Definition
A passive sensing technology used in some Enhanced Flight Vision Systems (EFVS) that detects natural electromagnetic energy emitted by objects and surfaces in the millimeter wavelength band (roughly 1 to 10 millimeters), and converts those emissions into a real-time image of the scene ahead of the aircraft. Because it senses energy at wavelengths that pass through fog, cloud, and precipitation more readily than visible light, it can produce a usable image of the runway environment in low-visibility conditions.
Plain English
A type of sensor that 'sees' the heat-like energy that everything naturally gives off at very short radio wavelengths. It builds a picture from that energy, and because those wavelengths cut through fog and cloud better than light does, the pilot can see the runway when the eyes alone cannot.
Context Anchor
Seen in discussions of enhanced flight vision systems used during instrument approaches, especially when comparing different sensor types that may help a pilot see the runway environment in reduced visibility.
Derivation
Millimeter refers to the wavelength of the energy being detected — about 1 to 10 millimeters long, much shorter than typical radio waves but longer than infrared. Radiometry comes from Latin radius (ray) and Greek metron (measure), so it literally means 'measuring rays.' The sensor measures the natural rays of energy coming off objects, rather than sending out a signal of its own.
Why Pilots Care
It supplies usable runway imagery when infrared is blocked by fog or precipitation, supporting EFVS operations to lower decision heights.
Analogy
Think of it like a camera that is tuned to a kind of energy your eyes cannot see. Instead of relying on sunlight or runway lights, it measures energy coming from objects and builds a display from that.
Grounding Statement
On a low-visibility approach, this type of sensor may help the aircraft display outside features by sensing energy that can get through some conditions that block normal sight.
Intuition Check
Do not read “millimeter” as meaning the system measures tiny distances. Here it describes the wavelength of the energy being sensed. Do not assume it is radar. Radiometry is passive: it listens for naturally emitted energy instead of sending out a signal and waiting for a return.
Example Sentence 1
The aircraft's EFVS uses millimeter wave radiometry to display the runway environment on the HUD when fog reduces natural visibility below approach minimums.
Example Sentence 2
Because millimeter wave radiometry is unaffected by certain cloud layers, the crew maintained visual reference to the approach lights.