Best bets for small setups are T8 two-bulb shop lights. Two of them spaced about 10" apart will illuminate four 10x20" plant trays (see picture). Suspend them 2-3" above the leaves by chains, allowing enough room to raise them as the seedlings grow. T12 bulbs are obsolescent and T5's are more expensive, but may be useful for larger greenhouses.
Use "cool white" (4100°K), "natural" (5000°K), or "daylight" (6000°K) bulbs. Check the prices and lumen outputs---manufacturers differ. Ignore the pricey "full-spectrum" bulbs. They are useful in permanent indoor gardens but seedlings don't need the red end of the spectrum to grow.
LED shoplights are becoming available, but check their specs carefully. Some have cooler color temperatures or lower lumen outputs. I tried one model that was comparable with T8 shop lights and have noted no differences in seedlings grown under either.
