Author Topic: What Is The Silver Coating In The Base Of HPS & SOX Lamps For?  (Read 287 times)
108CAM
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What Is The Silver Coating In The Base Of HPS & SOX Lamps For? « on: August 10, 2022, 08:25:34 PM » Author: 108CAM
This is something that I've always wondered. What is the purpose of the silver colour that coats the inside of HPS & SOX lamp bases? I've noticed that only sodium based lamps have it and have always wondered what it's meant to do.
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Re: What Is The Silver Coating In The Base Of HPS & SOX Lamps For? « Reply #1 on: August 11, 2022, 02:08:49 AM » Author: Medved
The outer bulb has vacuum in it, so in order to have really a complete vacuum there (so no glow discharge or so could start there), a "getter" is used to absorb the remaining gasses. Usually that is in the form of a mix of metals, initially sprayed out from their holder (a ring of U-shape reservoir, fast heated by induction) and then deposited onto larger surface, so it has sufficient surface area to do the absorbing...
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Re: What Is The Silver Coating In The Base Of HPS & SOX Lamps For? « Reply #2 on: August 11, 2022, 05:23:07 AM » Author: Rommie
Medved is correct, the getter is mostly comprised of barium.
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Re: What Is The Silver Coating In The Base Of HPS & SOX Lamps For? « Reply #3 on: August 11, 2022, 05:46:35 AM » Author: Econolite03
What happens if the coating turns white? Is the lamp useless?
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Re: What Is The Silver Coating In The Base Of HPS & SOX Lamps For? « Reply #4 on: August 11, 2022, 06:25:38 AM » Author: dor123
The getter turns white if oxygen leaked into the lamp, so the lamp is useless then.
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Re: What Is The Silver Coating In The Base Of HPS & SOX Lamps For? « Reply #5 on: August 11, 2022, 08:33:47 PM » Author: 108CAM
Thanks for the info! This is something that I had been curious about ever since I got my first HPS lamp.
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Re: What Is The Silver Coating In The Base Of HPS & SOX Lamps For? « Reply #6 on: August 12, 2022, 12:43:50 AM » Author: joseph_125
Interestingly Philips marketed a line of SOX lamps that used solid state zirconium-cobalt getter pellet instead of the traditional barium getter. They're quite rare and I think were only ever marketed in Europe. They look really unique though as the lamps are completely clear.

http://www.lamptech.co.uk/Spec%20Sheets/D%20SLP%20Philips%20SOXPSG35.htm

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Re: What Is The Silver Coating In The Base Of HPS & SOX Lamps For? « Reply #7 on: August 12, 2022, 09:00:50 AM » Author: Rommie
Yes, the PSG range. They were only made in 35W and 55W (26W/36W for SOX- E) as the position of the pellet meant that the getter was not as effective on longer lamps.

Quite rare, as you say, especially with the different coloured caps, after a while they changed to normal coloured ones, presumably as a cost-cutting exercise.
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Re: What Is The Silver Coating In The Base Of HPS & SOX Lamps For? « Reply #8 on: August 16, 2022, 12:17:04 PM » Author: Lampwizard
As far as I know some getters for certain lamp types (forgot which ones; quartz lamps?) also function as a hydrogen getter to prevent it from diffusing from the outer bulb through the glass of the discharge vessel. Apparently, the ingress of H2 increases the lamp re-ignition voltage up to the point when the lamps fails to re-ignite, thus rendering the lamp useless...
However, this kind of getter is placed inside a metal ring and does not come in the form of a silver-like layer.
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