And finally finally! This is a nitrogen spectrum. This tube was deliberately filled with circa 3mmHg of air to demonstrate so called chemical nitrogen afterglow. I believe all of the oxygen was consumed now, so no more afterglow effect, but nitrogen still displays its funny spectrum. At first, one may thought there is some kind of interference on the camera, but no! There is none, and as a sanity check nitrogen spectrum shows the same banded spectrum also on my HP320 spectrometer and even in optical spectroscope. Molecular gases have complex electron interactions resulting in funny wide spectrums. A lot of UV is radiated, too!

About that 589nm line seen in some lamp spectrums. 589nm of course screams Sodium!!! loudly ) So it is easy to understand we can see a marker of ever-present sodium impurity, with it's very intense resonant radiation, likely from oxide coating, as the lamps discussed here are mostly NOS.

Nice experiments, RRK. A few minutes proably won't hurt the tube. Even crap  chinese flyback power supply fixtures for 4, 6, 8 W tubes need several hours for a significant blackening.
If they had neon in the gas filling it would probably be easy to notice like in Nixie tubes and some CCFLs.

To see the neon glow through the phosphor coating is not easy.   you often have to place the lamp in a deep freezer to solidify the mercury and then it becomes much easier.  On the other hand, all of these gas fillings seem to have changed so frequently and between different manufacturers, it’s possible there is a different fill in your Osram lamp.

I am sure the detectability depends on gas fill Ratio as well, but I find with known neon filled lamps, you certainly can see the Neon glow through the Phosphor coating, for example when I tickle my GE F96T12/VHO with my Martindale I can very much see the Neon buffer gas being excited at the barrier discharge point



(as an aside I notice interestingly with Triphosphor lamps even those most certainly not Neon filled, the barrier discharge seems to preferably cause the red phosphor to glow, I wonder why that is!)


BTW @RRK which spectrometer program is that? looks like by the USB Device name your still using the little-garden spectrometer, but a different program? does that program offer an option to calibrate the spectometer against a known CCT Black body radiator?

Yep, I use a nice alternative program written in Rust:  https://github.com/DerFetzer/spectro-cam-rs

Spectro-cam offers calibration. *But* I do not believe a webcam spectrometer can be satisfactory calibrated by spectral response because camera definitely demonstrates strong non-linearity. May be if you take care of selecting a fixed gain (exposure) and disable all dynamic range adaptations in control panel. If this is possible at all...

Regarding reddish color with the cathode part of a barrier discharge. Yes I noticed this as well. A probable explanation is that cathode part of the discharge often shows significantly different excitation of different gas wavelengths due to much higher electron temperature. And different balance of UV + visible lines may mean slightly different color balance of triphosphor components radiation (plus visible light from the discharge). Remember classic Xe/Ne filled plasma balls, where cathode parts clearly show red Ne excitation and discharge columns are blue-gray xenon?

From the recent experiments: here you can compare a spectrum captured from TN20 neon lamp (mostly cathode glow, see orange 585nm Ne line is excited preferentially) and a spectrum captured off a discharge column of a neon sign tube, 585nm is weak and ~640nm + a line cluster around is radiated as much redder light. A confounding factor to note here is also TN20 is filled most likely with Penning mix while the sign tube is 100% neon - here i am sure as I filled it myself

Nice experiments, RRK. A few minutes proably won't hurt the tube. Even crap  chinese flyback power supply fixtures for 4, 6, 8 W tubes need several hours for a significant blackening.
If they had neon in the gas filling it would probably be easy to notice like in Nixie tubes and some CCFLs.

So bottom line is at least my Osram HO tubes are actually pure Ar filled. I do not have any other branded T5 lamps like Philips or GE or Sylvania on hand for now to compare. Longer lengths of T5 HEs really have a bit of Kr added, very likely to keep burning voltage reasonable, and may be to improve efficiency a bit as well. Economy types of T5 can be reasonably expected to have some even more Kr added to drop lamp voltage further.

But the biggest surprise for me was that I was really able to monitor cathode sputtering going on in the tube clearly by detecting barium and strontium lines in the light captured near the electrodes!