If so .. if you google , you will see that cannabis can indeed suffer from light burn , but there is alot of people that have they bulbs pretty close , my advice to you , is keep the bulb as you please , just keep an eye on the top leaf's(with your HPS off you can have a better look at the health of you plants , hps bulb's can trick your eyes while on) if they show any signs of light burn or any stunted grow , as long as they look fine i really think you are ok and you can keep it that close.
Just keep a close watch on your exhaust system , cause any malfuntion in the fan with the lamp that close can cause irreversible damage and ruin your ladys.
Do you use a reflector ? if soo watch if having the bulb too close you are not losing light in the sides of your canopy.
