I don't know if light decay or spread but I know the results I've got using both.
Fact is you can put the CFL closer mean you can use more lumens as there's less space to spread.
You place your HPS higher meaning it covers more space.
Thats all due to heat issues.
I like the chart I've added to calculate my lumens.
once more into the breach. it is just not accurate to say that because you "put the CFL closer mean you can use more lumens". you have the cause and effect mixed up - the fact that the cfl has so few lumens means that you have to keep it close to get a decent lumens per square foot. and because the lumens/watt ratio is so low with cfl's you need a lot more wattage of cfl's to cover the same square footage as an hps. it's basic physics - the superior efficiency of an hps will always make it a better choice if heat, space and budget are not issues.
as for the chart, it is extremely misleading. it suggests that light far in excess of 10,000 lumens per square foot (or footcandles, same thing) is ok to grow pot and that is not true. it also places the low end of light for weed at 5,000 lumens per square foot and it's more commonly accepted that you can go down to 3,000 lumens per square foot and get a decent yield. the chart also makes you do your own math to calculate how many square feet of coverage you get at different distances. and it doesn't give you cfl as a comparison. other than that it's a great chart.
