Keeping HPS lights cool

I just bought 400 watt HPS/MH light system and I was thinking of putting a 16'' or 20'' fan right beside it to blow cool air at the bulb so I can put it as close as possible to the plant. Have anyone did this before and how close do you think I can get without burning the plant?

if you use a air cooled reflector you can move really close to the glass plate of it but with your set up the heat just gone be blown into the grow so i would stills stay a minimum of 12" away from the plants.