1943 GEC MBF/V 80W

The development of high-pressure mercury lamps during the 1920s and early 30s led to the introduction of the first efficient high-intensity discharge lamps in 1932. However, these first models suffered from one major drawback that limited the use of this technology to industrial and street lighting: a poor color rendering due to the lack of red radiation in their emission spectrum. The increase of mercury pressure in the quartz lamps introduced in 1935 raised the red output to a mere one percent of the total radiation, which did not really solve the problem. Early studies revealed that the mercury spectrum lacked in fact both blue and red radiations in order to produce a well balanced white light needed for lighting people. A viable solution to this problem then quickly emerged in the use of fluorescent materials that can convert some otherwise useless ultra-violet radiations into visible light to fill in the gaps in the mercury spectrum.

Although the General Electric Company of England was the first in 1937 to use phosphor coatings in hard-glass mercury lamps, it applied this technology to quartz lamps only in 1939, two years after Philips did. The lamp featured here is the first model introduced in the UK, and as opposed to its Dutch competitor, GEC used a unique ternary mixture of fluorescent materials. A blend of zinc-cadmium sulphide activated with copper, and zinc beryllium silicate activated with manganese raises the red output five folds to 5 percent. The 20 to 40 % of cadmium sulphide in the first phosphor leads however to a strong optical absorption that only enhances the lack of blue light in the lamp’s output. This deficiency is then counterbalanced by a third blue-emitting phosphor of calcium tungstate activated with lead. Such arrangement permits a more efficient use of all mercury's UV lines transmitted through the quartz burner, therefore ensuring the same lumen outputs as those of clear, un-phosphored lamps. Moreover, GEC used a unique coating technique developed in 1934: the outer jacket was first heated near its softening point and the phosphor was then blown into the bulb from a nozzle. The high surface energy of hot glass ensured an excellent phosphor adhesion without the need for a chemical binder. An enamel powder might have been added in order to increase the adhesion though. Finally, the bulb was cooled quickly to prevent the risk of damaging the fluorescent material. The end result is a relatively transparent outer jacket that permits both a color correction and an efficient imaging of the mercury discharge by the luminaire's optics. This is a peculiar design approach that GEC followed for its fluorescent mercury lamps until the late 1980s.

The MBF/V 80W featured here is fitted with an outer bulb borrowed from 300 W incandescent lamps. Its large size keeps the phosphor temperature at around 100 °C for an optimum fluorescence output. A three-pin bayonet cap is used to prevent the accidental use of mercury lamps in non-ballasted circuits. The burner present inside has four quartz parts fused together and is of the very first design GEC introduced in 1937. Three gas-tight seals were formed by hand using quartz tubing that were collapsed onto molybdenum foils. Two of these seals are flanged and fitted with main coiled electrodes filled with a mix of thorium and silica for an efficient thermal emission of electrons. These main feedthroughs are then rim-sealed to a quartz tube that forms the body of the burner, while a side electrode is added as a starting probe.

This lamp design saw a major change a few years later with the use of a thicker, non-fused phosphor layer, and was then superseded in 1952 by lamps coated with magnesium fluorogermanate, a fluorescent material invented by Westinghouse. During their fifteen year of existence these early fluorescent lamps had little popularity outside the UK on account of their high cost price and the limited degree of color correction this technology provided.