||The Super Metalarc series of metal halide lamps was pioneered by Sylvania in the 1980s specifically to provide enhanced performance in the burning position in which they were designed to be operated.
The shape of a normal electric arc is strongly influenced by convection currents, caused by the effects of gravity in the gases and vapours in which it operates. A horizontal arc will always bow upwards at its centre, this being the reason why electric arcs are so named. Similarly a vertical discharge is usually broader at its upper end than the lower part, again due to the effects of convection. This non-uniform arc shape leads to non-uniformities in the temperature of the discharge tube. Since lamps must be designed so that no part becomes overheated, if the arc tube has a large temperature gradient over its surface it means other regions will be very much cooler. It is the temperature of the coldest spot that determines lamp performance, and the colder it is, the worse the performance. However the cold spot temperature can only be raised by making the overall thermal profile more uniform, otherwise it would cause the former hot-spots to become too hot and shorten lamp life.
The lamp pictured here is the Super Metalarc type designed for horizontal burning. The photos clearly show the curved shape of the arc tube, and how it fits the shape of the arc perfectly for the ultimate in temperature uniformity. The effect is that the the cold spot temperature is increased without increasing the hot spot temperature, and luminous efficacy and colour rendering are both considerably improved. To ensure correct orientation of the lamp its has a special cap with side pin which mates with a special holder.