http://www.reptilesmagazine.com/Reptile-Health/Habitats-Care/Reptile-Lighting-Information/

http://www.reptilesmagazine.com/Care-Sheets/Uromastyx-Lizard/

Uromastyx need a very hot basking spot, 120-130 degrees. It is very difficult to achieve a proper temperature gradient for them in a 40-gallon, once your basking spot is at the right temperature, your cool side will be too hot as well. Best enclosure size for a uro is a 4x2x2 type enclosure. I keep mine in melamine tanks and use 65-70 watt halogen bulbs for basking.

I agree with the above post in terms of caging, but you can mediate the heat-cool ratio to some degree by using a lamp stand and/or a chain to suspend the bulb and lamp over the cage and raise and lower it to reduce/add heat. The best recommended bulb for a uro or any dirunal animal that requires UVB would be a mercury vapor bulb or MVB. I'd recommend a Powersun MVB, the 120watt version, so as not to overheat your tank, plus a lamp stand or suspension chain so you can manually adjust the temps to get a 120 basking area and around 77-80 in the cool end. Uros won't suffer if the cool end is around 80F, but anything over that is no bueno. 

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