Motion sensing or intrusion detection is the event that starts everything in a well-designed camera-based security system. Currently, two methods of motion detectors are most common:


The camera watches an area and records at a certain framerate, each frame (or several frames) are analyzed by the camera’s internal software that can detect the difference between frames. If something is different, a motion is detected. They can be overcome with utilizing shadows and exploiting backgrounds with a solid color. That, of course, can be solved with using thermal imaging and/or installation-specific measures (eliminating shadows with controlled light).

Security systems that use attached or internal optic-based (or software-based) motion detection should be used with great caution in anything but the most basic applications, and never outdoors. The limitations of the types described for demanding external situations are as follows.

  • Will not cope with moderate changes in light levels.
  • Sporadic generation of alarms in high contrast scenes.
  • Will not cope with changing weather conditions.
  • Lack of size discrimination means compromise in setting up.
  • Non-uniform sensitivity with range.
  • Will not cope with size variation due to perspective.
  • Slow processing speed can miss moving action.
  • Inability to discriminate between small high contrast dark and large low contrast objects.
  • Prone to false alarm due to camera shake.
  • Cell measurements prevent accurate area discrimination.
  • Restricted to small areas of view.
  • Unlikely to detect a person at 10% of screen height.
  • Only simple algorithms can be computed.
  • Cannot distinguish between a person moving in a line and a waving object.
  • Single processor increases time between frame comparisons.

PIR (Passive Infrared)

PIRs measure the change in the energy of the area in view. They’re prone to false positives due to sensitivity to environmental changes like hot/cold air flow and sunlight. They are prone to missed or false alarms:

  • Moving plants and trees.
  • Reflected sunlight from windows, windshields, or other highly-reflective objects.
  • Facing in the direction of the sun.
  • Movement outside the intended control area.
  • Weather such as rain, hail, and snow.
  • Lack size discrimination.
  • Inconsistent range based on external factors.

In most environments, the only effective motion detection system for outdoor use is the detachable or remote PIR sensor. The sensor can be aimed at a fairly small area such as a gate or walkway that is not affected by the limitations of the PIR sensor. The camera can then be positioned to view a much broader area without being affected by these limitations.