Sensing Opportunities: Pros and Cons of PIR and Microwave Approaches
Chuck Ross
Lighting the great outdoors requires a bit more thought now than it did a couple decades ago. The goal used to be simply packing as many lumens as possible into a landscape fixture. That was the entire point. Photocell sensors might have been used to ensure lights didn’t stay on during daylight hours, but keeping outdoor space fully illuminated – even when no humans were around – was seen as a key safety measure.
These days, we’re much more conscious of the impacts of both light pollution and energy inefficiency. The natural rhythms of wildlife can be upset when night looks too much like daytime, and smaller animals can depend on the secrecy of dark skies to move about without being spotted by predators. And, with electricity prices climbing, no one wants to pay the power bill for lighting that’s not really needed – and many jurisdictions now have energy codes in place requiring outside lights to turn off when no one is around. Both of these factors have led to motion sensors becoming a common outdoor fixture feature – but with both passive infrared (PIR) and microwave technologies available, which approach makes the most sense in exterior applications?
As with many such decisions, the answer turns out to be “it depends.” The two systems use different kinds of information to decide when luminaires should be switched on. PIR devices act on changes in temperature within their sensing field. So, body heat from a person or animal triggers them to send a power-on signal to system controllers. Microwave sensors act like radar, emitting a constant stream of microwave energy that radiates off objects within their sensor field and then measure the bounce-back of that energy to determine when movement is occurring.
There isn’t a clear winner between these two options that’s best for every setting. For example, PIR devices are less expensive and more efficient, so they might be a better option in lower-cost battery or solar-powered fixtures. Plus, because they work by sensing heat, they won’t be triggered by false-positive signals, such as swaying tree branches. But PIR sensors also require a direct line-of-sight to operate, so they aren’t useful if there are any obstructions between them and approaching individuals. And they don’t work as well in high temperatures, because human body heat doesn’t stand out as well.
Microwave device signals, on the other hand, can penetrate through obstructions, with 360-degree functionality within their detection field. They also can be set to work constantly, unlike PIR products that stop sensing when lights turn on and need to reset when internal timers turn lights off again, even if someone is still nearby. However their increased sensitivity can lead to false positives if trees or other objects start moving and they are typically more expensive the PIR options.
As this explanation shows, placement is an important issue when installing fixtures with either sensing technology. For PIR lighting, consider what objects might obstruct sensors’ direct line-of-sight capabilities, or if ambient temperatures might pose a challenge to their operation. With microwave solutions, the challenge is whether nearby inanimate objects, like tree branches or blowing leaves, might trigger lights to be turned on unnecessarily. With these caveats in mind, PIR-based controls might be best in smaller, enclosed areas like patios or doorways, while microwave-based options might be a better choice in wide-open areas like parking lots and sports fields, or where barriers might otherwise cause problems for PIR operation.
These days, we’re much more conscious of the impacts of both light pollution and energy inefficiency. The natural rhythms of wildlife can be upset when night looks too much like daytime, and smaller animals can depend on the secrecy of dark skies to move about without being spotted by predators. And, with electricity prices climbing, no one wants to pay the power bill for lighting that’s not really needed – and many jurisdictions now have energy codes in place requiring outside lights to turn off when no one is around. Both of these factors have led to motion sensors becoming a common outdoor fixture feature – but with both passive infrared (PIR) and microwave technologies available, which approach makes the most sense in exterior applications?
As with many such decisions, the answer turns out to be “it depends.” The two systems use different kinds of information to decide when luminaires should be switched on. PIR devices act on changes in temperature within their sensing field. So, body heat from a person or animal triggers them to send a power-on signal to system controllers. Microwave sensors act like radar, emitting a constant stream of microwave energy that radiates off objects within their sensor field and then measure the bounce-back of that energy to determine when movement is occurring.
There isn’t a clear winner between these two options that’s best for every setting. For example, PIR devices are less expensive and more efficient, so they might be a better option in lower-cost battery or solar-powered fixtures. Plus, because they work by sensing heat, they won’t be triggered by false-positive signals, such as swaying tree branches. But PIR sensors also require a direct line-of-sight to operate, so they aren’t useful if there are any obstructions between them and approaching individuals. And they don’t work as well in high temperatures, because human body heat doesn’t stand out as well.
Microwave device signals, on the other hand, can penetrate through obstructions, with 360-degree functionality within their detection field. They also can be set to work constantly, unlike PIR products that stop sensing when lights turn on and need to reset when internal timers turn lights off again, even if someone is still nearby. However their increased sensitivity can lead to false positives if trees or other objects start moving and they are typically more expensive the PIR options.
As this explanation shows, placement is an important issue when installing fixtures with either sensing technology. For PIR lighting, consider what objects might obstruct sensors’ direct line-of-sight capabilities, or if ambient temperatures might pose a challenge to their operation. With microwave solutions, the challenge is whether nearby inanimate objects, like tree branches or blowing leaves, might trigger lights to be turned on unnecessarily. With these caveats in mind, PIR-based controls might be best in smaller, enclosed areas like patios or doorways, while microwave-based options might be a better choice in wide-open areas like parking lots and sports fields, or where barriers might otherwise cause problems for PIR operation.
