1993 Philips C1500/ED37

In the second half of the 1980s Philips developed a series of compact high-pressure sodium lamps built with two arc tubes connected in series. Although the concept was not new (first patent filed in 1971), the Dutch were the first to bring such a lamp to the market in the form of the C1000/ED37 released in 1993 in North America. This particular model featured two 500 W burners and was much smaller than the standard high-voltage 1 kW ANSI type S52 sodium lamp used in that region (292 vs. 383 mm overall length). Another advantage of this dual burner configuration is the smaller physical size of the optical radiator which allows a more efficient control of the emitted light.

The development of this lamp platform first took place in Belgium (Turnhout) and activities were transferred to Bath, NY, USA, in the early 1990s. During that period Philips designed and built a variety of series-burner lamps from 150 to 1500 W for testing and evaluation purposes. The largest model is shown here and features two 750 W arc tubes assembled in an ED37 bulb. Interestingly, this lamp is filled with nitrogen at low pressure for the proper thermal management of the burners. The lamp atmosphere is kept free of most impurities thanks to a barium mirrors getter flashed on the end cap side of the bulb. Incidentally, this gaseous fill also helps with ignition as the glow discharge excited upon the application of high voltage acts as an active antenna to the arc tubes. This mechanism complements the local enhancement of the electric field caused by the proximity of the two electrodes connected to the lamp terminals, which ensures a reliable starting below 5 kV.

The 750 W sodium burners where also a novelty back then, Philips developed this type shortly after Sylvania introduced the LU750 in 1990, but they never released it. In this particular lamp design the xenon fill pressure was adjusted so the ignition and glow-to-arc voltage characteristics of the two burners connected in series remained similar to those of standard single-burner lamps. For the C1000/ED37 this ensures a reliable starting on standard S52 control gears while two series-connected S111 750 W CWI ballasts could be used to run the 1500 W variant.

While the arc tubes must be placed close to each other in order to limit the effect of parallax in the projected light beam pattern, a minimum separation gap must be maintained to prevent overheating and light shadowing. The optimum distance between the burners was found to be 20 mm. The resulting light output in the present case is close to 200 klm, a level which is lower than that of a hypothetical 1500 W single-burner lamp. Beside the mutual shadowing of the burners, which accounts for ~4 % optical losses, the reduced light output is also caused by the higher electrode losses and the heat conduction and convection losses from the gaseous atmosphere in the bulb.

As opposed to the C1000/ED37 the 1500 W Ceramalux was never released on the market. Either the applications for the lamp were deemed too niche to generate enough sales, or the concept suffered possibly from thermal management issues due to its very high power density. The presence of an atmosphere in the lamp certainly complicates matters further as convection causes the formation of a hot spot on the outer bulb which, combined with the light reflected from the luminaire, may cause some local overheating of the bulb. Except for some special horticultural lamps released by the Chinese during the 2010s, the maximum wattage of commercial high-pressure sodium lamps never exceeded 1 kW, which was good enough for the most demanding industrial and outdoor lighting applications.