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Impact of mercury (and lack thereof) on the design of HPS lamps The first practical high-pressure sodium lamps made at GE in the early 1960s did not contain mercury at all, it is only a bit later in the development of the technology that mercury was added in order to build up voltage to sufficient level (~100 V) in order to ensure an efficient operation on electrical systems. It is only two decades later that lighting manufacturers researched ways of removing the mercury buffer for environmental reasons. The best approach was found in the increase of the xenon fill pressure, made possible with the introduction of the ignition antenna, combined with a longer electrode gap length and a narrower arc tube. The snapshot below shows the impact of those design changes on a 250 W GE Lucalox, with the standard type at the bottom. These changes made the first commercial Hg-free HPS lamp possible, introduced by Philips in 1992 as the EuroSON aimed at the European market (GE never released the Hg-free Lucalox shown here). Keywords: Lamps

Impact of mercury (and lack thereof) on the design of HPS lamps


The first practical high-pressure sodium lamps made at GE in the early 1960s did not contain mercury at all, it is only a bit later in the development of the technology that mercury was added in order to build up voltage to sufficient level (~100 V) in order to ensure an efficient operation on electrical systems. It is only two decades later that lighting manufacturers researched ways of removing the mercury buffer for environmental reasons. The best approach was found in the increase of the xenon fill pressure, made possible with the introduction of the ignition antenna, combined with a longer electrode gap length and a narrower arc tube. The snapshot below shows the impact of those design changes on a 250 W GE Lucalox, with the standard type at the bottom. These changes made the first commercial Hg-free HPS lamp possible, introduced by Philips in 1992 as the EuroSON aimed at the European market (GE never released the Hg-free Lucalox shown here).
2020-05-27_Narva_HgE213.jpg Philips_HPL4_NL_vs_PRC.jpg 20221120_130614.jpg 20231230_123547.jpg 09_25_IMG_0001.jpg
Lamp/Fixture Information
Manufacturer:General Electric
Model Reference:Lucalox 250W
Lamp
Lamp Type:Sodium high pressure: classic (bottom) and mercury free (top)
File information
Filename:20221120_130614.jpg
Album name:Max / Misc lamps and lighting
Keywords:Lamps
Filesize:628 KiB
Date added:13 Sep 2025
Dimensions:1500 x 741 pixels
Displayed:14 times
Software:Adobe Photoshop CS2 Windows
URL:https://trad-lighting.net/gallery/displayimage.php?pid=974
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Tuopeek   [Sat 13 Sep 2025 at 09:39]
Interesting Max to hear the first lamps were developed with out Hg. It's always intrigued me how little of the mercury shows in the spectrum of the HPS lamp. I think it seems intuitive that the light is a blend, being a whiter light, but the fact it is mainly due to the pressure change and spectral line broadening of sodium is one of the amazing facts of the applied physics. I believe this is also achieved with not a particularly high pressure in the lamp.
Max   [Sun 14 Sep 2025 at 05:25]
The physics behind the high-pressure sodium lamp is truly fascinating indeed. Mercury lines do not show up during the lamp operation at full regime because the arc is essentially cooled down by sodium vapor, which has a much lower ionization potential and a greater optical emissivity than mercury. This is what keeps the HPS plasma temperature at around 4000 K, which is too low to excite the mercury vapor, which thus acts mainly as a buffer gas useful for building up voltage through elastic electron-atom collisions.

When the first discharge experiments with sodium at elevated pressures were done at the end of the 1950s nobody expected to see such a dramatically broadened spectrum, which was later found to arise from quasi-molecules in the plasma (Na-Na, Na-Xe, and Na-Hg in the case of commercial HPS lamps). So, while mercury does not emit light by itself, it does contribute indirectly to the lamp's peculiar spectral output, particularly by broadening the red side of Na's optical emission around the element's resonant transition. As you point out, this is all the more interesting since the formation of sodium quasi-molecules do not require an overly high pressure. The standard HPS lamp operates under 1.5 bars (0.8 for Hg, 0.1 for Na, about 0.5 for xenon), while the high-pressure mercury lamp of same wattage runs at about 4.7 bars!

Comment 1 to 2 of 2
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