I'm no director of research - far from it... but my limited understanding tells me that plants do not use the entire light spectrum. In fact, it uses quite limited amounts of white, yellow, orange and green. So an HPS that pumps out tons of "white" light (not to mention heat) is wildly inefficient. It is successful due to the high photon flux rate across the spectrum.

Khyber get's crazy props for his in depth knowledge and understanding. We need to thank him for the time he puts in here educating nOObs like myself. I'm happy to address my next question to The Director.

I've seen these lights LEDs with dimmers and I've wondered why you would use them. OK, with young seedlings you don't need to raise the light if you can dim it. And I guess you could tweek them to better mimic sprning / summer / fall lighting but really, the major factor is time phase (12/12 vs. 18/4) so once your seedlings are into veg wouldn't you just crank the LEDs all the way up and leave them there to maximize output / penetration?