Using HPS Bulbs for lighting will require a reflector otherwise over 50% of your light will be wasted!
When your grow light emits photons, those photons travel in a straight line until they hit something — in this case, that something is your grow light reflector. More specifically, they are hitting the reflective surface of your reflector.
To maximise the efficiency of your reflector, choose one with as high a reflection index as possible. High quality anodised aluminium and vapor deposition aluminium are the choice for most serious growers which include the adjusta wings and maxibright refelctors.