Infrared radiation (IR) is electromagnetic radiation with longer wavelengths than those of visible light. This means infrared radiation is invisible to the human eye. That said, most of the thermal radiation emitted by objects near room temperature is infrared. Therefore, at times when visible light is scarce, infrared detectors may be used to establish an image based on the infrared radiation emitted by objects.
Passive Infrared Vs Active Infrared: The passive infrared detector does not emit an infrared ray – it just detects the infrared radiation emitted by objects within its range. Active infrared detectors require a transmitted and receiver. The transmitter actively projects an infrared ray which reflects off a target, and is picked up by the receiver.