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Dimming a SOX-Plus 35W Interestingly, it is possible to dim a fully warmed-up SOX lamp without causing neon emission from the discharge. Here I'm running a SOX-Plus 35W at 15.9 W, which is less than half of the lamp's nominal wattage. The lamp had stabilized at this power level when I shot this picture, and neon could not be found spectroscopically in the light emitted by the plasma, which is important for the durability of the lamp as this means that the electrodes are not being sputtered excessively in this regime. This is quite remarkable given that standard SOXs are not the best thermally insulated low-pressure sodium lamps (the present lamp is a 2001 model with no gap in the ITO coating). Nevertheless, the lamp efficacy decreases strongly as power is reduced. While it is 130 lm/W at 35 W, it is only 78 lm/W in the present case at 15.9 W. Keywords: Lamps

Dimming a SOX-Plus 35W


Interestingly, it is possible to dim a fully warmed-up SOX lamp without causing neon emission from the discharge. Here I'm running a SOX-Plus 35W at 15.9 W, which is less than half of the lamp's nominal wattage. The lamp had stabilized at this power level when I shot this picture, and neon could not be found spectroscopically in the light emitted by the plasma, which is important for the durability of the lamp as this means that the electrodes are not being sputtered excessively in this regime. This is quite remarkable given that standard SOXs are not the best thermally insulated low-pressure sodium lamps (the present lamp is a 2001 model with no gap in the ITO coating). Nevertheless, the lamp efficacy decreases strongly as power is reduced. While it is 130 lm/W at 35 W, it is only 78 lm/W in the present case at 15.9 W.
20200822_185927.jpg DSCF0265m.jpg DSCF0281m.jpg DSCF0289m.jpg Gassiot_Cascade.jpg
Lamp/Fixture Information
Manufacturer:Philips
Model Reference:SOX-Plus 35W
Lamp
Lamp Type:Sodium low pressure
Filament/Radiator Type:Nonthermal discharge in neon, argon, and sodium vapor
Base:BY22d
Shape/Finish:Tubular clear with ITO coating
Electrical
Wattage:15.9 W
Voltage:138.1 V
Current:0.142 A
Optical
Lumen Output:1235 lm
Lumen Efficacy:77.7 lm/W
Colour Temperature:1770 K
Colour Rendering Index:-45 Ra8
Physical/Production
Factory Location:Hamilton, Scotland
Fabrication Date:November 2001
File information
Filename:DSCF0281m.jpg
Album name:Max / Misc lamps and lighting
Keywords:Lamps
Filesize:718 KiB
Date added:06 Apr 2026
Dimensions:1500 x 1000 pixels
Displayed:26 times
DateTime Original:2026:04:06 19:05:42
Exposure Time:1/30 sec
FNumber:f/3.2
File Source:Digital Still Camera
Flash:No Flash
Focal length:18 mm
ISO:160
Model:X-E4
Software:Adobe Photoshop 25.7 (Windows)
White Balance:0
URL:https://trad-lighting.net/gallery/displayimage.php?pid=1190
Favourites:Add to Favourites

Comment 1 to 9 of 9
Page: 1

Ria   [Mon 06 Apr 2026 at 22:42]
How was the dimming done..? Just by reducing the supply voltage or another means..?
Max   [Tue 07 Apr 2026 at 10:59]
Current was adjusted just by varying the ballast's input voltage. An interesting characteristic of leakage-flux transformers is that they are essentially voltage-to-current converters in the sense that the regulated current output is directly proportional to the input voltage. This property is very convenient for experiments such as the one shown here!
Ria   [Tue 07 Apr 2026 at 14:46]
Thanks, I'll have to give it a try sometime LPS
AgentHalogen_87   [Tue 07 Apr 2026 at 21:44]
Very cool setup Max! How would the lamp react to a dimming schedule? Like 100% for 2hrs; 75% for 1hr; 50% for 1hr; 100% for 1hr Wonder
Max   [Wed 08 Apr 2026 at 06:05]
Thanks! The lamp reacts pretty much instantaneously to the power change, and then there is the thermal inertia which affects the electrical characteristics in the course of ~10 minutes. As opposed to metal halide lamps, sodium and mercury lamps are quite easy to dim as the discharge's re-ignition spike does not grow out of control as a result of a sudden decrease in applied current. This is quite convenient and makes those lamps relatively easy to operate.
AgentHalogen_87   [Wed 08 Apr 2026 at 20:27]
That's good to know. It's a shame the dimming never made into SOX ballasts apart form a couple of seldom examples. Dimming SON was of course very common, but I think it was over used, and excessively dimmed in places as well.
Max   [Thu 09 Apr 2026 at 09:52]
SOX was considered old tech and on the decline in the 2000s when dimmable electronic control gears for HID lamps became a thing. Moreover, I think the impact of dimming on the lumen efficacy is just too great also to justify this function in electronic SOX ballasts; maximum efficacy was the main selling point for low-pressure sodium.
AgentHalogen_87   [Thu 09 Apr 2026 at 17:59]
It's a shame really, but I do get it. No point spending loads of money on something that won't be all that useful and replaced soon anyway.
Max   [Thu 09 Apr 2026 at 20:44]
Light sources and gears manufacturers are (usually) very pragmatic, they don't have the emotions we -collectors and enthusiasts- usually associate with those lamps and ballasts.

Comment 1 to 9 of 9
Page: 1