GE Lucalox LU70/BU with Molybdenum Heat Pipe
Following the introduction of the original 400W HPS lamp by GE in 1965-66, the range was quickly broadened to offer higher and lower lumen packages. The 275W rating followed in 1966, being redesigned to 250W in 1969, and followed by 150W in 1973 and 100W in 1974. There was strong demand for a still lower rating of 70W, but GE's arc tube design with an external amalgam reservoir presented an obstacle. The external reservoir, of course, operates slightly cooler than the rest of the arc tube, ensuring that excess amalgam condenses in the reservoir instead of the arc tube - but the cold spot temperature of the reservoir must be sufficiently high to ensure a high enough sodium vapour pressure for satisfactory efficacy and colour. When moving to gradually lower lamp powers, the quantity of heat available from the arc tube also decreases, and for the small 70W rating it was found impossible to keep the reservoir warm enough.

The problem was solved in 1975 by including a small molybdenum heat pipe inside the niobium tube that contains the amalgam reservoir. This serves to conduct more heat from the electrodes towards the reservoir, and increases amalgam temperature to satisfactory levels. The heat pipe has been fixed in place by a 4-sided X-shaped crimp of the niobium tube, and is visible in the X-Ray image.

Rather embarassingly for GE, competitors did not suffer this problem due to their use of an internal amalgam reservoir. The GE design was simplified with the 1977 shift from niobium cup to tube and wire seals, which provided better thermal management and allowed the complex heat-pipe solution to be eliminated - as such this particular lamp was only marketed for about two years.
Manufacturer: General Electric Company U.S.A.
Lamp Power: 70 Watts
Lamp Current: 1.6 Amps
Lamp Voltage: 52 Volts
Cap Type: E39s/41 Brass + blue glass
Bulb Type: ED-75 ED-23½ in eighths/inch
Bulb Finish: Clear Lead borosilicate glass
Electrodes: Double Tungsten Coils Ba2CaWO6 Emitter
Arc Length:
Atmosphere: Inner: Na,Hg | Xe Outer: Hard Vacuum
Luminous Flux: 5,800 lm (@ 100 hrs) 5,220 lm (average)
Luminous Efficacy: 82.9 lm/W (@ 100 hrs)
Colour Temperature & CRI: CCT: 2100K CRI: Ra 22
Chromaticity Co-ordinates: CCx: 0.522 CCy: 0.423
Rated Lifetime: 20,000 hours to 50% survival
Warm-up & Re-strike Time: 3-4 minutes 1 minute
Burning Position: Vertical Base Up ± 95°
Overall Length: 197 mm 73/4 inches
Light Centre Length: 127 mm 5 inches
Factory: Cuyahoga, NELA Park Cleveland OH, U.S.A.
Date of Manufacture: June 1976 Date Code: 54 -
Original Value: Unknown
 
References: 1) General Electric USA Large Lamp Catalogue, Form 9200, 1977
2) HPS Lamp Design - a Diversification in Design Wattage & Optical Control, B.R.Collins & S.A.Mule, Lighting Design & Application March 1977 pp.26-31