It readily depends on the glass and its thickness ..
The benefits of almost NOHEAT far outway a loss of max 10-15% light ..
Thus meaning you can have a 1000w or even a few in a small cabinet if you have the proper intake and outtake for it
So you can infact have a hell of alot more light that usuall ..
BTW- I got all these pics from overgrow .. I havent made it myself .. But the results from their discussions are very good
Many members have made theirs for as little as $40 - While online versions cost anywhere from $150-300
The glass is pyrex .. It came from a Bread-Baking kit im fairly sure
RC- Main reason i didnt put it in lighting is cause its exacly 50% Lighting and 50% Construction hehe But i get your point ..
Im thinking about making a cooltube though .. cause no-one so far i dont think has on HC yet !!