For the whole grow or just for flowering?
For the whole grow. Flowering is accomplished by changing the length of the night (long nights induce flowering in most cannabis strains). The problem with this is short days don't give as much light as long days and we want as much as possible for maximum growth (we aren't growing leaves during flowering but more light still makes bigger flowers). Since this is a phytochrome response, we can try to a) avoid visible red or 2) supplement with far red around 730 nm. I've never tried either. There is a range of response for some plants (all the way into the blue) but others require specific wavelengths (around 660 nm) to induce flowering and likewise the reverse conversion isn't always exactly at or exclusively 730 nm. I don't know the specifics for cannabis.

Google book - scroll down to chapter 1, page 3:
Marijuana Botany: Propagation and ... - Google Books

More on phytochrome:
floinduc

How many watts of LED do you find necessary per square foot?
I put together this spreadsheet a while ago to figure out exactly that. Start on the 22moles/m2/day (E,39) and adjust from there. You will need to know the light output of each LED in watts and the area in square meters (C60).

https://spreadsheets.google.com/ccc?...5MzhzZlE&hl=en


Also, the path oxygen takes to the thylakoids is irrelevant to the point. Rubisco will react with either oxygen (photorespiration) or carbon dioxide (photosynthesis). When the availability of either gas is altered it directly alters the probability of each function. I was wrong, however, on the 1 for 1 ratio. I meant to say 3 CO2 for each oxygen but in actuality I can't recall if it is exactly 3 or a little higher. And the LEDs too: I've never really looked at yellow LEDs too much I just (ass)umed they were like white LEDs.

You got me again, khyberkitsune. :P