Great info about the â??rightâ? color of light to use. About lumens/radiant flux...remember that humans can â??seeâ? light in the wavelengths that plants use. So, realistically, luminous flux is adequate for determining light plants can use too. Light wavelengths between 380 and 760nm are, in general, visible to the human eye. Plant chlorophyll mainly absorbs light in the 400 to 700nm range (400 to 450 is red wavelength; 625 to 700 is blue). So, while radiant flux is technically correct, luminous flux does the trick awfully well for determining if you've got enough light.

This kind of gets back to the PAR thing...PAR is a measure of radiant flux (active photons) over a specific area. You need about 135 PAR watts per square meter for a really good grow. A typical 400w HPS gives off about 125 PAR watts, which means it should do a good job on .926 of a square meter. If you look at lumens...a typical 400s HPS gives off about 50,000 lumens. You need about 5,000 lumens a square foot to get a very good grow, and a square meter is 10.76 square feet. Using lumens, you get a figure of .929 of a square meter. Very, very close. I always tell people to keep it simple, and since lumen output is given for a lot of bulbs, it's the easiest way to figure if you've got enough light. Just my .02 on that.

And, like you said, you just want the right type of light with enough output. Factors like closet size, ventilation, etc. may make one type of light better or worse for someone.