Most HID lamps use about 75% of there watts to get up to illumination temp. Lumens are more the light intensity the lamp produces and par watts are the number of watts that the plants acually use. 1500 lumens per square foot is the average intensity growers are told to use from my reading. The dull side of tin foil is not to bad but mirror like surfaces absorb the light. Mylar is easy to come by at any grow shop. You can order it online at ww.wormsway.com but they are not open today, as I tryed calling them when I woke up. Personally I prefer flat white paint.

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