The First MB Lamp

In 1936 Philips announced the successful development of the first kind of MB Mercury lamp. It was called the Philora HP300, illustrated to the right, and it was made possible through a development of the quartz-to-metal seals that were described on the previous page. Instead of employing just one intermediate glass between the tungsten and quartz, several were used in a stack from high to low expansion, and this allowed the seal to operate at higher temperatures and without the slug of mercury in place to protect it. Removing the excess mercury also allowed the lamp to revert to the unsaturated vapour design invented by GEC with the MA lamp, in which the entire mercury dose is vaporised, and a stable discharge could again be developed. Owing to the use of a quartz bulb which could withstand higher operating temperatures, a relatively short arc could be adopted and the high power density which accompanied this led to a lamp of high luminous efficacy.

The first lamp consumed 75W and the efficacy was a remarkable 40 lm/W, the same as a 400W MA lamp. Its luminous flux was 300 dekalumens, the mercury pressure 20 atmospheres, the arc length 18mm and tube diameter 4mm. Its had a relatively high arc voltage of 230V (lamp current 0.4A) so the lamps had to be operated from transformers with an open circuit voltage of 410V. System efficacy was thus lower at 33 lm/W.

Although the internal auxiliary striking electrode had been introduced by GEC for its MA lamps three years earlier, it was not included in the first MB lamp shown above.  It was already pushing the quartz to its very limits to get it to accept a single tungsten wire for the main electrode, and to seal two wires through side by side was absolutely out of the question.  A few turns of nickel wire were therefore looped around one end of the arc tube, and connected to the electrode at the opposite end.  Part of the reason for designing a lamp with such a high voltage was that the 410V open circuit voltage of the transformer was required to make this external electrode work to reliably strike the discharge.  For a lamp designed to work at ordinary mains voltages and frequencies, the external auxiliary electrode technique is not always reliable. (An interesting exception exists in the case of the small 40W MBM mercury lamp found in British coal mines, the first ever discharge lamp to be driven on a high frequency power supply, featured here).

Not only had Philips been successful in giving the world a low wattage and efficient mercury vapour lamp, the lamp was found to offer considerably better colour rendering properties than could be attained with MA lamps. This is because the MB types operate at much higher pressure, and colour rendering always improves with increasing mercury pressure. A very small amount of continuum radiation was found in the spectrum of this 20 atmosphere lamp, and its red ratio increased from the 1% level of the MA lamp to about 2.5%, with no loss in efficacy. The spectral power distribution of the MB lamp is shown in Figure 38.