The big this is measuring the air temp. This should not be influenced by any heat source. RHPs emit IR radiation, this is how they heat exactly the same way as the sun does. The issue is taking air temps with out reading the heat waves from the RHP. Often these must be blocked as Skeletor mentioned. You can just set up something temporally take the reading and then return it to the way it was.

A UTH and RHP is over kill generally they are both radiant heat sources and both heat basically the same way. If you need to increase the air temps you could either add lighting (fluorescent light tubes add a fair bit of ambient temps and LED fixtures add more than you might expect.) The other method is much like the sun, add something dark matte and heavy to the enclosure. The RHP will heat this up (disproportionally than white, glossy, and light) much like a black chair in the sun. This can be used to slightly adjust the ambient air temps if needed.