Which motion detector works best to catch intruders who are walking across its field?

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

Answer: (D)

Motion detectors rely on sensing changes in the environment, and for someone walking across an open field, a sensor that monitors heat changes works best. A passive infrared detector detects infrared energy emitted by bodies. As a person with warm body heat moves through the sensing field, the pattern of infrared energy shifts across the detector’s zones, and that change is interpreted as motion, triggering the alarm. This makes infrared ideal for catching human-sized movement across a wide, unobstructed area since it responds directly to the moving heat signature rather than requiring light, sound, or a camera image. Other types rely on vision, sound reflections, or microwave energy and can be more prone to false alarms from weather, animals, or lighting changes. Infrared’s heat-based approach aligns naturally with detecting someone walking across the field.