Definition
A sensing method in which a device emits its own infrared energy toward a target and then measures the infrared radiation reflected back from that target. The system supplies the infrared source rather than relying on heat naturally radiated by the object being detected.
Plain English
The device shines its own invisible heat-light onto something and then reads what bounces back. Because it provides the light itself, it can detect objects that aren't warm enough to be seen by a passive heat sensor.
Context Anchor
Seen in discussions of aircraft or airport sensing equipment, security detectors, and some obstacle-detection systems.
Derivation
Active here means the system itself produces the energy used for detection, as opposed to passive systems that only receive energy already given off by the target. Infrared comes from the Latin infra (below) and red, meaning the band of light just below red on the visible spectrum -- invisible to the eye but felt as heat.
Why Pilots Care
Active infrared systems can detect objects that emit little heat of their own, but because they transmit energy, they can also be detected by other sensors. Knowing whether a system is active or passive matters for understanding its capabilities and limitations.
Analogy
It is like using a flashlight in a dark room: the flashlight sends out light, and your eyes see what reflects back. Active infrared detection does a similar thing with invisible infrared light.
Intuition Check
Do not read active as simply “turned on.” Here, active means the system sends out its own infrared light and then detects the result.
Example Sentence 1
The aircraft's active infrared detection system illuminated the area ahead with infrared energy and read the reflected return to identify obstacles in the dark.
Example Sentence 2
Active infrared detection allowed the crew to identify a small drone against the dark sky.