Lighting is very important for turtles in general.  All turtles need UVA and UVB rays to absorb calcium and vitamin D, without proper lighting terrapins can develop calcium deficiency.   It can cause improper shell growth and even death.  Terrapins should have two light sources.  A basking bulb (UVA) to provide the heat necessary to dry out their shells and a florescent light fixture to provide UVB rays.  Most of the heat lamps contain UVA rays but not UVB, which is most important.

I really like the ZooMed PowerSun which offer both UVA and UVB in one basking bulb (these are mercury vapor bulbs).  The UVB rays penetrate better and are stronger than the UVB florescent bulbs.  Another advantage to the UVA/UVB combination bulbs is that they last longer than both the florescent bulbs and the separate basking bulbs.

One very important side note about basking bulbs or the UVA/UVB combination bulbs is make sure that they are a safe distance above the basking area. Hatchlings have been known to cook themselves instead of leaving an area that is too hot.  I keep my 100-watt basking bulbs around 10 inches above the basking area.  I keep my bulbs about 8 inches above my adult basking platforms.

In general you want the basking area temperature to be around 90F.