Lumens is actually a poor figure to use because lumens takes into account of the sensitivity of our eyes to different lighting wavelengths. That being said, with HPS the photosynthesis net saturation point for marijuana without carbon dioxide enhancement is in the ball park of 6000-6500 lumens per square foot (in photobiology we would say the Pn for a C3 plant is 1200 umol/sec/meter^2). This is the level that I run my plants.thanx...one more question about lighting. how any many lumens should i have for 3 plants?