Hi rshin,
Screening ONLY filters out as much UV as it filters out visible light.
So.....
If the screening filters (ie: blocks) out say... 25% of the visisble light....
then it will ONLY be blocking out 25% of the UV light.
Diffenent mesh sized have different amouts of "open space" in them to pass light through.
If you have a real restrictive mesh like the tight weave fiber glass screening, then you would want to consider adding another bulb.
But if light is getting through, then so is UV.
However...
If you are using weak or cheapie UV bulb ... you may not be getting enough.
The true way to tell is to check it with a Solarmeter.
Hope this helps!
Sincerely,
Todd
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