Here’s an interesting question that comes up from time to time in home automation and home security circles: if I use a security camera with an infrared illuminator, and also use an infrared motion detector (like a PIR sensor) in the same space, will the two interfere?
First off, let’s begin with some background on what this question means. Humans (sadly) can’t see in the dark. Cameras, though, can. If you’ve ever watched Cops, Zero Dark Thirty, or pretty much any reality TV show from the late ’90s, you’re familiar with so-called “night vision” systems. These systems work by using a camera which is sensitive to infrared light. Humans can’t see infrared light, but there’s usually at least a bit of it out there, even in a space which looks totally dark to us. “Night vision” cameras turn this light into visible light, letting you see in the dark.
Here’s the great thing about infrared light, though. Since (again) we can’t see it without a special camera, you can flood a space with infrared light, and the space will still appear totally dark to a person. This is what an infrared illuminator does. It’s like a giant spotlight, filling a space with light that’s invisible to the naked eye. Put a camera in the space, and suddenly the camera can see great, even if people in the space are totally blind.
Understandably, people often use infrared cameras as part of home security systems. Install a camera in your hard, use an infrared illuminator to light up the space, and suddenly you can see the guy crouching in your azalea bush.
Lots of home security cameras now come with infrared illuminators (or IR illuminators for those in the know) built in. You’ve probably seen these as a ring of faintly-glowing red LEDs around a surveillance camera.
So what’s the issue with these illuminators? Well, most home security systems use another infrared tool to detect intruders: infrared motion sensors. These sensors work by reading the “passive” infrared energy (mostly heat) emitted by living beings, just as a night vision camera uses this energy to make those beings visible. Infrared motion sensors usually contain two sensing elements, and a lens which divides incoming infrared light between the elements.
If the level of infrared light hitting one element is higher the the level hitting another, it probably means that something in front of the sensor is moving around. If the two sensing elements are reading the same level, then the infrared light in the whole space in front of the sensor is probably constant, and the sensor infers that nothing is moving around.
Already, you can probably see where the trouble arises when you use infrared illuminators and infrared motion detectors as part of the same home security system. One system is pumping out tons of infrared light, and the other is looking for subtle changes in infrared light.
So do they interfere? The short answer is: sort of. In general, the systems don’t totally cancel each other out, and many people do use them together. Remember, an infrared motion detector is looking at changes in infrared light across a scene. Flooding the scene with infrared light doesn’t necessarily result in one part of a scene having more light than another; all parts of the scene have move, so there is no contrast in light for the sensor to detect. Infrared illuminators do, however, tend to make infrared motion detectors react slower to movement.
Why? Imagine, for a moment, that you’re part of an experiment. Someone tells you that you’ll stare at a room, and when the room gets brighter, you should raise your hand. When you first start, the room is dark. The experimenter turns on a light. You raise your hand immediately. Now, imagine that the same room is bathed in sunlight. Again, the experimenter turns on a light. Since the room is so bright to start with, it’ll probably take you a lot longer to notice the little bit of extra light that the experimenter has added. You might take a lot longer to raise your hand, or you might miss the subtle change entirely.
The same thing happens when you point an infrared illuminator at an infrared motion detector. When the detector is bathed in infrared light to begin with, it’s a lot harder to detect the subtle change in infrared light caused by a person moving in front of the sensor. The sensor might be slower to respond to movement, or if it’s a cheap sensor, it may not respond at all.
So if you’re trying to use both systems as part of your home security system, what should you do? One obvious solution is to not use both systems together in the same space. But if you really need motion detection and night vision cameras as part of the same security system, you can always use another motion detection technology. The two biggies are microwave and ultrasound systems. Both work by using sound or radio waves, the Doppler effect, and fancy math to detect motion. Both are pricier than infrared motion detectors, but they should work more reliably.
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