AilSnail- 4.2 mm is not too bad for reflectors > 5"-6" but begin to look longish for 4" reflectors.
It is the lamp that cannot be turned on instantly, not the ballast.
The real instant on ones are the Xenon short arc lamps, the smallest of which is the ARC 20W lamp
ARC . This is not very efficient though. It is probably the lamp in use in the Xenonics Nighthunter II. We all know that this flashlight is rather poor in its light output. I do not know of any quasi-instant on lamps under 35W. Only the automotive industry can make this happen. So for the moment, even if a very small 15W ballast can be made, there is no lamp to use it with.
The Maxa Beam uses a 75W Xenon short arc lamp with a ballast of thier own design. The luminous output is only 1000lm. As a reference, a 55W H3 gives 1600 lm, a 100W version 2300 lm. A D2/1s at 50W gives some 5000lm!
Hope that answers some of your doubts /ubbthreads/images/graemlins/blush.gif