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Evaluate LEDs for Possible Use in Outdoor Safety and Security Lighting

By Maryellen Lo Bosco - April 2013 - Lighting


An important error to avoid is ignoring developments in technology. That doesn't mean trying unproven products. Rather, it means evaluating new choices to determine if they are a good choice for a specific application.

A good example is LEDs (light emitting diodes). According to Eric Richman, senior research engineer at the Pacific Northwest National Laboratory, there is a wide range in the quality of LED products on the market, but high-quality LED lighting does afford a number of advantages in an outdoor setting.

For one thing, LED lights come on immediately, unlike HID (high-intensity discharge) lights such as metal halide or high- or low-pressure sodium, which need time to warm up. Some HID lights can take up to 30 minutes to get to full luminescence. As a result, HID lights generally do not work well with occupancy sensors, because once the lights go off, they need re-strike time to come back on. "There are some metal halide lights with quartz re-strike lamps that can work with occupancy sensors," says Richman. "If the light goes out and comes back on, the quartz lamp provides a reasonable level of light until the metal halide fully kicks in."

Quality LED lighting also performs well in cold temperatures, which can be an advantage over HIDs or fluorescent lights. In moderate climates, where the temperature is cool to cold in the evening, LEDs are performing in their optimum environment. By contrast, HIDs need more time to warm up in colder climates. Fluorescent lights also have trouble warming up in the cold and may not reach their full output when they do. "In really cold climates you need specifically rated fluorescent bulbs for colder temperatures," Richman notes.

LED lights can also provide an advantage when it comes to uniformity of illumination. Unlike any other type of light, LEDS have many point sources of light within the fixture. Thus, it is possible to take the diodes and aim them a row at a time or even one diode at a time. "You can tilt them so that the ones on the outside [of the board on which the diodes are arrayed] are aimed at an outward angle, while others are aimed straight down," says Richman. "This provides more uniform illumination across a wide area." It's harder to do that with a single-point source of light. "You can bend the reflector that's behind the light source," explains Richman, "to try to get more rays to shoot out to the side and fewer to shoot straight down, and you can put a refracting lens in front of [the light source], but this decreases light efficiency."

Every light source will depreciate over time. Technically, LEDs can work for a very long time, but they will degrade like other lights, says Richman, although at a slower rate. Still, longevity is another reason LEDs are a good choice for outdoor lighting, despite their higher cost. "They have more longevity than HIDs — either sodium lamps or metal halides," says Richman.





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