That depends on your light. HPS and MH are a lot hotter than say T5 or LED, or even cfl for that matter. I am not exactly sure about HPS and MH but the others can be about a foot or so away while they're just popping out and until they get a little older, then you can move T5 or cfl right up to it as long as your temps and air flow are good. I believe there is a point of being too close with the LEDs though. Hope this helps.