As soon as tomato seedlings sprout, they need a good, strong light source provided right away.
Newly sprouted seedlings can be placed in a window, however the sunlight coming through a window does a funny thing to tomato seedlings. They tend to grow fast and spindly, bending toward the light from the window. Tall, spindly, bending seedlings are not likely to grow into strong plants that can support lots of tomatoes.
A light source placed within a couple of inches of the tops of the seedlings is best. This can be as simple as one inexpensive shoplight installed over a table holding the tray of seedings or as sophisticated as a multi-tier lighting cart. It depends on the aesthetics you desire, how much space you have, how much money you wish to spend and the number of trays of seedlings you are dealing with.
Gardens Alive! has a "Jump Start Light System" that is perfect for two standard seed-starting trays. has both table-top and floor light systems.
Any of these would work great. We chose to save money and build our own lighting rack.
This is a shelving unit outfitted with shoplights from the local hardware store. It's not very pretty, but I keep it out of sight in my laundry room. It does look a lot better when it's full of trays of new green seedlings.
Here's a close up photo of two shelves. Notice how the lights hang between shelves, shining light to the shelf below.
Our plant rack was built by my husband, Don.
--Building a light rack is easy and relatively inexpensive.
--The frame is built from 2x2s; the shelves are built with 1x2s.
--Basic Fluorescent Shop Lights are hung from the bottom of each shelf.
--Lightweight plastic chain is used to adjust the light height.
--The cheapest florescent light fixtures are not the best. They tend to fail quickly (sometimes after one season). I believe this is due to poor quality ballasts in the fixtures. Spend just a few dollars more and get the next best fixture. You will save a lot more on replacing lights and fixtures in the long run.
--The lights are hung to a height just 1-inch above the seedlings and moved higher as they grow. We have the lights plugged into a timer so that the seedlings get a rest period of 6 hrs each night. Too much light can be just as detrimental to your tomato seedlings as too much water.
--Our seed starting rack is in our laundry room in the basement. This works because it is out of the way as far as esthetics, but is still convenient since someone typically visits the laundry room at least once a day and can check on the plants, water them and make adjustments.
So that's all there is to it. Using this type of rack makes the whole seed starting process so much easier for me. With just a small amount of time to devote to my garden, I love how this rack allows me to multitask by keeping my plants organized together in one place that I visit often (since I'm always doing laundry).