How IR sensor works?

IR detector

(InfraRed detector) A sensing device that picks up radiation in the infrared band. Used extensively for still and video night vision cameras, cooled IR detectors can sense the makeup of nebulae light years in the distance. The IR industry is maturing rapidly and is expected to make devices that detect a myriad of conditions, including irregularities in machinery, ice on aircraft wings and faults in circuit boards.

Infrared detection works because most everything in our known world gives off heat, which emits radiation in the infrared band.

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