High-intensity discharge (HID) lamps are similar to fluorescent lights . The main advantage of HID lighting is its high efficacy when used for high power floodlighting. As is the case with fluorescent lighting, HID lighting technology has advanced rapidly in recent years, with improvements in both light quality and efficiency. Although some HID lamps are significantly more efficient than fluorescent lamps, the improvement is not very great in the low wattage sizes that are most useful for homes. Of the three HID light sources that are suitable for residential settings (mercury vapor, high-pressure sodium,and metal halide) mercury vapor is the most common, but the light quality is relatively poor and the energy efficiency the lowest, High-pressure sodium lights offer the greatest efficiency, but the light is yellowish in color. Metal halide is slightly less efficient than high-pressure sodium, but the light is much whiter— closer to the light from incandescent lights. All HID lamps require ballasts to operate, and most of these ballasts require relatively long warm-up periods,these lights are not practical for short intermittent use.