Metal Halide

Metal halide lamps evolved from the high pressure mercury vapour lamp.  Various metallic salts (a compound of a metal and a halogen) are added to the discharge to produce light that is lacking from the mercury spectrum alone. Mercury provides the blue, green and to some degree yellow spikes of the spectrum and the rest is mainly Ultra-violet, meaning phosphors need to be used to boost the red and orange wavelengths if a white light is desired, resulting in lower efficiency. In the metal halide lamp, scandium and sodium salts for example are added to produce different spectral peaks helping to balance the light output.  Depending on the cocktail of chemical compounds added the colour and colour temperature can be tailored to suit different requirements. There are two main types of metal halide lamp, firstly the long established quartz lamp and secondly the ceramic lamp. In the case of the quartz lamp the construction of the discharge envelope is very similar to that of the high pressure mercury lamp but most European lamps do not have a third auxiliary starting electrode as seen in the high pressure mercury lamp and rely on other forms of ignition. Quartz is fairly resilient to attack from chemical corrosion but never the less devitrification occurs and weakening of the arc tube.  A newer version is the ceramic lamp where the arc tube is constructed from a translucent ceramic. This material is more suited to the task of enclosing the high heat, high pressure and volatile arc of a metal halide lamp. It still has its problems, but with some clever ingenuity and engineering in the form of longer seals and ‘cermet’ ceramic conductors, invented by some of the leading lamp designers and scientists, problems have almost entirely been eliminated. Due to the nature of the ceramic arc tube and its increased durability, lamp efficiency can be improved further. Metal halide lamps can be retrofitted into most luminaires intended for high pressure sodium and even some high pressure mercury fittings. But before installation, compatibility MUST be checked! Sometimes it’s a straight swap, but in other scenarios a super imposed ignitor is required or even a whole new set of control gear has to be installed. It is paramount that the luminaire housing a metal halide lamp is equipped with a shield, lens or visor capable of holding extremely hot fragments of lamp due to the  (low but still possible) risk of explosion  that could so easily cause injury or fire! If there is no physical space or the addition of some form of screening would be disagreeable to the design or appearance of the luminaire a shielded or shrouded lamp MUST be used. Metal halide lamps are available in a vast range of ratings from a sparkling 20w lamp to an eye watering 2Kw. Their high efficiency, wide range of colours and compact size means these lamps are perfect for numerous applications – too many to list here! Unfortunately these lamps do have a warm up time and re-strike time like most discharge lamps and come with a high price tag, couple these drawbacks with the need for control gear and its clear why it’s unlikely to find this amazing lamp in a domestic setting.