1981 Philips SON-ST 150W

The SON-ST 150W is the first commercial high-pressure sodium lamp designed with a high xenon fill pressure in order to improve both the lumen efficacy and the lumen maintenance. Although it was GE who first built HPS lamps with a high xenon pressure (to compensate for the absence of a mercury buffer) during the 1960s, they never developed this concept into a product. The following decade Philips investigated this design approach (but with a mercury buffer) in order to improve the performance of low-wattage sodium lamps, which eventually resulted in the introduction in Europe of the SON/ST 150W in 1979.

Increasing the cold xenon fill pressure from 30 to 250 mbar increases the initial output flux from 14.5 to 16.0 klm (+10 % gain) and results in a significantly better flux maintenance. At 16 kh the lamp output is raised from 12.5 klm (86 % of initial level) to 14.4 klm (90 % initial level), corresponding to a 15 % overall improvement. The change in start gas pressure also affects the color of the emitted light. The higher density of Na-Xe quasi-molecules in the plasma enhances the lamp's output in the green, which shifts the color point upward in the chromatic diagram and results in a less natural yellow light color. Interestingly, this change is mitigated by the high mercury vapor pressure which causes a large amount of red light being emitted from the sodium discharge (thanks to Hg-Na quasi-molecules), thereby ensuring still a relatively pleasing light color. Such consideration on the color/efficacy balance no longer prevailed when the lamp was improved further during the 1990s and after.

Because the higher xenon pressure also causes the lamp's ignition voltage to increase, Engineers at Philips devised a starting aid so as to ensure that the improved lamps can start reliably on standard 4–5 kV electronic ignitors. This aid consists of a coiled tungsten wire wrapped around the burner and tensioned between an insulator (left) and a ceramic capacitor (right). The capacitor serves as passive commutation device that grounds the antenna at high frequency only. Such scheme enhances the antenna’s effectiveness since this increases the electric field strength around the electrode driven by the high-voltage pulses (i.e., the left electrode connected to the central terminal of the end cap). Without this antenna the xenon fill pressure (cold) in the 150 W SON lamp could not be increased beyond 60 mbar without resulting in the impossibility for standard ignitors to start the lamp.