The metal halide lamp is essentially a development of the high pressure mercury vapour lamp. It had been known that adding trace amount of other metals to the discharge could improve the spectral output by both filling the gaps in the emission spectra of the mercury discharge, and decreasing the output at the ultraviolet end of the spectrum. This also helps increase the overall luminous efficacy of the lamps too. These metals however could not simply be added directly to the discharge vessel as the vapour pressure would be too low to make them take an active part in the discharge. As a result, halides of these metals are used, which are far easier to vaporise. They are however rather corrosive towards the quartz of which the lamps are constructed. The first commercially practical metal halide lamps appeared in the 1960s.
Ceramic metal halide lamps are a somewhat newer invention, first seeing commercial success when Philips launched the CDM series of lamps in 1994. Ceramics in this application have a number of advantages, primarily in that they are more resistant to the halide salts present in these lamps than quartz envelopes traditionally employed, and that they are far more resistant to heat. This means that a considerable increase in wall temperature is possible from the quartz lamps. This in turn beings about an increase in efficacy and colour rendering. The ceramic discharge vessel was not without teething problems however, particularly that the end seals of the lamps were particularly vulnerable to corrosion. Mainly this is because there are only certain metals which are suitable for sealing to ceramic - and none of these metals are resistant to the halides used here. In the 1980's cermet end seals were trailed by the English lighting company Thorn as a solution to this (cermet is effectively an electrically conductive ceramic - though is somewhat more complicated than that! The description in detail of how cermets work goes beyond the scope of this page - for now - though I may research it and add more data later), but were not really a practical solution. The ceramic metal halide lamp had to wait until Philips came up with an entirely new sealing technique. Their solution was radically different to anything which had come before - actually positioning the seals a long, long distance away from the discharge itself. This far away from the heat of the discharge itself, the normal combination of niobium and molybdenum can be used to form the electrical connections into the discharge vessel itself.
For a more detailed description of the evolution of metal halide (and all other types of discharge lighting), I suggest a visit to Lamptech. In my opinion, THE place to look for everything relating to discharge lighting.
|Philips HPI-T Plus 400W|
|Osram Powerstar HQI-T 70W/WDL UVS|
|Philips MasterColour CDM-R 35W/830 PAR30L 10°|