I disagree with some of the facts above.
For example, while people do it often, it is not good to force a chameleon to have intense UVB exposure in order to warm itself. Yes, basking in nature, they get exposed to UVB, but they can seek out temperature gradients that include UVB gradients, in filtered light and shade. I always provide basking sites that don't include UVB, though use of Mercury Vapor bulbs is a great idea for a free range situation.
I also think that you need to determine why the chameleon doesn't prefer the free range and make some attempt to confine him, so he doesn't get lost, trodden on, mashed by a door, or into other trouble. Use a smooth "fence" around the bottom of the free range area, so that if he climbs down, he can't go wandering into unintended areas. If the free range is properly set-up, the chameleon should prefer to be there and not inclined to wander off, frequently, though some wandering is expected.
As far as your basking temps go, you could try a narrower dome, that focuses the beam in a smaller area, look into other bulb options, measure the distance between the basking spot and fixture and typical ambient temps, then consult Todd at lightyourreptiles.com.
How are you providing enough hydration and oppertunity for eye flushing, in the free range?
Some chameleons are better caged, if the animal or environment does not suit a free range situation.