Metal halide lamps

The colour rendering of a mercury lamp can be improved by the addition of another
metal. It is, however, necessary that the metal used should have a sufficiently low vapour
pressure at the operating temperature of the lamp, and that it should not react with the
material of the arc tube. This can be achieved by using the metal in the form of its halide
salt.
The general design of metal halide lamps is similar to that of mercury lamps. When
one of these lamps is first ignited the output is due to the mercury. As the temperature
rises the metal halide, which is initially solid, melts and vaporizes. The high temperature
causes it to dissociate into metal and halogen, and emission of light commences. The tube
walls are cooler than the interior of the tube and the metal and halogen recombine on the
surface of the walls; this has an important effect in preventing chemical attack on the
silica walls.
The metal halide lamp with a clear glass outer envelope is designated MBI. When the
outer envelope has a fluorescent coating the designation becomes MBIF. A linear version
for use in floodlighting and in television studios is designated MBIL.
These lamps have a better colour rendering than MBF or incandescent lamps. They
also have better colour rendering than SON lamps but are not so efficient and therefore
SON lamps are still preferred where colour rendering is not so important. Metal halide
lamps are finding use in offices, supermarkets and large stores. They can also be used for
high bay warehouse lighting and floodlighting, but it is in these applications that the
higher efficiency of the high-pressure sodium lamp is generally thought to be more
important than the colour rendering.