Thanks to everyone for the information. Unfortunately, I can't mount the bulb horizontally because it won't fit. This is not your cheap glass aquarium. It's a custom made enclosure about 20 X 20 X 48 and the 24" bulb will not fit at the top. And even if it did there's no guarantee he will be directly under it. Normally, he would be basking on a vine, but right now climbing is not his thing (as you can imagine). If I mount it vertically he gets UVB at most levels and it fits. Any beardie is capable of staring at the bulb whether its on top or not. I don't see any other way to do it.