This has to be provided indoors by a fluorescent light tube and quality is paramount. Incandescent lights cannot provide UVB no matter what is printed on the packet. UVB is filtered by glass and plastic so any source has to be positioned to be free of these barriers. The tubes designed to emit UVB use quartz instead of glass to avoid this filtering effect , but in doing so greatly increase the cost. UVB is required by dragons and monitors and many other reptiles so that they can synthesise vitamin D into a more usable form.
D plays an important role in the skeletal growth and it correct formation in reptiles.
These lights at various distances are compared to 5 minutes of natural sunlight on Melissa Kaplan's site . Her table at the bottom will give you a good idea as to how they lose their effectiveness as the distance is increased. Although table 1 lacks the information on what these percentages refer to they do show the Zoo Med products to be the most effective at providing UVB which I tend to agree with. These tubes are recommended to be changed every 12 months because they lose their effectiveness over time also. Which ever product you choose its placement is crucial.