[saalweachter]

Yeah, even with the 4:1 lumen:watt advantage for LED bulbs, that's still a nearly 6x brightness advantage for halogen.

Is the limiting factor the technology to make a 4500W LED bulb in the same form-factor/focus as the 18kW halogen, or does it just become prohibitively expensive to do so?

[kuschku]

LEDs produce 80% less heat, but they also fail at much lower temperatures. Which is the major limiting factor for LED brightness today, as the cooling solutions get larger and more esoteric the more power you concentrate in an area.