This is my understating of Metal Halide lamps... I'm not an expert so take this info with a grain of salt. I may be completely wrong.
A metal halide bulb doesn't have a filament. Instead it contains 2 bulbs. An inner quartz heater bulb and an outer Arc bulb. The inner bulb heats up to a certain temperature. The outer bulb contains Metalized gasses. Each gass produces a different spectrum of light. Once the heater bulb hits temperature an electrical charge is applied to it and the outer bulb archs to it. This arching causes the the gasses to expand increasing the pressure inside the bulb. The pressure combined with the Arch produce light. The spectrum is dependent on the gasses in the bulb.
My guess is that they are heating up the inner bulb and then varying the voltage to the arch bulb.
This is VERY similar to how vacuum tubes work. vacuum tubes used in audio amplifiers sound a lot better when cranked all the way up. Many musicians rig their amps in such a way that they can heat the heater plate (Metal Halides have bulbs instead) as if it were cranked but then lower the filament voltage to have lower volume. This alows you to play an amp that sounds like it's cranked, but the actual volume is much lower... you can play a small club without blowing everyones hearing. The problem this causes in tube amps is shorter vacuum tube life (they are expensive) and sometimes circuit failure in the amp itself. Eddie Van Halen was one of the pioneers in doing this. Few know that he was actually an extremely proficient electrical hobbiest.
The fact that they pulled these ballasts off the market kind of leads me to believe that the problems may also be similar. The solution to the amp problem is to let the tube go full blast but then blead the power with a giant resistor after the transformer outout. Then, if anything goes, it'd be the resistor and not the amp itself.
Unfortunately, a light bulbs output is already out in the environtment as light after it's created. If I were going to build a varible Metal halide light source, I'd leave the lamp alone. I'd instead make a variable shade. I'd take a clear LCD screen and then slowly add more and more pixles to the screen as night approached... then have a timer shut the lamp off. In the morning the lamp would pop ack on and the screan would slowly remove more and more pixels untill it was clear at noon. The parts would only be about $100 but you'd have to know how to program the LCD controller. They already make window shades like this for rich people. You hit a button and 'wham' your window is black.
Hope this helps.
