Here are some pics showing how screening can block UV light.
You can compare the readings side by side to see percentage wise how much this particular "average" black metal mesh screening cuts down on the UV light readings.
And screening does not absorb UV light.
It just mechanically blocks it the same way the mesh blocks any visible light.
That is to say UV Light behaves just as regular "visible" light does....
so...
If screening blocks(shades) ...say ...20% of the visible light, then it will also block about 20% of the UV light.
I hope that made sense.
Cheers~!
Todd
PS. If the bulbs are fairly strong (like these Arcadia HO t5's) then screening realy DOES NOT matter much, and is nothing to worry about. Plenty of UV and light would be getting through.
However, with the weaker cheaper bulbs, it would become a factor.
Especially if they are just barely giving enough UV to begin with, then deduct 20 - 35% blocked by screening and its "hello MBD". 