ДНаО 140 (МЭЛЗ)
Illustrated here is an unusual Russian low pressure sodium lamp, manufactured by MELZ in 1966. The discharge tube has a U-shape, and it is assembled within an outer vacuum flask. Normally on European lamps it is possible to remove the discharge tube and replace that while re-using the outer vacuum flask, but on this Russian lamp it has been assembled with a powerful spring which is almost impossible to remove. Clearly it was the intention to replace the whole lamp when the discharge tube failed. The discharge tube has sodium-retaining dimples, and unusually these are all positioned on one side. European lamps have the dimples around the outer edge, the reason for the difference is not known. The electrodes are technically similar to an LL lamp but in beehive-shape. They are welded to small glass stems, like used on small incandescent lamps. The stems are sealed into the ends of the discharge tube. The wires passing through the glass of the stems look like tungsten - this would indicate that the discharge tube is made from borosilicate hard glass, which is unusual because the low pressure sodium lamp is normally made from soda-lime soft glass. It would appear that technical development of sodium resistant glass types took quite a different direction in the soviet countries than the rest of the world. If this lamp was made using European glass types, it would not be possible to have sodium-resistant glass on the outer surface of the glass stems, so it is not clear how MELZ solved that problem. Maybe they did not - which could explain why the life of this lamp is very short, only 2000 hours! According to the instruction leaflet delivered with this lamp, there was another type DNaO 140a, which has lower lumen output than this DNaO 140. It is not known what the technical difference is. It is also not known when MELZ started producing these lamps or when the production was ended, but they seem not to have been very popular in Russia.