I would like to add that the webpage referred to a few posts back is very wrong when LED is concerned. The lumen-requirements stated there are based on non-directional sources. LED is extremely directionally focussed (usually beaming 120 degrees or less). Meaning the lumen reaching an aquarium bottom surface would be much much lower than the requirements stated on that page, yet would be more than enough for plants and the human eye.
With LED there is no loss at the back (top of aquarium) or sides, so you need no reflectors either. It is also noticeable that the light from LED does not reach out of the sides of the tank, as other lighting does.