600w Powerplant SuperVeg Metal Halide Bulbs are used for the veg-phase of plant growth. When in vegetative growth plants prefer blue spectrum light which these bulbs put out. For flower plants need red spectrum light which these bulbs do not put out. The benefit of metal-halide lamps is that plants do not stretch as much and instead remain very squat and develop more shoots.
Metal halide bulbs run on the same equipment as your main sodium flowering light so all you need to do is swap the bulbs. Just remember that you can only use the same wattage as your sodium light though.
600w Powerplant SuperVeg Metal Halide Bulb will fit any reflector with an E40 lamp holder and require a 600w ballast or powerpack to run.