depends on the type of light . if cfl or flro you can put them 1-3 inches from the plant. if hps or mh put your hand over the top of your plant then lower your light till it get too hot for your hand rase the light a little till its conforable for your hand. you can get air cooled lights closer but use the same methid to check.not now .il sort one out maybe later .not too good on pc . how far should light be from plant