I don't think there are any magic numbers.... As bulbs diminish in intensity a great deal over their lifetime(several years). Most give a range that starts with a minimum, and doubles to a 'maximum', as most bulbs will diminish in intensity by about half over their full life.

Plants don't do as well with narrow light sources. They require a fairly broad spectrum to grow their best. And 400-700nm(classical PAR range of radiance) wavelengths aren't the only useful ones. UV(below 400nm) has been shown to increase flowering yield in many species of plants, including cannabis. And even a little bit of NIR(just barely over 700nm, perhaps to as much as 800-900nm) has also been demonstrated to aid flowering. HPS emits a negligible amount of UV light(plenty of IR!). MH and fluoros emit about 5-10% of their radiance as plant-useful UV(CMH having the most besides specialty UV lights). Also, plants use different wavelengths of light for different functions. Providing a full-sprectrum allows it to be fully functional, and growth will be optimal given the other various environmental conditions are properly addressed. This makes for the highest quality smoke.

You don't need fluoros at all. A combination of MH and HPS is perfect for flowering, and HPS can do it alone quite well. 1 part MH/fluoro, 2 parts HPS(watts) would be ideal for flowering.

Look on youtube. There are plenty examples of HPS grows, and a few that mix MH with HPS. The plants speak for themselves. Extremely healthy and the most dense shining buds... anyone could hope for.

Fluoros are great for side lighting and 'in canopy' lighting in some HID grow rooms as they can be much closer/more adjustable and get to those 'weak points' in a room's light coverage.

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i have tried hanging cfl's under hps light and the results are not worth the time, a cfl looses light at a rapid rate as distance increases, so what happens is that you have these cfl's blocking light from the hps without throwing much of there own so its a real negative, i have had good results hanging t12 tubes along side the room where its dark or using them under the canopy where the hid cant penetrate, if you do something like this i suggest using warm white bulbs as the cool white tend to make the buds stretch

I obviously don't have as much experience with this as TeaTree, but I love the science part of it. Yes, cfls hanging vertically are very effective at getting light to the lower branches that the hid has a hard time penetrating. This will definately increase your yield on the bottom portion of your plants, therefore increasing your overall yield. Typically, 2,500 lumens per square foot for vegatative growth is effective, yet efficient. However, for best results, the lumen per square foot output during flowering can range from 8,000-10,000. For reference, the sun emits 10,000 lumens per square foot on a sunny day. Lumens per square foot is also know as foot candles. So if someone says 5,000 foot candles, they just mean 5,000 lumens in a one square foot area.

An important thing to remember when considering light is the inverse square law. Light intensity diminishes pretty rapidly as the source is moved farther away. The formula is pretty simple. It is 1/X squared. X is the distance from the plants to the light, you will then need to multiply this fraction times your total lumen output.

For example, say you have a 400w light that has a 36,000 lumen output rating that is 3 feet from the top of your plants. The equation would be as so:

1/3(squared)=1/9

1/9 x 36,000=4,000 lumens at the top of your plants.

This law is universal and can be used whenever your calculating lumens and footcandles.

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for a first round/trail run. CFL's for the veg state and a 150W HPS can give good yeilds. (the CFLs are easy to deal with and this gives time to work out some of the HPS issues ie:heat)

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