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2. Passive infrared sensor - Wikipedia

   en.wikipedia.org/wiki/Passive_infrared_sensor

   A passive infrared sensor (PIR sensor) is an electronic sensor that measures infrared (IR) light radiating from objects in its field of view. They are most often used in PIR-based motion detectors . PIR sensors are commonly used in security alarms and automatic lighting applications.

3. Microwave radiometer - Wikipedia

   en.wikipedia.org/wiki/Microwave_radiometer

   A microwave radiometer consists of an antenna system, microwave radio-frequency components (front-end) and a back-end for signal processing at intermediate frequencies. The key element is the Dicke switch, which alternately switches between the antenna and a cryogenic load at a known temperature.

4. Atmospheric sounding - Wikipedia

   en.wikipedia.org/wiki/Atmospheric_sounding

   There can be a variety of sources for a passive instrument, including scattered radiation, light emitted directly from the sun, moon or stars—both more appropriate in the visual or ultra-violet range—as well light emitted from warm objects, which is more appropriate in the microwave and infrared.

5. Motion detector - Wikipedia

   en.wikipedia.org/wiki/Motion_detector

   A passive infrared detector mounted on circuit board (right), along with photoresistive detector for visible light (left). This is the type most commonly encountered in household motion sensing devices and is designed to turn on a light only when motion is detected and when the surrounding environment is sufficiently dark.

6. Microwave imaging - Wikipedia

   en.wikipedia.org/wiki/Microwave_imaging

   Microwave imaging is a science which has been evolved from older detecting/locating techniques (e.g., radar) in order to evaluate hidden or embedded objects in a structure (or media) using electromagnetic (EM) waves in microwave regime (i.e., ~300 MHz-300 GHz). [1]

7. Security alarm - Wikipedia

   en.wikipedia.org/wiki/Security_alarm

   A passive infrared sensor used to detect motion The passive infrared (PIR) motion detector is one of the most common sensors found in household and small business environments. This sensor does not generate or radiate energy; it works entirely by detecting the heat energy given off by other objects.