Infrared Detection Devices

Infrared detection devices are sensors that detect radiation in the infrared portion of the electromagnetic spectrum (≅10¹² to 5 × 10¹⁴ 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 × 10¹⁴ to 7.5 × 10¹⁴ Hz) is scarce or absent. Because the warmer an object it 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. Many devices used by police investigators, including forensic examiners, exploit some form of infrared detection technology.

Infrared—"below-red"—light consists of electro-magnetic 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 × 10¹⁴ to 4.0 × 10¹⁴ Hz) is termed near IR, while light farther from the visible spectrum is divided into middle IR, far IR, and extreme IR.

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 been developed 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 the manufacture of focal-plane arrays with good resolution feasible. 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.

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. Scrutiny of the recorded data from such surveillance cameras can be useful in piecing together the course of nighttime events in a forensic investigation.

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 determine precisely the time of death of a body less than 15 hours old 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.