well, with no glass, no cool tube, i could get 3 1/2"-4" away no issues, but if they can rest on the glass for a prolinged amount of time without being moved expect some burnt parts even if its cooled. those that are air cooled are in the 2" range (hanging vertical is what im talkin about) so thats 1 1/2 closer then me. test out diff positions til you find one that maximizes in your space. seedlings wise im not worrying about having them too close to the light( all 7 in sig are under it now) but when i put one closer on top of a cup so it could catch up you could see the heat effect on it being closer, but, i have a shit ass round bell type reflector that creates a central hotspot that im using for veg, flower i get rid of it.