If you're planning to have algae growing in the fuge, such as chaetomorpha, then you'll need a light. Almost any light can grow chaeto, such as a standard incandescent bulb in a reflector, to CFL or LED spotlights. I like the new LED spotlights at big box stores like Home Depot or Lowes since they provide a lot of light without the heat, and are more directional. The amount of light needed depends on the size of your fuge, but you basically want to cover the entire fuge with light.
If you haven't built your sump/fuge yet, my recommendation would be to use black plexi between the fuge and sump -- this will block out light that may end up growing algae in the sump.