I believe Lexan allows UV to pass through, but it's usually thicker than the flimsy plastic used as a cover on the Zilla fixture from what I've seen.
So If I remember right, It blocks quite a bit of UVB still, Lexan, Lexan is just a brand name of Polycarbonate, when you buy a bottle of soda, that is poly carbonate. It can be had in thick sheets and paper thin.
I think Lexan doesn't block UVA at all though, neither does Glass.
I read a few reviews. People who called zilla, where told to remove the plastic if you are placing the the hood on screen. So it does block "some" UVB. Its just so hard to get zilla information vs the bigger players. I dont even know how they get 25 watt out of a 18" T8.
Oh ya it defiantly blocks some. However, I would think there numbers account for it.
However the addition of screen is likely not factored in. So while the plastic blocks 5-10%, the screen blocks 20% now you are at 30% less. Screen blocks an Awful lot of light sadly.
Still be neat to try it. And in the case of an Arcadia 6% or 12% I would think you could leave it on.
I looked into it for LEDs. As While UVB LEDs are ungodly expensive and not worth it, UVA is very obtainable. And as more and more keepers start using LED lights, UVA might be something we should aim to add to our fixtures. Reptiles can see UVA, and use it.