Interesting read, this thread....

I wanted to comment something:
In my grow cabinet, I have one 400W MH, and one 600W HPS.
So... let's say we've got 35,000 lumens in the MH... and, say, 90,000 lumens in the HPS... (figures are "conservative", I would think).

The eternal question::: Can these lumens be added?
If not, I would be having with the HPS
90,000 lumens / 5.55sq.ft. = 16,216 lumens/sq.ft.

With the MH only
35,000 lumens / 5.55sq.ft. = 6,306 lumens/sq.ft.

(cabinet dimensions: 20"x40" = 800sq.in./144 = 5.55 sq.ft. )

Ideas?