how close can a 600watt bulb be if tempture isnt a problem

hey guys

just woundering how close a 600watt hps bulb can be to the plants, tempture wount be a problem

i think i read 8 inchs cos any closer than that then the light gets to intence

any thoughts

nar not cool tube just some big fuck of extraction fans and the bulb is hung verticaly with no shade on top, i have a 400watt hps have no problems, the plants dont get any closer than 8 inchs to the bulb