On 13/08/2023 09:05, alan_m wrote:
> On 12/08/2023 21:29, Animal wrote:
>
>> Those lm/w figures are for new fl tubes, over decades they fall a lot.
>> Last time I replaced fluoros their output was way down, but no-one
>> realised until the new LEDs went in.
>
>
> Many years ago at my former place of work they replaced all the fluoro
> tubes with new fluoro tubes over the weekend.  When we came in on the
> Monday the office/factory was very much brighter!
>
>
>
Yes. I think that you can be well down on output and still have a
functional tube.

Here is a post from quora that seems fairly authoritative:
===============================================
A fluorescent lamp is a long-arc light source. There are tiny
incandescent filaments in each end of the lamp which are covered
(smeared, really) with mercury emissive paste (mep). When the tiny
filaments heat up they cause the mep to vaporize, which increases the
conductivity of the atmosphere in the tube, and allows the arc to
establish from one end of the tube to the other. The arc produces very
high frequency, short wavelength light, very high in the spectrum and
well into the UV range. The interior of the glass tube is lined with
clay phosphors which glow in the visible spectrum when excited by the UV
light.

In general, there are four separate forces that cause the lamp to become
dimmer as it ages.

1) The incandescent filament gradually loses particles of tungsten which
deposit on the interior of the glass tube;

2) The mep also migrates to the interior of the tube;

These tungsten and mercury deposits appear as black smudges, which, over
time, usually becoming gradually worse towards the ends of the tube,
which is where the filaments are located.

3) The eroding lamp filaments and mep cause the arc to gradually
deteriorate so the light output from the arc itself is reduced;

4) At the same time as all of these other factors are coming into play,
the clay phosphors begin to lose their ability to glow and
simultaneously start to erode off of the interior tube walls.

This is why the life of a fluorescent isn’t measured until the last
photon is released from the lamp (like an incandescent lamp is) but
rather at an arbitrary point in loss of intensity, usually around a 60%
loss compared to new. You see lamps still in service all the time which
are actually burned out, but still “glowing”.

So, the TL/DR answer: Yes they lose a lot of their brightness over their
life and this is why the a new lamp appears SO much brighter than the
old one did.

--
“It is dangerous to be right in matters on which the established
authorities are wrong.”

― Voltaire, The Age of Louis XIV

On Sunday, 13 August 2023 at 09:33:07 UTC+1, The Natural Philosopher wrote:
> On 13/08/2023 09:05, alan_m wrote:
> > On 12/08/2023 21:29, Animal wrote:
> >
> >> Those lm/w figures are for new fl tubes, over decades they fall a lot.
> >> Last time I replaced fluoros their output was way down, but no-one
> >> realised until the new LEDs went in.
> >
> >
> > Many years ago at my former place of work they replaced all the fluoro
> > tubes with new fluoro tubes over the weekend. When we came in on the
> > Monday the office/factory was very much brighter!
> >
> >
> >
> Yes. I think that you can be well down on output and still have a
> functional tube.

anyone with experience of old tubes knows it's so

> Here is a post from quora that seems fairly authoritative:

I have some questions

> A fluorescent lamp is a long-arc light source. There are tiny
> incandescent filaments in each end of the lamp which are covered
> (smeared, really) with mercury emissive paste (mep). When the tiny
> filaments heat up they cause the mep to vaporize, which increases the
> conductivity of the atmosphere in the tube, and allows the arc to
> establish from one end of the tube to the other.

for that to work the mercury would need to condense onto the filaments every time. How do thy get it to do that?

AIUI linear fls used elemental mercury. Not until very thin tubes (T4, cfl etc) did non-elemental get used.