Spectra change due to dimming

[artis sideley]

I would like to know if the light spectra of a fluo tube used at max
power differs from that delivered by the same tube dimmed at 5% , 10%
...80 %.

[Ian Stirling]

Slightly.
The emissions from the filliments will change.
The fill gasses other than mercury may be dominant over the emission
from the phosphors. Especially if the tube is not run at full power to
vapourise the mercury first.

I've got one CFL tube that at low currents glows a brilliant neon
orange.

[Victor Roberts]

Yes. In a fluorescent lamp the light produced is a
combination of light produced by the phosphor, which is
excited by 185nm and 245nm UV radiation from the discharge,
and so-called direct visible radiation from the discharge,
mostly at 365nm 405nm , 436nm, 577nm and 579nm.

The lamp is dimmed by decreasing the discharge current,
which causes the intensity of all the lines to decrease.
However, reducing the current also causes the electron
temperature to increase, which in turn causes the 185nm and
254nm lines to decrease slightly more than the visible
lines. That is, the ratio of the visible lines to the UV
lines increases. The net effect is that the net color of
the lamp will shift toward the blue, and usually away from
the black body curve.

--
Vic Roberts
http://www.RobertsResearchInc.com
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[artis sideley]

Victor Roberts a écrit :

> On Sun, 17 Dec 2006 09:12:14 +0100, artis sideley
> <artis-poi...@atfreepoint.fr> wrote:
>
>> I would like to know if the light spectra of a fluo tube used at max
>> power differs from that delivered by the same tube dimmed at 5% , 10%
>> ...80 %.
>
> Yes. In a fluorescent lamp the light produced is a
> combination of light produced by the phosphor, which is
> excited by 185nm and 245nm UV radiation from the discharge,
> and so-called direct visible radiation from the discharge,
> mostly at 365nm 405nm , 436nm, 577nm and 579nm.
>
> The lamp is dimmed by decreasing the discharge current,
> which causes the intensity of all the lines to decrease.
> However, reducing the current also causes the electron
> temperature to increase, which in turn causes the 185nm and
> 254nm lines to decrease slightly more than the visible
> lines. That is, the ratio of the visible lines to the UV
> lines increases. The net effect is that the net color of
> the lamp will shift toward the blue, and usually away from
> the black body curve.
>

Thanks you all.