You have a zoomed mini dome with a polished reflector inside. Normally a compact CFL is recommended. Did you get a Zoomed Reptisun 13watt #5.0 UVB? Something sounds off at 12" ? There instructions here :
http://www.drsfostersmith.com/Media/PDF ... struct.pdfOr what brand and type and size do you have ? Are you looking at the burn in distance or normal distance after burn in ?
Also the UVA puts off white light on most , what size and wattage is it. If you are trying to put a 100 watt UVA bulb in that same fixture with the #5 UVB [ it the UVA bulb will be too close and have problems] . Could burn turtles eyes. A UVB #5 is set close , a #10 would be setup at more distance and other types even farther away , then you when using a double fixture must balance that distance for the UVA wattage. That's why a lot of long time turtle keepers use separate fixtures to control lighting better. It can be done in a double fixture But you must balance both bulbs to the distance !
This is me and me only " I am not saying for you to use , only an example , the tank setup is different :
with my #5.0 UVB at 8" I have a 50 watt UVA
" " #10 UVB at 20" I have a 75 watt UVA
And so on , I have different lighting for different size tanks.
One thing I will say if you keep that screen top I would put any "UVB" about three to four inches closer than the recommended distance due to what is blocked by the screen. Unless some charts give the distance "for" screen cover already.