Updated 22-VI-2003
Mercury Vapour
Mercury Pressure
Mercury Spectrum
Lamp Nomenclature
Timeline of Developments
Mercury Vapour
J.T. Way
Küch and Retschinsky
MA Medium Pressure
MB High Pressure
MC Low Pressure
MD Water-Cooled
ME Super Pressure
UHP Ultra High Pressure
Mercury Vapour
Fluorescent Coated Lamps
Tungsten Ballasted Lamps
Lamp Electrodes
Additives to the Arc
Electrodeless Designs
Future Developments
Mercury Vapour
High Pressure Circuits
Low Pressure Circuits
Electronic Operation

Germanate Phosphors

The development of the mercury lamp was to take a significant turn in the late 1940's with the development of a sensational new phosphor, based on magnesium germanate activated with manganese (Mg2GeO4:Mn).  The key feature of this material was its temperature characteristic, which made it supremely useful as a phosphor to be coated on the outer jackets of high pressure mercury lamps. 

It will be remembered that traditionally, phosphors tend to lose their efficiency as they reach higher temperatures.  Thus the first mercury fluorescent lamps with sulphide phosphors had to be made with grossly oversized bulbs to keep the phosphor temperature as low as possible.  Magnesium germanate, however, exhibits a peak in its efficiency when it is at a temperature of 300°C - precisely the kind of temperature at which the outer bulbs of mercury lamps operate.  It is also stable over long periods at elevated temperatures so lamps could be made in which the phosphor maintained a high efficacy throughout life.

Since 1936 it had been known that Mg2GeO4:Mn gave a strong red fluorescence.  The phosphor was invented by Humboldt Leverenz at the Radiophone Corporation of America (now GE's NBC Television company) and he intended it to be a more durable coating for the fluorescent screens of his company's cathode ray tubes (Leverenz, 1936).  At the time it was a rather inefficient, but very durable generator of red light.  Then in 1947 it was discovered that by adding an excess of MgO, phosphors could be made which were five times more efficient (Patten & Williams, 1947).  But the real breakthrough came in 1950 when Luke Thorington of Westinghouse made a further improvement which was to make the material suitably efficient as a phosphor for mercury lamps.

He found that if part of the MgO (0.5 moles) in the composition 4 MgO.GeO2:Mn was substituted with magnesium fluoride, MgF2, then the light output could be doubled yet again (Thorington, 1950).  It is believed that the presence of fluorine results in a more thorough reaction of the components from which is phosphor is manufactured, yielding the new material magnesium fluoro-germanate, MgFGeO4:Mn.  The combined high efficacy, thermal stability and strong red radiation peaking at 658nm saw MgFGeO4:Mn hold is place as the principal mercury lamp phosphor for nearly two decades.  Despite the thermal stability the colour of fluorescence does shift towards orange at higher temperatures, so it is still desirable to keep the bulb wall temperature fairly low, certainly no more than 200-250°C.  The spectral emission at typical lamp operating temperatures is shown in Figure XX.

Magnesium Fluoro-Germanate Lamps

The first lamps to make use of the new phosphor were launched of course by Westinghouse, its inventor, around 1950.  They offered a sizeable increase in the red ratio of up to 7%, nearly doubling again the red content that had been attained with earlier British developments with the sulphide phosphors.  The spectral power distribution of a lamp made with this phosphor is illustrated in Figure XX.  Despite the extra red luminescence, the increase in luminous flux is still almost negligible.  Although magnesium fluoro-germanate is not nearly so strongly coloured as the earlier sulphides, it does still have a slightly off-white yellowish hint.  The coating also has to be applied in a somewhat thicker layer than with earlier sulphides, and consequently once again, the coating absorbs as much light as the extra it generats.

The slight yellowish tinge is a result of the absorption of blue light at around 420nm, once again sapping up some of the light generated by the mercury arc, although the amount absorbed is considerably less than with earlier materials.  Nevertheless, this blue absorption is still sufficient to impart a very slightly greenish hue to the light from lamps bearing this coating.  The blue absorption also increases at higher operating temperatures so despite the high temperature stability of this phosphor, it is still desirable to keep its temperature low.  The blue absorption is sufficiently small though that for the vast majority of applications the greenish tint proved to be acceptable, in just the same way as the light from an incandescent lamp appears somewhat yellowish.

Manufacture of Magnesium Fluoro-Germanate

This phosphor is prepared essentially from a high temperature reaction between MgO, MgF2 and GeO2 with the addition of a trace amount of the activator, Mn.  The performance of the finished material is highly dependent on the precise firing time and temperature, as is the case for most phosphors.  The actual time and temperature required are depending on the starting particle sizes of the raw materials, but the best phosphors in general are obtained after firing at a temperature of between 1100°C and 1200°C for eight hours.

During the firing process the grains all grow in size as the reaction proceeds.  The higher the temperature and longer the firing time, the larger the grains will become and this is good for a high efficiency final phosphor.  Incidentally it is also found that firing the phosphor in closed crucibles is greatly beneficial, this leading to a much coarser grain structure.  When open crucibles are used the rate of fluorine loss is quite rapid, and since fluorine is the active agent in grain growth it is beneficial to keep its concentration high, and the closed crucibles assist greatly in this matter.

The importance of the coarse large-grained structure is simple to understand.  If the grains are large, then a relatively thick coating can be applied but the amount of light absorption within the coating is small - this is because absorption principally occurs as a result of the boundaries between one grain and the next, and if the grains are larger, then there are fewer boundaries hence light absorption will be reduced.