Infrared Detection Devices
█ LARRY GILMAN
Infrared detection devices are sensors that detect radiation in the infrared portion of the electromagnetic spectrum (>10 12 to 5 × 10 14 Hz). Often, such devices form the information they gather into visible-light images for the benefit of human users; alternatively, they may communicate directly with an automatic system, such as the guidance system of a missile.
Because all objects above absolute zero emit radiation in the infrared part of the electromagnetic spectrum, infrared detection provides a means of "seeing in the dark"—that is, forming images when light in the visible portion of the spectrum (>4.3 × 10 14 to 7.5 × 10 14 Hz) is scarce or absent. Because the warmer an object is, the
more infrared radiation it emits, infrared imaging is also useful for the detection of outstanding heat sources that may be invisible or hard to detect even when there is ample visible light (e.g., exhaust heat from ships, tanks, jets, or rockets). Many devices used by police, security, and military organizations, including user-wearable, gunmounted, vehicle-mounted, missile-mounted, and orbital systems, exploit some form of infrared detection technology.
Principles of infrared detection. Infrared—"below-red"—light consists of electromagnetic radiation that is too low in frequency (i.e., too long in wavelength) to be perceived by the human eye, yet is still too high in frequency to be classed as microwave radio. Infrared (IR) light that is just beyond the human visual limit (>1.0 × 10 14 to 4.0 × 10 14 Hz) is termed near IR, while light farther from the visible spectrum is divided into middle IR, far IR, and extreme IR. Military and security systems utilize mostly near IR and a narrow band in the far IR centered on 3.0 × 10 13 Hz, because the Earth's atmosphere happens to be transparent to IR radiation primarily in these two "windows."
All objects above absolute zero glow in the far IR, so no source of illumination is needed to image scenes using such radiation; to image scenes in near IR, illumination from a light-emitting diode or filtered light bulb must be supplied. Near-IR imagers, however, are still cheaper than passive, far-IR imagers.
There are two basic designs for electronic IR imagers. The first is the scanner. In this design, light from a tiny portion of the scene to be imaged is focused by an optical and mechanical system on a small circuit element that is sensitive to photons in the desired IR frequency range. The intensity of the signal from the IR detector element is recorded, then the mechanico-optical system shifts its focus to a different fragment of the scene. The response of the IR detector element is again recorded, the view shifts again, and so forth, systematically covering the scene. Many scene-covering geometries have been employed by scanning imagers; the scanner may record horizontal or vertical lines (rasters), spiral outward from a central point, cover a series of radii, and so on.
The second basic type of IR imaging system is the "starer." Such a system is said to "stare" because its optics do not move like a scanner's, scanning the scene a little bit at a time; instead, they focus the image onto an extended focal plane. Located in this plane is a flat (planar) array of tiny sensors, each equivalent to the single IR sensor employed in a scanning system. By measuring the IR response of all the elements in the flat array simultaneously (or rapidly), the system can record an entire image at once. Image resolution in a staring scanner is limited by the number of elements in the array, whereas in a scanning system it is limited by the size of the scanning dot.
Hybrid designs, in which partial or entire scan-lines are sensed simultaneously by rows of sensors, have also been developed. Chemical films have also proved useful for IR imaging, but these are rarely used today.
The earliest IR imagers, built in the 1940s, 1950s, and 1960s, were scanners. Starers were not technologically feasible until the early 1970s, when large-scale circuit integration made possible the manufacture of focal-plane arrays with good resolution. As integrated-circuit technology has been refined, focal-plane arrays have become cheaper. Starers have many advantages, including greater reliability due to the absence of moving parts, quicker image acquisition, and freedom from internally-produced mechanical vibration.
Formerly, both scanners and starers needed to be cooled by liquid nitrogen in order to keep the sensor from blinding itself with its own IR radiation. In recent years, however, uncooled IR imagers, both scanners and starers, have been increasing in quality and decreasing in price.
Military applications. Aircraft, ground vehicles, surface ships, human beings, industrial facilities, rockets, and warheads entering the atmosphere are some of the objects of military interest that emit IR radiation in telltale patterns. To exploit these patterns, a wide array of military IR systems have been developed. For example, "heatseeking" missiles that home in on the IR-bright gasses emitted by jet-aircraft engines have been commonplace since the 1950s. Heat-seeking missiles have also been developed for use against surface vehicles and ships. Also, starting in the early 1960s, military IR-imaging satellites have been observing the Earth to detect the IR emissions of rocket and missile launches, and modern proposals for ballistic-missile defense depend heavily on space-and ground-based IR detectors that will track missiles and warheads as they arc through space. The U.S. military is currently designing a new system of satellites dedicated to tracking missiles using IR imaging, the Space Based Infrared System. According to the United States Air Force, this system will have a "unique capability to track missiles throughout their trajectory—not just during the 'hot' boost phase [when IR emissions from the missile are most intense]—allow[ing] the system to effectively cue missile defense systems with accurate targeting data."
Various IR camera systems for "seeing in the dark" are also commonplace. These may be mounted on vehicles or at stationary locations to allow nighttime surveillance of a fixed area. Night-vision systems worn on helmets and mounted on portable weapons usually do not operate by sensing IR; instead, they amplify the visible light already present in a dark scene. Hence they are sometimes called "starlight" vision systems.
IR imaging is being investigated for use in the detection of landmines. Antipersonnel mines are typically buried only a few centimeters below the surface, so the heat radiation (IR) pattern of an area can, under some conditions reveal their presence.
Police and security applications. The security of a building or area of land from intruders is often enhanced by cameras that image the perimeter of the secure area and can be monitored by personnel in a central office. At night, such systems must either be supplied with illumination or must be capable of IR imaging. Visible-light camera systems are cheaper and easier for human users to interpret; however, because excess illumination of an area by visible light ("light pollution") is sometimes a concern, and because security forces may wish to keep an area under surveillance without making their presence known, IR systems are widely used for perimeter security and other surveillance tasks.
IR imaging has many other uses in police and security work besides surveillance. Aerial IR imaging can track vehicles, show which vehicles in a parking lot have arrived most recently, distinguish heated buildings, and locate buried structures (e.g., clandestine chemical laboratories) emitting heat through vents. IR images can be used to precisely determine the time of death of a person deceased for less than 15 hours or to detect document forgery by revealing subtle mechanical and chemical disturbances of the original paper and ink. The power consumption in a building can be estimated in real time by observing the IR radiation emitted by the power transformer on the pole outside; modifications to walls or automobiles are often obvious in IR images; and IR images can reveal such visually inconspicuous features of crime scenes as use of cleaning solvents to remove blood, drag-marks across carpets, fresh paint, and explosives residues.
Countermeasures. IR imaging, like all surveillance and targeting technologies, has given risen to a thriving countermeasures field. IR countermeasures fall into three broad categories: blinding, decoys, and concealment. Blinding refers to the use of IR lasers to overload an enemy's imaging detectors, as for example those of an approaching missile. Decoys are heat sources released in the vicinity of a target heat source (e.g., aircraft or ship) in order to reduce the chances that an approaching missile will home in on the true target. For example, a system named the AN/ALQ-156(V)2 Missile Approach Detector is standard on several U.S. military aircraft. This system uses radar to scan for approaching heat-seeking missiles. When one is detected, the AN/ALQ-156(V)2 automatically activates the M-130 General Purpose Dispenser System, which releases a flare from the aircraft. The flare emits more IR radiation than the aircraft itself, hopefully distracting the missile from the aircraft.
Since most proposals for ballistic missile defense include interceptor missiles that home in on the heat signature of ballistic warheads approaching from space (and thus IR-bright against a cool background), thought has also been given to the question of "infrared stealth" measures for ballistic-missile warheads. One possibility is "shrouding," which would involve the placement of a close-fitting cap over the cone-shaped warhead. The cap would consist, essentially, of a hollow aluminum shell filled with liquid nitrogen and kept aloof from the warhead itself by insulated supports. The liquid nitrogen would cool the exterior of the warhead, reducing its IR emissions to low levels and making it difficult or impossible for the heat-seeking system of an interceptor missile to locate. The exterior coating of the warhead could be made of a radar-absorbing material, making the warhead radar-stealthy as well.
█ FURTHER READING:
Schlessinger, Monroe. Infrared Technology Fundamentals. New York: Marcel Dekker, Inc., 1995.
Carter, L. J., et al. "Thermal Imaging for Landmine Detection," in proceedings from Second International Conference on the Detection of Abandoned Land Mines, IEEE 110–114, 1998.
Maki, M. C., and M. C. Dickie. "New Options in Using Infrared for Detection, Assessment and Surveillance," in proceedings from the International Carnahan Conference on Security Technology, IEEE 12–18, 1996.
Riedel, R. B., J. S. Coffin, and F. J. Prokoski. "Forensic Uses of Infrared Video," in proceedings from the International Carnahan Conference on Security Technology, IEEE 108–112, 1992.