Atlas MB/U High Pressure Mercury
The MB/U Mercury lamp was the direct successor to the earlier MA lamps having hard glass aluminosilicate arc tubes. Quartz had been employed as an arc tube material for the 80W and 125W lamps since their inception, it being a requirement for the smaller ratings since mercury lamp efficacy falls off as wattage is reduced, and a higher pressure, hotter arc tube was necessary to make those ratings efficient enough to be commercially viable.

It was not until the cost of quartz fell and the pinch-sealing technique of lamp manufacture was developed, that quartz also began to be adopted for the higher wattage lamps as well. Here it also allowed mercury vapour pressure to be increased to similar levels found in the low wattage lamps, and consequently the luminous efficacy of high power lamps was significantly enhanced with the introduction of quartz. Quartz lamps also had better red colour rendition, owing to the higher pressure mercury discharge.

A further advantage of the quartz lamp was its universal burning position. MA lamps were only suitable for vertical burning, although they could be burned horizontally if a special electromagnet was used above the lamp to prevent the arc bowing upwards and causing the arc tube glass to soften. Quartz can more than withstand the heat of the arc bowing upwards in the horizontal position, and as such most of the MB lamps were rated for universal burning.

Both MB and MA lamps were offered in parallel for many years, the later MA versions being less efficient types which were also suitable for universal burning. They were sometimes preferred because MA was a cheaper lamp.
Manufacturer: Thorn Lighting Ltd
Lamp Power: 400 Watts
Lamp Current: 3.2 Amps
Lamp Voltage: 120-150 Volts For 220-250V Mains
Cap Type: E40s/45 Ni plated brass + vitrite
Bulb Finish: Clear Borosilicate hard glass
Bulb Type: TD-51 TD-16 in eighths/inch
Overall Length: 330 ± 8 mm 13 inches
Light Centre Length: 177 ± 5 mm 7 inches
Arc Length:
Electrodes: Backwound tungsten Double carbonate emitter
Atmosphere: Inner: Hg | Ar (5 atm) Outer: Nitrogen
Luminous Flux: 20,500 lm @ 100 hrs 18,800 lm @ 2,000 hrs
Luminous Efficacy: 51.3 lm/W @ 100 hrs 47.0 lm/W @ 2,000 hrs
Colour Temperature & CRI: CCT: 5900K CRI: Ra 16
Chromaticity Co-ordinates: CCx: 0.320 CCy: 0.379
Burning Position: Universal Horiz. lumens 5% less
Rated Lifetime: 7,500 hours Rated to 70% survival
Warm-up & Re-strike Time: 3 minutes 5 minutes
Factory: Leicester England
Date of Manufacture: August 1970 Date Code: ..... ....
Original & Present Value: £4 0s 0d (1969) £44.07 (2005)
References: 1) Thorn Lighting Comprehensive Catalogue, UK, 1969/70, p.CF5