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MH Retrofit and UV lens shielding



I'm in the process of constructing a DIY MH Pendant.  My question concerns 
the use of either a glass or acrylic lens for this fixture.  My 
understanding is that nearly all single-ended MH bulbs available in the 
U.S. have a U.V. barrier as part of the bulb construction.  If this is true 
the only purpose of a lens would be to protect the hot bulb from moisture 
and possible explosion from rapid cooling?  I consulted a large commercial 
supplier of metal halide fixtures who had heard of this "exploding bulb 
theory" and commented by calling it "baseless in practical experience".

So I ask those of you using MH do you shield your bulbs and why?  Is it 
because it came from the manufacturer with a shield?  Those of you with 
retrofits or DIY units do you use a lens and why?  Do you use it on a 
pendant fixture or on a enclosed hood?  What material do you use or recommend?

Lastly, what are the implications of UV light slightly above natural 
conditions on fish and plants?


______________________________________________
 From Andrew Lester                     atlester at home_com