Turtles need several hours of exposure to natural sunlight every day. This
  helps them synthesize vitamin D3. If you keep your pet outdoors, and there
  is sun and shade available in the enclosure, you don't have to worry.
  If your turtle is indoors all the time, you have to provide a source of
  full spectrum lighing or supplements.
  There are constantly new lights on the market, and there is a lot of 
  discussion on this issue. 
  There are 3 bulbs on the market that produce significant
  levels of UVB: Zoo-Med's ReptiSun 5.0, ESU's Desert Sun 7.0, and Reptile
  D-light (usually available only by mail order). All must be replaced every
  4-6 mos (timing depends on which research you think was most elegantly
  designed, but they all agree on the brevity of the useful life) because a
  coating builds up on the inside of the bulb which blocks the UVB. Also, they
  must be no more than 12" (or 14", or 18", again depending on whose research
  was best designed) from the animal in order to affect blood calcium levels.

* Some of my turtles enjoy a walk outdoors every once in a while. Watch your
  turtle at all times, so he won't get hurt or lost.Turtles can get lost
  very quickly, if they want to.

* Turtles need a basking light. The silvery shop lights from the hardware
  store are great. Place it on a screen top or hang if (high enough that
  the turtles cannot touch the bulb). I find that a 60W bulb is about the
  right strength. I tend to use a 40-60 Watt bulb in summer and 75W in winter.

* The lights should be on between 10 and 14 hours a day, depending
  whether you use a yearly cycle, or not, and depending on where your
  turtle comes from.