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I have a ~60 cm fluorescent tube light that is failing. ~10 cm on one end glows orange, ~10 cm on the other end glows blue-white (normal fluorescent light color) and the middle 40 cm is dark.

Question: From an electrical point of view what exactly is going on such that this particular graded pattern of three colors is produced?

my fluorescent light

There seems to be a "starter" just behind the tube, near the "orange" end.

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    \$\begingroup\$ Potentially of interest: en.wikipedia.org/wiki/Glow_discharge#Regions \$\endgroup\$
    – Hearth
    Jul 30, 2019 at 14:10
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    \$\begingroup\$ Does the light fitting have an old fashioned removable 'starter'? \$\endgroup\$
    – HandyHowie
    Jul 30, 2019 at 14:15
  • \$\begingroup\$ @HandyHowie yes I think so, I've updated the drawing and described it. \$\endgroup\$
    – uhoh
    Jul 30, 2019 at 14:21
  • \$\begingroup\$ The first, and easiest, step would be to change the tube. Take a known good tube from another fixture (if available) if you don't want to buy a new one. \$\endgroup\$
    – Peter Bennett
    Jul 30, 2019 at 15:54
  • \$\begingroup\$ Have you tried changing the starter? \$\endgroup\$
    – HandyHowie
    Jul 30, 2019 at 19:00

1 Answer 1

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Your fluorescent tube has filaments on both ends. Those filaments are heated by the starter current and then emit electrons into the gas filling of the tube. To assist that, they have a coating of lanthanides.

Over time, those lanthanides amalgamate with the mercury gas filling and cannot serve their function any more. The tube cannot be started any more then. It will be heated up through the filaments until the starter opens, but as nothing happens then, the starting process is repeated.

What's still there is the filaments. Depending on the amount of lanthanides still in function, electrons are still emitted and excite the mercury gas filling in the surroundings of the electrode. That's the green or white end. At the orange end, what you see is the glowing of the filament.

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  • \$\begingroup\$ I have used thoriated tungsten wire in the past. While thorium is not a lanthanide, is that the same basic idea (work function, electron emission...) \$\endgroup\$
    – uhoh
    Jul 30, 2019 at 14:25
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    \$\begingroup\$ If the tube does not flicker, the starter is defective, too. Often, that's the reason why a tube dies prematurely. Always replace the starter, too. \$\endgroup\$
    – Janka
    Jul 30, 2019 at 20:08
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    \$\begingroup\$ I think thorium-coated wire got out of fashion because the inadvertible radioactive hazard. \$\endgroup\$
    – Janka
    Jul 30, 2019 at 20:13
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    \$\begingroup\$ @uhoh Either the gas ions in the glow switch tube in the starter have plated so much metal from the electrodes onto the glass (a process known as sputtering) that it has shorted itself out, or the capacitor across it is shorted. \$\endgroup\$
    – Peter R. McMahon
    Aug 5, 2021 at 3:00
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    \$\begingroup\$ @Janka What blackens the ends of tubes and shortens their lives is the bad design of starters allowing the tube to blink on before the cathodes are fully heated. The high voltage between the gas & the cathode causes ions to blast it, dislodging atoms (a process known as sputtering). There is an English made electronic starter in a transparent green case, that fully preheats the cathodes. The 1st tube in my 1984 range hood lasted about 10 years. The 2nd one with one of these starters is still going. I don't remember the brand, but they are available from electrical wholesalers. \$\endgroup\$
    – Peter R. McMahon
    Aug 5, 2021 at 3:17

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