Hello Mayo,
Bulb types are explained here:
http://en.m.wikipedia.org/wiki...
And here:
http://www.gelighting.com/Ligh...
I've grown a variety of plants, including daylily seedlings, under lights, and I've always used one daylight tube and one "Grow" tube in each fixture, but for strictly sprouting and growing daylily seedlings to transplant size, I'm not sure how important the flowering wavelengths are, or if a growth-only setup might actually be better.
If you were growing flowering plants, such as cactus, succulents or African Violets you would need to pay more attention to that end of the spectrum.
Lumens are a unit of light measurement that specifically relates to the parts of the spectrum which can be perceived by the human eye. I think its origin is in consumer protection, but also has the secondary effect of encouraging efficiency in light design. Lumens may not be the best way to measure light output for horticulture, since plants also react to light in the non-visible parts of the spectrum, but it's the most commonly used rating system.
The configuration of your growing area will play a part in which way you go regarding lamps. How many seedlings are you going to grow, and how big is the growing area? Is it going to be inside the home or in a cold garage? If the latter, do you think you'll need some bottom-heat?
The HID 400W lamp throws a lot of light (and heat), and offers the simplest set up, but in order to evenly illuminate a larger area it will need to be held up a few feet from the plants. Light intensity follows the inverse-square law, meaning that the intensity decreases as the square of the distance from the source, so a lot of that light will attenuate by the time gets to the leaf surface. Also, it's a 400 watt all-or-nothing proposition. If you run several growing stations of 22" X 22", you can shut off any unneeded lamps.
The high-power 75W fluorescents you're looking at are probably a little too strong to double-up in fixtures. Compared to conventional (T8) fluorescent tubes of similar length, they provide about 3x the light.
My basic grow station uses two 24" T8 tubes at 20w each per fixture, with 2 of these fixtures spaced to cover a 22 inch X 22 inch growing area. This holds 24 3.5" pots, with up to 10 seedlings in a pot, and uses 80 watts of power. I have two of these in my garage. Growth is good, but I run the lights right down on the foliage tips. I think fluorescent light is most effective and efficient this way. If you put 2 of those 75W lights in a fixture, it might be a bit more concentrated than necessary, and when using fewer but more powerful lights, you might start to see areas which are not as well-lit as others, and might even see leaf damage. I think 2 single-lamp fixtures using the 75W "Cutting" lamps would light the same area
extremely well. (150 W vs. 80W) I also have white board surrounding the growing area to bounce light back into the plants. A sheet of cardboard or plywood painted flat white is highly efficient.
Keep in mind that if you "under light", you can go closer or add more lamps, but if you "over light", you have to raise the lights and still pay for the extra electricity.
It's an interesting "problem", and if I was starting over, I might be tempted to try some of the newer technology.
Good luck, and be sure to have a plan in place to effectively deal with fungus gnats before you get started.
Ken
