I use 4 x 600 watt hps XTRASUN aircooled hoods with a 6" centrifugal pushing out and a double 4" vacuuming in. With these aircooled hoods i can seriously get away with about 6 inches away...i will take pictures when the light comes on today, and put them on here for you to see. But this is a very talked about subject, and alot of people are mis-informed on what you can and can't do. But i reccomend watching the "seemorebuds" videos...He goes through several different gardens in each video and it gives you different ideas of different techniques to use. Every grower is diff with our own ways you know. But i used to do the reccomended 18 to 30 inches away with my HID's and did great. Had the average results everytime, never had weak yields, always average and slightly above. Moved my lights down and started moving the air more...guess what? DOUBLED MY YEILD EVERY TIME!!!!! But with anything you should take special care and consideration according to YOUR room and plants. I grow Sativa doms, and supplement co2 witch allows me some higher temps, and the strains geno plays a matter into this...but to answer, you can get closer than the usual 18 to 30. I'll show you how close later.