with only 1 or 2 of the 3 legs of the lamp emitting light. The odd thing
was that this was indoors in normal ambient temperatues and in an open
6-lamp fixture (one of those high-bay types) with about 1,000 hours on the
lamps. Ordinarily, I would suspect a lamp about to fail; but there were a
dozen or so fixtures in the installation and each one had a similar lamp.
My next guess is a low system voltage situation since the total light output
of the fixtures was also reported to be low.

Any other situations that could result in such a lamp appearance?

Terry McGowan

 

[Victor Roberts]

This sounds like something recently reported by a colleague who described a
42 watt triple-tube CFL operating at low output with an orange color and
with only 1 or 2 of the 3 legs of the lamp emitting light. The odd thing
was that this was indoors in normal ambient temperatues and in an open
6-lamp fixture (one of those high-bay types) with about 1,000 hours on the
lamps. Ordinarily, I would suspect a lamp about to fail; but there were a
dozen or so fixtures in the installation and each one had a similar lamp.
My next guess is a low system voltage situation since the total light output
of the fixtures was also reported to be low.

Any other situations that could result in such a lamp appearance?

Terry McGowan

If an amalgam lamp has been off for a long enough time, all the
mercury will diffuse to the amalgam and be captured there. When the
lamp is then switched back on, it takes time for the mercury to
diffuse back throughout the lamp. The diffusion time is especially
long in multi-tube CFLs, due to their high length-to-diameter ratio
and the narrow diameter of the bridges between the tubes. If there is
a lack of mercury, the buffer gas will be excited. Normally the buffer
gas is argon which has only a weak bluish glow, not unlike the direct
visible emission from mercury, so sections of the lamp that do not
have enough mercury look "dead". However, neon has a distinctive
red-orange color that cannot be confused with mercury emission.