Narva Natralox: original and later lamp designs

The development of high-pressure (HPS) sodium lamps in East Germany was a slow and tedious process, limited by a lack of knowledge and experience in ceramic engineering, combined with raw materials of poor quality. VEB Narva, the state-owned lamp manufacturer, began development work on that technology around 1971 and relied heavily on Tesla of Czechoslovakia for the design and manufacturing techniques of the new arc tube. In 1973 Narva released its first commercial sodium lamps, the Natralox NA400 and NA250, the latter is shown in the picture above (left).

The design of the first Natralox lamps was defined by key limitations that characterized the East German lamp industry, such as a limited tooling capacity which led to the reuse of standard components from high-pressure mercury lamps (bulb and frame), or the poor raw material quality and processing that went into the production of arc tubes. The latter was particularly problematic as it led to a short lamp life due to an accelerated reaction of sodium with burner components (alumina wall, seal frit glass, etc). Several strategies were implemented in order to mitigate that issue, such as reducing the burner wall temperature to lower reaction rates with sodium, and decreasing the initial lamp voltage so as to provide more room for the arc voltage rise over time. In fact, those changes resulted in lamps with quite different characteristics than those of equivalent wattage produced in the West. The NA250 shown here has a short and wide arc tube which is in fact similar to that of color-improved (deluxe) Western HPS lamps, but without the improved light color. Narva kept the sodium pressure low and increased the mercury pressure so as to meet a minimum arc voltage requirement (90 V here). This results in a light color which is noticeably redder than that of other HPS lamps, caused by an enhanced Na-Hg quasi-molecular emission in the red end of the spectrum. Such design and characteristic has a negative impact on the lamp performance, the lumen efficacy of the present NA250 is below 100 lm/W, which is mediocre for this particular lamp type.

Limitations in material and processing qualities is what prevented Narva from releasing Natralox lamps smaller than 175 W. Further developments in arc tube manufacturing eventually enabled them to produce lower wattage sodium lamps with a sufficiently long mean service life (4000 h or more) permitting an economically viable usage of the technology. Production of the NA70 began in 1982, followed by the NA150 shown here (right) the following year. Thanks to a higher quality of the (PCA) ceramic material, they finally could make long and narrow arc tubes with a sufficiently high wall load and sodium vapor pressure to realize a high lumen efficacy at low wattages (100 lm/W initial for the NA150). Because voltage buildup in the arc does not require as much mercury as in wider arc tubes, the light color of the newer, second-generation Natralox lamps became similar to that of standard HPS lamps from the West. Sodium consumption at the electrodes and the burner end seals remained problematic however, so Narva kept its strategy of a low initial lamp voltage (80 V here) to ensure a long enough mean service life (8 kh).

Interestingly, the measured lamp currents are very similar to those of 250 and 400 W high-pressure mercury lamps. It is not unlikely that Narva also used electrodes from those lamps (but with different activator material) in their 150 and 250 W Natraloxes.