How does Metal Halide work?


I appreciate your attempt to help me out and I now realize I could have explained what I was looking for a little better.
In a basic light bulb electricity passes through filament causing the electrons in the atoms that make up that filament to fly away from their nucleus, when they return to their natural orbit they release their photonic energy (light).
The same happens in a fluorescent bulb except that the phosphorus coating reacts to heated mercury atoms causing more light with less initial energy cost.
My question is why are Metal Halide bulbs so much brighter/efficeint and give off so much more light? How does the technology work and what drives the cost up so much?
Can anyone help?