UVB light is critical for Iguanas and Bearded Dragons. Without it, their bodies are unable to produce Vitamin D3, and they will develop painful and eventually fatal bone problems. Simply put, these reptiles will become sick and die without proper amounts of UVB light.
UVB bulbs are available in a few different varieties, like for fluorescent or screw-in fixtures. These bulbs are not cheap, and should be replaced every six months, but they are absolutely necessary to keep iguanas and beardies.
Read more about housing for your reptiles here:
Bearded Dragons - http://www.drsfostersmith.com/pic/article.cfm?c=6016&articleid=820&d=160&category=635
Iguanas - http://www.drsfostersmith.com/pic/article.cfm?c=6016&articleid=2384&d=160&category=293