The idea of motion detection is a fairly simple one. A sensor is set up to detect a certain disturbance in the area, and when that disturbance is found, the motion detector sends out an electronic signal. Often that signal will travel through a security system to alert the other technology there's motion present. This signal can do everything from setting off alarms and turning on lights to flicking the switch on a camera and turning it to face the direction of the motion that's been sensed. Just what variety of motion sensor is used by a security system, or by a stand-alone camera, depends on what sort of motion will be captured.
One of the most popular forms of motion detection is infrared detection. Commonly called "heat vision," this type of motion detector works by monitoring an area for the ambient heat that every living thing gives off. If enough infrared radiation is detected by a sensor, then the sensor will activate a camera. The camera may also be swivel mounted; it will either begin to scan the area, or it will focus in on the place where the infrared sensors said there was an unusual amount of heat.
Another variety of motion detector that's been paired with security cameras is microwave motion detection. A microwave sensor sends out microwaves, and when those microwaves encounter an object, they get bounced back to the sensor to help create an image of the area that the sensor is viewing. When there is motion in the area, the signals the microwave sensor receives change. This change causes the sensor to activate the camera to view and record what's happening. There are some motion detector cameras that use a combination of both infrared and microwave sensors. These models are more expensive than either technology individually, but they're also much more likely to catch intruders than either technology on its own.
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