OK, lets see here. Quality light meter. 2 of the exact same hoods (Blockbuster 6"s). One with no glass. 1K watt bulbs, both changed to new. The actual measured difference in lumins comes out to only 4% +/- .5% at 18" from glass (bottom of the hood). Borosilicate glass is the glass used in the hoods we use, due to it's heat tolerance. BTW the glass will reduce UV lighting below the 400nm range a lot! But the spectrum seen by the eye is not by any real meaningful amount overall.
Learn about it here.
http://en.wikipedia.org/wiki/Borosilicate_glass
Now then, as I take readings at differing distance's from the glass. The usable light drops at almost a exponential rate! So then. One MUST deduct that;
A: The difference between glass in and glass out is not worth the exchange in cooling needs in an enclosed environment.
B: That increasing the distance between the plant canopy and the lighting will reduce the OVERALL effectiveness of the lighting. Thus you would require more lighting to obtain the same result closer in.
The picture of the large grow is NOT done the way I do mine.....I run cooled lighting for the ease in temp control. I run side lighting on TALL (10-12ft) plants to have them reach their true potential (these are not cooled and DO effect temps control) ! The average plant height is 6 ft and are topped, Super cropped and lolly popped for overall light yield and light effectiveness. The farther you get your lights from your canopy. The taller more lanky and longer internodal spacing problems you get!!!!
Tex, it may have had a minimal effect on your grow....BUT, like was said earlier. You did not do it side by side with the same age clones from a single mother/strain. Therefor, you can not compare one grow to another with any science to back up your claim.
How many posts did you put in this thread anyway? Are you simply padding your post #'s?