Narva NF125-00 Mercury with Sintered Pellet Electrode

For many years the low wattage mercury lamps of German Osram and Narva were manufactured with unique electrode designs, consisting of a sintered cermet pellet. Whereas Osram lamps followed this design during the 1960s and 70s, Narva continued well into the 1980s.

The electrodes of most mercury lamps comprise a coil of tungsten wire wound around a thin tungsten shank. The purpose of the coil is to radiate sufficient heat to keep the tip temperature within safe limits, and to reduce heat conduction losses towards the end seals. To maximise lamp efficacy an electron emissive material is also required. Originally a small strip of thorium metal was located under the tungsten coil, but that is quickly sputtered away and causes poor lumen maintenance due to end blackening. The more common solution is to fill the spaces between the coils and shank with a double or triple oxide emitter, usually consisting of barium, calcium and either thorium or yttrium oxides. These oxide coatings are also gradually lost, which limits lamp life, but they instead form a white deposit at the arc tube ends whic greatly improves lumen maintenance.

The Germans made extensive efforts to slow the loss of the emissive material by binding it firmly in a sintered pellet. Whereas Osram electrodes were made entirely of such materials, Narva employed a separate emissive pellet directly behind the electrode coil. The pellet is thus protected from the intense heat of the arc root to extend its life, but remains sufficiently warm that a layer of its constituent metals diffuses over the surface of the tungsten. EDX analysis of the emissive pellet revealed a composition of 17.5% Tungsten, 39.5% ThO2, 21.5% BaO, 21.5% CaO.
Manufacturer: VEB Berliner Glühlampenwerk
Lamp Power: 125 Watts
Lamp Current: 1.15 Amps
Lamp Voltage: 125±15 Volts
Cap Type: E27s/30 Ni plated brass + vitrite
Bulb Finish: Clear Borosilicate hard glass
Bulb Type: ED-75 ED-23½ in eighths/inch
Overall Length: 175 mm 6 7/8 inches
Light Centre Length: 115±5 mm 4 3/8 inches
Arc Length: 25mm 1 inch
Electrodes: Tungsten Coil Sintered pellet emitter
Atmosphere: Inner: Hg | Ar (8 atm) Outer: Nitrogen
Luminous Flux: 5,250 lm @ 100 hrs
Luminous Efficacy: 42.0 lm/W @ 100 hrs
Colour Temperature & CRI: CCT: 5980K CRI: Ra 16
Chromaticity & Red Ratio: CCx: 0.319 CCy: 0.389 Red Ratio: 0%
Burning Position: Universal
Rated Lifetime: 12,000 hours to 70% survival
Warm-up & Re-strike Time: 4 minutes 4-5 minutes
Factory: Warsaw Bridge, Berlin East Germany
Date of Manufacture: 1984 Quarter 3 Date Code: 43
Original & Present Value: Unknown
References: 1) Narva Lamp Catalogue, 1983 Germany pp. 6-7.
2) Narva Brochure Quecksilberdampf-Hochdrucklampen Naviflux, 1980 Germany
3) Narva Technical Manual High Pressure Mercury Vapour Lamps Naviflux, 1981 Germany
4) DDR Fachbereichstandard TGL 9563-02 Quecksilberdampf-Hochdrucklampen 1981-07
5) SEM-EDX analysis of emissive pellet composition from a broken lamp