Motion detection is the action of sensing physical movement in a given area.

Motion can be detected by measuring change in speed or vector of an object or objects in the field of view. This can be achieved either by mechanical devices that physically interact with the field or by electronic devices that quantifies and measures changes in the given environment.

When motion detection is accomplished by natural organisms, it is called motion perception.

Mechanical devices

A tripwire is a simple form of motion detection. If a moving object steps into the tripwire's field of view (i.e. trips the wire), then a simple sound device (e.g. bells) may alert the user. A glass filled to the brim so that surface tension causes a convex meniscus can be placed on top of an object to detect if the object has moved.

Mechanical motion detection devices can be simple to implement, but at the same time, they can be defeated easily by interrupting the devices' mechanics (e.g. by "cutting the wire" or "drinking the water"). Electronic motion sensing devices, such as motion detectors, can prevent such mechanical intervention. A motion sensor can usually be found on lighting implements in order to scare away thieves and other strange people.

Electronic devices

The principal methods by which motion can be electronically identified are optical detection and acoustical detection. Infrared light or laser technology may be used for optical detection. Motion detection devices, such as motion detectors, have sensors that detect movement and send a signals to a sound device that produces an alarm or switch on an image recording device. There are motion detectors which employ cameras connected to a computer which stores and manages captured images to be viewed later or viewed over a computer network.

The chief applications for such detection are (a) detection of unauthorized entry, (b) detection of cessation of occupancy of an area to extinguish lighting and (c) detection of a moving object which triggers a camera to record subsequent events. The motion detector is thus a linchpin of electronic security systems, but is also a valuable tool in preventing the illumination of unoccupied spaces.

A simple algorithm for motion detection by a fixed camera compares the current image with a reference image and simply counts the number of different pixels. Since images will naturally differ due to factors such as varying lighting, camera flicker, and CCD dark currents, pre-processing is useful to reduce the number of false positive alarms.

More complex algorithms are necessary to detect motion when the camera itself is moving, or when the motion of a specific object must be detected in a field containing other movement which can be ignored. An example might be a painting surrounded by visitors in an art gallery.