Which detector type relies on sensing changes in infrared energy from moving objects?

Which detector type relies on sensing changes in infrared energy from moving objects?

• A. Photoelectric detector
• B. Ultrasonic detector
• C. Passive infrared detector
• D. Microwave detector

Answer: (C)

Movement is detected by sensing changes in infrared energy, which is the heat radiated by objects. A passive infrared detector does exactly that: it monitors the infrared radiation in its field of view and triggers when a moving person or animal causes a noticeable change in the heat pattern. It’s called passive because it doesn’t emit energy itself; it only receives energy already present in the environment. In contrast, the other detectors use different principles: photoelectric detectors rely on light to detect smoke or obstructions, ultrasonic detectors use sound waves to sense movement, and microwave detectors emit microwaves and detect motion through reflections and Doppler shifts.

Movement is detected by sensing changes in infrared energy, which is the heat radiated by objects. A passive infrared detector does exactly that: it monitors the infrared radiation in its field of view and triggers when a moving person or animal causes a noticeable change in the heat pattern. It’s called passive because it doesn’t emit energy itself; it only receives energy already present in the environment.

In contrast, the other detectors use different principles: photoelectric detectors rely on light to detect smoke or obstructions, ultrasonic detectors use sound waves to sense movement, and microwave detectors emit microwaves and detect motion through reflections and Doppler shifts.