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Energy-efficient outdoor lighting takes two approaches: using the most efficient types of lamps (bulbs), and using efficient controls to determine when the lighting is used.
Metal halide lighting and high-pressure sodium lights are a better option for outdoor lighting because they are 3 to 4 times as efficient as incandescent lights. Metal halide is the best choice when you want lighting that produces natural colors. If color doesn't matter, choose high-pressure sodium lamps. They produce a yellowish light, but they are one of the most efficient types of lamp and they also has the longest life.
Automatic controls can save 20 to 90 percent of lighting costs. The most common type of automatic control is the motion sensor. If you only need light for someone walking by or to discourage prowlers, choose off-the-shelf incandescent light fixtures with built-in motion sensors. In this case the inefficiency of occasionally using incandescent lighting isn't as important as minimizing the on-time of the light.
Both metal halide and high-pressure sodium lights have ballasts that require a warm-up period, and so aren't compatible with motion sensing controls that require the light to come on instantly. These lights should be fitted with photocells or time clocks for efficient automatic control.
Photocells turn lights on when it is dark outside. They work well for controlling individual lights and are cheaper than time clocks. Be sure to shield photocells from other nearby lighting sources that could cause them to turn off during the night.
Time clocks are more expensive, but are the best solution for controlling large groups of lights. Be sure to choose one that compensates for seasonal day-length and doesn't need to be reset after short power outages.
Metal halide and high-pressure sodium are from the high-intensity-discharge lighting family. The lamps have an arc tube that produces a fiery arc.
For more information on lighting, see Residential Energy: Cost Savings and Comfort for Existing Buildings Chapter 7.