IT's all about the Zoo-med Repti-Sun and Iguana light 5.0 bulbs.....I have seen some charts and graphs kicking around, plus anecdotal evidence from working for a reptile dealer in the past, and the Zoo-med Repti-sun 5.0 and iguana light 5.0, outperform all other fluorescent reptile bulbs. I cut corners, and find good cheaper alternatives for many things, but the UVB bulbs, and rep-cal vitamins are two things I do not find a cheaper alternative for.
I have taken an excerpt from one of the links thsat Edwin posted, and I think it says it all:
"How strong is a reptile light? A Vita-Lite at 12 inches produces about 1.1 µW/cm2 [21], which would produce 20 mJ/cm2 over any exposed skin in 5 hours. That should be sufficient to begin vitamin D3 production. Gehrmann [22], gives the Zoo Med Iguana light 5.0 at 10 µW/cm2. This would produce 20 mJ/cm2 in less than an hour. (A reptile D-light 8% produces about 9 µW/cm2, and a Reptile D-light 3% produces 4 µW/cm2.) Winter sunlight in Boston is a non-vitamin-D-producing 5mJ/cm2 over three hours; the Zoo Med 5.0 would beat this figure in just a few minutes and go on to vitamin D3 producing levels, sustaining them for the rest of the day. Although this looks very strong on paper, I have personally found no problems with iguanas living about 12 inches away from Zoo Med tubes for 6-10 hours per day over the past two years."
Here is the link to the article anyway:
http://www.anapsid.org/uvbanne.html