A screen will block a lot of the UVB. There was a study done on this (UV Guide UK) and this is what was learned:
"If 5mm zinc plated mesh is placed in front of the tube, a 15-20% reduction in output is seen.
25-35% losses occur with fly screen of the type often used for chameleon vivaria."
If your screen has big holes (around 1/2" across) then it won't block too much of the UVB. If it has smaller holes, you should cut a hole in it over the basking area to allow the light to come on through.


