G'day,
Does anybody know what do the colours mean on fluro cathodes? some are green, blue or orange. AFAIK the colours are not a good thing and the tube is probably EOL but I'd like to know what exactly is happening with the cathodes.
Cheers

The colors come from the different materials in the arc stream, bringing their characteristic spectral lines. So if anyrhing but Hg, Ar Kr or Xe appears there, it means heavy sputtering from the electrodes. Normally it is brief flash at cold electrode start, but if it persist for long time, it means tye cathode is collapsing (and if used, the electronic ballast suffering).

The colors come from the different materials in the arc stream, bringing their characteristic spectral lines. So if anyrhing but Hg, Ar Kr or Xe appears there, it means heavy sputtering from the electrodes. Normally it is brief flash at cold electrode start, but if it persist for long time, it means tye cathode is collapsing (and if used, the electronic ballast suffering).

So... colours like blue, green and orange are not good. I think orange is if the filament is melting, blue is if there is an arc but im not sure what green is?

The green is probably spectral emission lines caused by the electron emissive coating on the filament entering into the arc. If I remember correctly, standard oxide coated filaments are made by coating a tungsten wire in a mixture of barium carbonate, calcium carbonate, and strontium carbonate, and then hearing it during the exhausting process to convert the carbonates into oxides.

For reference, here’s the spectral lines of the alkali earth metals that I mentioned:

Barium:
https://upload.wikimedia.org/wikipedia/commons/thumb/6/6c/Barium_spectrum_visible.png/640px-Barium_spectrum_visible.png

Calcium:
https://upload.wikimedia.org/wikipedia/commons/thumb/e/ec/Calcium_spectrum_visible.png/640px-Calcium_spectrum_visible.png

Strontium:
https://upload.wikimedia.org/wikipedia/commons/thumb/8/86/Strontium_spectrum_visible.png/640px-Strontium_spectrum_visible.png

As you can see, Calcium has very strong green lines, and Strontium also has some.