The use of ceramics in metal halide lamp construction has long been recognised as having many advantages - namely that it is more resistant to chemical attack by the halide salts present within the lamp, and that it is able to withstand higher temperatures than the equivalent quartz structure.   The greater corrosion resistance also allowed adequate lifetimes to be reached without requiring as thick (and heavy) a wall on the arc tube.  This meant that ceramic arc tubes had a smaller thermal mass, allowing lamps to run up/re-strike more rapidly than their quarts equivalents.  It was not a design without at least some problems to overcome however - not least figuring out how to get a metallic conductor into the ceramic arc tube to carry the discharge.  The conventional metals for doing this (usually niobium, such as found in the similar arc tubes used in high pressure sodium lamps) can not be used, as they are rapidly attached by the halide salts at the operating temperatures of the arc tube.  Philips however were the first to come up with a truly practical solution, which resulted in the launch of their CDM range of lamps in 1994.  The design was so successful that 15 years on, virtually all ceramic metal halide lamps are still using arc tubes in some way based on the Philips CDM design.  The key to this success was to move the fragile seals as far away from the intense heat of the arc as possible, therefore reducing their temperature and as a result the rate of attack.  The very long seal region also allows the seal to be made of conventional niobium wire tucked away in a thin enough tube that the halide salts cannot diffuse towards the seal reasons.

The style of lamp shown below was the first of the CDM range to come on the market, and the excellent quality of light produced and variety of lamp styles and ratings which have since become available have ensured that it proved to be very much a success for Philips.

I actually use one of these lamps in an up-lighter in the corner of my room as one of the main sources of light.  In normal use, the colour really is quite hard to distinguish from a good halogen source of the same colour temperature (well, unless you get the diffraction grating out anyway!).

There are a number of lamps in the CDM family which can be operated in an open fixture, this however is not one of them, and must be operated in a suitably rated fixture which is fully capable of containing any fragments of lamp in case the arc tube should rupture, resulting in a lamp explosion at the end of life.  This is unlikely during the normal lifetime of the lamp if operated as specified - however if the lamp is operated beyond its rated lifetime (usually indicated by a sudden shift in colour, a lamp becoming hard to start, or flickering), it becomes ever more likely.