Which detector technology is most effective for intruders moving toward a motion detector?

• A. Microwave
• B. Infrared
• C. Ultrasonic
• D. Photoelectric

Answer: (A)

Movement toward a sensor changes the reported signal in a way that microwave radar detects most reliably. Microwave detectors emit microwaves and listen for the Doppler-shifted reflections created by a moving object. When someone moves straight toward the detector, the component of their velocity along the beam is largest, producing a clear, strong Doppler signal that rises above background noise and clutter, making the device most effective at detecting approaching intruders.

Infrared detectors sense changes in heat across their sensing elements, so they’re more sensitive to motion that crosses the field of view rather than directly toward the sensor, which can produce a weaker response. Ultrasonic detectors use sound waves and can detect movement in any direction, but they can be more susceptible to room conditions and interference. Photoelectric beam sensors rely on interrupting a light beam between two points rather than monitoring a room for moving heat or sound, so they aren’t the same kind of motion detector and aren’t specifically optimized for an intruder approaching a detector.