Reptile UV
From internet  Jan 2010
In the USA it is becoming fairly common for iguana owners to include blood tests for serum calcediol levels in their regular veterinary health checks.
We all know that reptiles need a "heat gradient" and part of their normal behaviour involves moving in and out of the warmth so that they maintain a healthy body temperature. Since some reptiles are also aware of ultraviolet light "gradients" and will actually move in and out of them according to their bodily vitamin D3 status and requirements, it would thus seem very appropriate to provide our animals with UV light in the same way we provide heat, ensuring they can choose how much, or how little UV they absorb by establishing a UVB gradient in our vivaria. 
When a mercury vapour lamp with a higher UVB output was introduced to the  bearded dragons, however, a change in behaviour was seen. Their activity levels increased and they chose to bask directly under the mercury vapour bulb, rather than their incandescent basking lamp, for much of the day. In particular, one adult bearded dragon recoverinFig.4. Recovering from MBD under high UVB lampg from metabolic bone disorder (the result of a lack of UVB in the past) basked directly underneath the UV source for most of the day, sitting as close as she could safely get (which with this particular lamp, was 12"). This dragon was never seen basking under the fluorescent tube that was provided for her before the mercury vapour lamp was installed. (Figure 4)