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SOX - the last production year I am lucky to have gotten my hands on one of the last low-pressure sodium lamps produced in Europe. In 2017 Philips announced the end of its SOX production for 2020, the last one of this kind outside China. Signify (formerly Philips Lighting) eventually closed its Hamilton (Wellhall road, Scotland) SOX lamp plant in late 2019 after 72 years of operation. The lamp I manage to get is a very clean SOX-E 26W from March 2019. Its infrared-reflecting ITO coating is spotless and I could not find any trace of helium in the lamp fill. There was a helium problem in some lamps in later production batches, but this one doesn't seem to be affected. However, I don't remember seeing such a large gap in the ITO coating near the electrodes. This feature was introduced in the early 2000s to reduce the rate of sodium migration away from the hot electrodes and to disconnect this electrically-conducting film from the barium mirror at the base of the lamp (which sometimes is at one of the electrodes' potential, causing a radial electrolysis and migration of sodium that eventually resulted in severe blackening of the bulb due to a reaction with indium of the ITO film). That gap was initially much smaller than that if I recall correctly. Another interesting characteristics of this "last" lamp is the fact that its nominal power dissipation is 27 W instead of the original 25 W for the SOX-E 26W model. The buffer gas fill must have been adjusted a bit, either to reduce sodium distillation further, or/and to boost the light output in order to compensate for the higher IR losses near the lamp base (due to the wider gap in the ITO film) and the greater amount of barium getter inside the burner around the electrodes. The latter became problematic during the 2000s when the process of electrode flashing became less well controlled, resulting sometimes in too much blackening at the burner extremities. Keywords: Lamps

SOX - the last production year


I am lucky to have gotten my hands on one of the last low-pressure sodium lamps produced in Europe. In 2017 Philips announced the end of its SOX production for 2020, the last one of this kind outside China. Signify (formerly Philips Lighting) eventually closed its Hamilton (Wellhall road, Scotland) SOX lamp plant in late 2019 after 72 years of operation.

The lamp I manage to get is a very clean SOX-E 26W from March 2019. Its infrared-reflecting ITO coating is spotless and I could not find any trace of helium in the lamp fill. There was a helium problem in some lamps in later production batches, but this one doesn't seem to be affected. However, I don't remember seeing such a large gap in the ITO coating near the electrodes. This feature was introduced in the early 2000s to reduce the rate of sodium migration away from the hot electrodes and to disconnect this electrically-conducting film from the barium mirror at the base of the lamp (which sometimes is at one of the electrodes' potential, causing a radial electrolysis and migration of sodium that eventually resulted in severe blackening of the bulb due to a reaction with indium of the ITO film). That gap was initially much smaller than that if I recall correctly.

Another interesting characteristics of this "last" lamp is the fact that its nominal power dissipation is 27 W instead of the original 25 W for the SOX-E 26W model. The buffer gas fill must have been adjusted a bit, either to reduce sodium distillation further, or/and to boost the light output in order to compensate for the higher IR losses near the lamp base (due to the wider gap in the ITO film) and the greater amount of barium getter inside the burner around the electrodes. The latter became problematic during the 2000s when the process of electrode flashing became less well controlled, resulting sometimes in too much blackening at the burner extremities.
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Lamp/Fixture Information
Manufacturer:Signify
Model Reference:SOX-E 26W
Lamp
Lamp Type:Sodium low pressure
Filament/Radiator Type:Nonthermal discharge in argon, neon, and sodium vapor
Base:BY22d
Shape/Finish:Tubular coated (ITO)
Service Life:18 kh
Burning Position:Base up ±110°
Electrical
Wattage:27 W
Voltage:69 V
Current:0.37 A
Optical
Lumen Output:3,700 lm
Lumen Efficacy:137.0 lm/W
Colour Temperature:1770 K
Colour Rendering Index:-45 Ra8
Physical/Production
Factory Location:Hamilton, Scotland
Fabrication Date:March 2019
Application/Use:Streetlighting
File information
Filename:DSCF0262m.jpg
Album name:Max / Misc lamps and lighting
Keywords:Lamps
Filesize:734 KiB
Date added:27 Mar 2026
Dimensions:1500 x 1000 pixels
Displayed:42 times
DateTime Original:2026:03:27 00:25:19
Exposure Time:1/240 sec
FNumber:f/3.6
File Source:Digital Still Camera
Flash:No Flash
Focal length:35.8 mm
ISO:160
Model:X-E4
Software:Adobe Photoshop 25.7 (Windows)
White Balance:0
URL:https://trad-lighting.net/gallery/displayimage.php?pid=1163
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Comment 1 to 9 of 9
Page: 1

Tuopeek   [Fri 27 Mar 2026 at 12:53]
That's a significantly historic lamp then Max. Did you measure the power consumption to ascertain the wattage difference? I was wondering if the wattage may vary slightly with aging or even bedding in getting closer to the design spec. That is a significant gap in the ITO coating, I've seen variations but never that much. Perhaps possible with little radiated loss behind the electrodes. Did always feel the SOX-E versions seemed to have longer lead-in wires on their electrode as if to slightly reduce the discharge length and perhaps to make use of the glow that occurs behind them. Latterly, I felt the internal getter around the electrodes could be so excessive as to reduce the overall light output. Sadly, although living close to 50 miles from the Hamilton plant I unfortunately never got to visit it while production was running.
Ria   [Fri 27 Mar 2026 at 13:25]
The end of an era, it was a sad day when Hamilton closed, even more so when it was demolished Crying or Very sad Crying or Very sad Crying or Very sad
Sammi   [Fri 27 Mar 2026 at 18:50]
We didn't get the chance to visit the factory either Mark.! Crying or Very sad Crying or Very sad Crying or Very sad
Max   [Sat 28 Mar 2026 at 08:52]
I feel a great disturbance in the force coming from the UK. It's a shame indeed that none of you guys managed to pay Hamilton a visit before it closed down... Low-pressure sodium lamps certainly defined an era in lighting that ended in late 2019. It really was a special and unique technology whose manufacturing depended to a large extent on the expert skills of a handful of craftsmen. At least we can be grateful that we appreciate the value of this lost art, that we saw this technology in widespread use, and that we even have (historical) samples of it.

Tuopeek - This SOX-E 26W has a great historical value indeed and I sure will keep it safe in the Collection. I've already run it once but I haven't measured its exact electrical characteristics yet. I'll do that next week and I'll let you know about my findings.
Ria   [Sat 28 Mar 2026 at 13:26]
No-one wishes we could have got there more than us Max, we just left it too late; story of our lives Crying or Very sad LPS
Tuopeek   [Sat 28 Mar 2026 at 16:06]
Out of interest I dug out a 26W Hamilton SOX I have dated 2011 and it has the same gap in the ITO coating. Quite hard to spot as the coating is very transparent. Wonder
Ria   [Sun 29 Mar 2026 at 00:57]
Hmmm, I'll have to look at some of ours now Wonder
Tuopeek   [Sun 29 Mar 2026 at 12:38]
Max, was the helium issue in the outer jacket or with the discharge gas? Did it affect efficiency or life expectancy?
Max   [Mon 30 Mar 2026 at 10:54]
That was a contamination in the burner gas fill as a result of a problem on the gas supplier's end. It mainly affected the life expectancy (excessive sputtering and emitter evaporation). It went unnoticed in the production of SOX lamps for some time because Hamilton did not check the nature/quality of the gas fill with any advanced techniques, such as spectroscopy. The problem is that low-pressure sodium lamps became less important to the company after the late 1980s, so production suffered from a lack of investments and that resulted in some quality issues in later lamp batches.

Interesting that your 2011 SOX-E 26W also has such a large gap in its ITO coating (it is exactly 1.5 cm-wide in mine). This means that this design feature is likely to have been unchanged since its introduction in the early 2000s. I still have to check my lamp's electrical specs, and if it dissipates 27 W indeed, then I'm wondering if that change was not actually introduced together with the ITO film gap. That would not be unlikely given the heat losses introduced by that gap. To be continued...

Comment 1 to 9 of 9
Page: 1