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Although the origins of the metal
halide lamp can be traced back to a 1912 patent
taken out by GE's Charles Steinmetz, the practical
lamps based on current technologies first appeared
in the labs of Osram in Germany, and of GE in USA,
in the late 1950s. It was not until the mid
1960s that commercial products appeared.
The technology is a development
of the high pressure mercury lamp, to which trace
additions of various other metals are added in the
form of their halide compounds. These are
vaporised into the discharge and contribute their
spectral output, and materials are selected which
fill in the gaps in the mercury spectrum to
deliver a light source having considerably
improved colour properties. The luminous
efficacy of the lamps is also increased owing to a
reduction in the generation of invisible
ultraviolet radiation.
It is necessary to add the metals
in the form of halides because the metals
themselves usually have too low a vapour pressure
to participate in the discharge, but the halides
are vaporised very much more readily. In
addition the additive metals are rather corrosive
towards the quartz discharge tube wall, but when
halides are employed, they always exist in
compound form on the quartz surface in which state
they are very much less reactive, thus minimising
the corrosion
problems. | 
