A HMI/MSR lamp uses mercury vapour mixed with metal halides in a quartz glass envelope along with two tungsten electrodes of medium arc separation. Unlike traditional lighting units using incandescent light bulbs, HMI/ MSR luminaires need electrical ballasts. These are separated from the head via a header cable to limit the current and supply the proper voltage. The lamp operates by creating an electric arc between the two electrodes within the bulb that excites the pressurized mercury vapour and metal halides and provides very high light output with greater efficiency than Incandescent lighting.