What are the two classes of IR detectors?

The correct classification of infrared (IR) detectors is based on how they detect IR radiation, which falls into two main categories: active and passive.

Active IR detectors emit their own infrared energy and detect changes in the reflected energy from objects. They require a power source to send out signals, such as lasers or LEDs, and can detect alterations in the environment based on the changes in the return signal.

Passive IR detectors, on the other hand, do not emit their own energy. They rely on detecting the infrared radiation that is naturally emitted by all objects with a temperature above absolute zero. These detectors are commonly used in security systems and occupancy sensors, as they can sense the presence of persons or changes in the heat signature of an area.

The other options—photoelectric and thermal, as well as mechanical and electronic—do not accurately describe the fundamental classification of IR detectors. Photoelectric sensors are related to light detection rather than specifically infrared, and mechanical versus electronic classifications do not align with the operational principles of IR detection. Therefore, selecting active and passive provides the accurate distinction in classes of IR detectors based on their functionality.