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PCA UV phosphorescence I’ve taken pictures of this before and have been trying to figure out the mechanism. Some later manufactured PCA tube in HPS lamps demonstrate phosphorescence as shown here. It relates to a dopant used in the production of the tube and may be magnesia, yttria or a few other dopants used for enhancing fabrication in onw way or another. The emission from the PCA is a pale blue as shown here but there is also a UV component in this phosphorescence as can be seen by the re-emission of light from some powder taken from a fluorescent tube at the lower right side of the tube. I was interested in what stimulates this emission as it only occurs with the initial start pulse on the lamp. The glow continues for quite a few minutes after stimulation. From a few experiments I’ve tried UV light from the initial gas discharge is not the driver. I suspected it was electron bombardment due to the enhanced behaviour with higher pulse voltages. I intended to find a broken tube and subject it to electron bombardment under vacuum to test this, but AI confirms it is cathodoluminescence, so stimulated by electrons in the same way as an old CRT screen. AI seems to be getting quite smart lately. Keywords: Miscellaneous

PCA UV phosphorescence

I’ve taken pictures of this before and have been trying to figure out the mechanism. Some later manufactured PCA tube in HPS lamps demonstrate phosphorescence as shown here. It relates to a dopant used in the production of the tube and may be magnesia, yttria or a few other dopants used for enhancing fabrication in onw way or another. The emission from the PCA is a pale blue as shown here but there is also a UV component in this phosphorescence as can be seen by the re-emission of light from some powder taken from a fluorescent tube at the lower right side of the tube. I was interested in what stimulates this emission as it only occurs with the initial start pulse on the lamp. The glow continues for quite a few minutes after stimulation. From a few experiments I’ve tried UV light from the initial gas discharge is not the driver. I suspected it was electron bombardment due to the enhanced behaviour with higher pulse voltages. I intended to find a broken tube and subject it to electron bombardment under vacuum to test this, but AI confirms it is cathodoluminescence, so stimulated by electrons in the same way as an old CRT screen. AI seems to be getting quite smart lately.

neon_question2_copy.jpg railtube.jpg PCA.jpg tubeson_HVHF4.jpg crookes.jpg
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Filename:PCA.jpg
Album name:Tuopeek / Misc
Keywords:Miscellaneous
Filesize:446 KiB
Date added:08 Mar 2026
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Comment 1 to 7 of 7
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Max   [Sun 08 Mar 2026 at 21:22]
Really nice capture, Mark! PCA is known to exhibit fluorescence from the UV-B domain up to the deep blue, and that is put to use in UV enhancers made by Philips. I also don't know the exact mode of PCA excitation or which dopant is responsible for this light emission, but I've always heard that deep (vacuum) UV radiation (below 200 nm) from the discharge in noble gas (xenon in HPS arc tubes, argon in UV enhancers) was the primary source of material excitation. Electrons may play a role, but I have never heard this being considered when the PCA UV enhancer was developed in the very early 2000s.

On the subject of AI, it is certainly looking more and more impressive, but what's available to us remains language models i.e., codes optimized for conversation and data presentation with a bias for positive social engagement, so I certainly would take the information provided with a grain of salt (check with external sources at the very least!!!). This looks like a very promising tool, but we should not forger than the primary goal of Gemini, Copilot, ChatGPT, etc, is more to hook us up on a shiny new product rather than provide absolutely accurate information (the large customer base that was promised to AI investors won't materialize itself from thin air)... so be very critical of it.
Tuopeek   [Sun 08 Mar 2026 at 22:11]
Thanks. Absolutely Max, I spend a bit of time trying to catch AI models out with misinformation and it is very noticeable the algorithms seek to support the questioner rather than be critical. They do seem to be improving at an alarming rate although still remain just a language model. I will still try to prove this at some point if I can find a suitable broken PCA discharge tube. I have tried on a number of occasions to stimulate the emission with UV of different wavelength but have never managed to reproduce the glow to any real extent. May be I just haven't hit the right wavelength. I'm sure I've tried it under a deuterium lamp too.

Was this intentionally added as a photoelectric assist in starting the discharge?
Max   [Sun 08 Mar 2026 at 23:05]
Yes, the PCA UVE was added to most Philips MH lamps around 2011-12 following the mandatory removal of 85Kr from all metal halide arctubes. Below is a closeup shot of the ceramic UVE in a Philips CDM-TC Elite 35W/942:



About the UV excitation of PCA, the issue with resonant radiation from low-pressure noble-gas discharges is that the peak emission lies well below the optical cut-off of the usual materials used in lamps (quartz, ceramic). There is a reason why this particular sub-UVC domain is referred to as "vacuum UV", because it is also absorbed by gaseous atmospheres and is best propagated in vacuum. The resonant emissions for argon lie at 104.8 and 106.7 nm, and for xenon it is at 129.6 and 147.0 nm. Those lines do not pass through air nor glass. As for deuterium lamps, the shortest wavelength obtainable is around 190 nm, which is not as energetic as the resonant emission inside a low-pressure argon or xenon discharge. If you want to go to lower wavelengths then excimer lamps are the way, but you'll gain 20 nm at most (there are shorter-wavelength sources but those are highly specialized ones. Now, the best way to test the electronic excitation hypothesis is under high vacuum with an electron beam...
Ria   [Sun 08 Mar 2026 at 23:40]
Very impressive, Mark Very Happy

Regarding AI, I confess the concept of it scares the pants off of me, it's not something I want anything to do with, thanks all the same Sad
Max   [Mon 09 Mar 2026 at 09:09]
AI is here to stay and it can certainly be useful. In the end it's just another tool, a very powerful one indeed, and its outcome will depend on what exactly we do with it.

Let's go back to the main subject. Mark, I dug a bit into this interesting topic and I think I've found the actual source of this luminescence: According to Lee et al. (https://journals.aps.org/prb/abstract/10.1103/PhysRevB.19.3217) color centers in sapphire resulting from oxygen vacancies - which can be formed when the ceramic is fired in a reducing atmosphere - present two luminescence bands when excited with shortwave radiation: one peaking at 3 eV (413.5 nm) for the F center (oxygen vacancy with 2 electrons, featuring an optical absorption centered at 6 eV), and the other one peaking at 3.8 eV (326.4 nm) for the F+ center (oxygen vacancy with 1 electron, featuring an optical absorption centered at 4.8 eV). So, it seems that what we see here is optical in nature, although I would not rule out the possibility of an excitation upon electron impact from the discharge in contact with the ceramic wall (didn't find any information about that though).
Tuopeek   [Mon 09 Mar 2026 at 11:35]
Thanks for looking into this further Max. Your image of a PCA mercury halide burner and UV starter is interesting, as I haven't seen the UV phosphorescence in these so far, only in some HPS lamps. This makes me wonder if it was also used in halides the external starter may be made redundant. I have seem some smaller halide lamps with UV fluorescent additives to the outer jacket again now thinking this may be an alternative photoelectric starting aid for the lamp. I wasn't sure how transparent PCA was to shortwave UV either. I am still fascinated to see if this type of PCA can respond to electron bombardment but don't want to destroy a lamp to test it as I have only a few that demonstrate this property. Thanks for the link BTW but I think my old uni email has expired for access.

Max   [Tue 10 Mar 2026 at 21:15]
You raise a good point about the phosphorescence of PCA, it greatly varies so it must also involve the influence of dopants and impurities on the characteristics of color centers.

Comment 1 to 7 of 7
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