HMI lights were invented in 1960 by the lighting manufacturing company ORSAM. They were created in order to make less-expensive lighting for the film industry.
HMI lights, which are a substitute for incandescent lights, are two to five times more efficient. This means they require at least half of the electricity of incandescent bulbs and do not give off nearly as much heat.
Instead of using an incandescent bulb like other lighting sources, HMI lights use arc lamps. These create light via an electrical arc set off inside of the lamp.
HMI lights generally end up being a fairly expensive light source. While HMI lights are used by many (if not most) major productions, those with a smaller budget may not always be able to afford them.
HMI lights are run by ballasts, which create the electrical arc that acts as a light source. There are two types: the less expensive magnetic ballast and the smaller, more-efficient electrical ballast.