Second chance

What do you do when your sodium lamp has been run into the ground and cycles? You discard it, right? Do you have any other choice? Yes. You can squeeze more life out of it. The 50 W SDW-T shown on the left has been run well past its mean rated life expectancy and cycles on a 50 W HPS choke ballast. So, it's officially dead! The reason why it behaves this way is because it has lost sodium to the point that a too high mercury/sodium vapor pressure ratio in the discharge results in a lamp voltage that exceeds what the ballast can sustain. A neat way around that is to increase the ballast impedance so as to reduce the current and wattage at the stable point of operation (i.e., the crossing between ballast load line and lamp V-I characteristic). This way the burner operates at a lower temperature, which results in a stable operation due to the reduced mercury vapor pressure.

The picture above shows the "dead" 50 W SDW-T running stably on a 35 W electronic driver without the typical voltage regulation found on SDW gears, with a new 35 W SDW-T next to it (same driver) to show the difference in light color (constant camera white balance set to 4000 K). The "dead" 50 W lamp delivers a pleasant extra warm-white light color that is intermediate between the Deluxe and White HPS colors (closer to the latter by my estimate). Its arc voltage is 108.4 V, meaning that it now has plenty of life left as it can got up to about 150 V before it starts cycling. However, its light output is 30 % lower than that of the new 35 W lamp on account of its severe blackening. The latter is the only drawback of the present method... it extends the life of spent sodium lamps, but it does not recover its initial performances (the case presented here is quite extreme given the small size of SDW burners).

The last picture on the right is that of a brand new 50 W SDW-T running on that 35 W electronic driver, showing its much oranger color (of a Deluxe HPS kind). A lower operating temperature combined with a sodium-rich discharge results in 62.4 V arc voltage, which means that the lamp can be run for much longer than its rated service life, about 35 kh by my estimate. The lack of blackening combined with a more optimal sodium vapor pressure (from an efficacy point of view) results in the highest light output of the three cases, 33 % higher than that of the 35 W SDW-T. That solution thus delivers the highest efficacy and longest service life, but this comes at the expense of light color quality.