Metal halide light bulbs and high pressure sodium light bulbs are in a class of bulbs called “HID” or “high intensity discharge” bulbs. According to LightBulbsDirect.com, in HID light bulbs, arcs of electric current pass between electrodes in a tube that is pressurized, which vaporizes metallic additives resulting in large amounts of light emissions.
Despite the large amounts of electricity needed to maintain the reactions in HID lighting, the amount of light produced per watt of power used in unparalleled by any other artificial light source.
The intensity of light given off by a light source is measured in lumens. A 250-watt metal halide light bulb can have a mean lumen output of around 58 lumens per watt of power used. A 250-watt high-pressure sodium light bulb can have a mean lumen output of around 87 lumens per watt of power used.
Both metal halide and high-pressure sodium light bulbs are rated to last for many hours. Metal halide light bulbs are rated to last for anywhere between 10,000 and 15,000 hours. High-pressure sodium light bubs are rated to last for anywhere 12,000 and 24,000 hours.
While both types of light bulbs are efficient for the amount of power they draw, high-pressure sodium light bulbs have the slight advantage in terms of efficiency, light output and lifespan.