Definition
Passive millimeter-wave (PMMW) imaging is a method of forming images through the passive detection of naturally occurring millimeter-wave radiation from a scene. Although such imaging has been performed for decades (or more, if one includes microwave radiometric imaging), new sensor technology in the millimeter-wave regime has enabled the generation of PMMW imaging at video rates and has renewed interest in this area. This interest is, in part, driven by the ability to form images during the day or night; in clear weather or in low-visibility conditions, such as haze, fog, clouds, smoke, or sandstorms; and even through clothing. This ability to see under conditions of low visibility that would ordinarily blind visible or infrared (IR) sensors has the potential to transform the way low-visibility conditions are dealt with. For the military, low visibility can become an asset rather than a liability.

In the commercial realm, fog-bound airports could be eliminated as a cause for flight delays or diversions. For security concerns, imaging of concealed weapons could be accomplished in a nonintrusive manner with PMMW imaging. Like IR and visible sensors, a camera based on PMMW sensors generates easily interpretable imagery in a fully covert manner; no discernible radiation is emitted, unlike radar and lidar. However, like radar PMMW sensors provide penetrability through a variety of low-visibility conditions (moderate/heavy rainfall is an exception). In addition, the underlying phenomenology that governs the formation of PMMW images leads to two important features. First, the signature of metallic objects is very different from natural and other backgrounds.

Second, the clutter variability is much less in PMMW images than in other sensor images. Both of these characteristics lead to much easier automated target detection with fewer false alarms. The wide range of military imaging missions that would benefit from an imaging capability through low-visibility conditions, coupled with its inherent covertness, includes surveillance, precision targeting, navigation, aircraft landing, refueling in clouds, search and rescue, metal detection in a cluttered environment, and harbor navigation/surveillance in fog. Similarly, a number of civilian missions would benefit, such as commercial aircraft landing aid in fog, airport operations in fog, harbor surveillance, highway traffic monitoring in fog, and concealed weapons detection in airports and other locations. This article introduces the concept of PMMW imaging, describes the phenomenology that defines its performance, explains the technology advances that have made these systems a reality, and presents some of the missions in which these sensors can be used.

Overview of millimeter wave radiometry:
The regime of the electromagnetic spectrum where it is possible for humans to see is that part where the sun's radiance peaks (at about 6,000 K): the visible regime. In that regime, the human eye responds to different wavelengths of scattered light by seeing different colors. In the absence of sunlight, however, the natural emissions from Earth objects (at about 300 K) are concentrated in the IR regime. Advances in IR-sensor technology in the last 30 years have produced detectors sensitive in that frequency regime, making night vision possible. The exploitation of the millimeter-wave regime (defined to lie between 30 and 300 GHz, with corresponding wavelengths between 10 and 1 mm) follows as a natural progression in the quest to expand our vision. The great advantage of millimeter-wave radiation is that it can be used not only in day and night conditions, but also in fog and other poor visibility conditions that normally limit the "seeing" ability of both visual and IR sensors.