Last year my husband installed a 2 strip set of LED lights over the front bench in our attached greenhouse. They’re just regular LED’s lights (except they happen to be 12v), mix of bright white and warm white. I’m not trying to grow anything exotic, just wanted to extend our short days, short sun period to help the greens grow, and allow starting seedlings earlier. Didn’t know how it would work but he had some sets leftover from making house lights so we thought we’d give it a try. They worked well and use little power (important to us as we produce our own power) (solar). I’m real happy with them. The lettuce and other greens were the healthiest I’ve had and they grew better. So did some early seeding I did. Now I have on my wish-list a similar set over the bed along the back side of the GH.
(more info and photos on the lights in the GH at http://www.manytracks.com/Garden/greenhouse.htm)
Steven, as others have mentioned I don’t think lights will help in hardening off. I used to haul things in and out of the greenhouse getting them used to the outdoors, but now I simply transplant on a cloudy day (usually enough of those here) and cover with sheets (or something) during the sunniest part of the days until they seem acclimated. Tender crops go in cold frames, similarly covered. It’s worked well for me and is a lot easier.