Thermal imaging cameras gained military interest late in the Vietnam era. The sensitivity of the detectors to IR radiation allows them to acquire images in darkness and through airborne obscurants such as dust and smoke. These same capabilities are creating interest for thermal imaging within law enforcement and related emergency services.
The visible spectral region, which runs between 400 and 780 nm, is significantly smaller than the IR spectral range, which runs roughly from 780 nm to 100 µm. For purposes of this discussion, we can further break down the IR spectrum into four regions: near IR (NIR; 780 nm to 1.3 µm), mid-wave IR (MWIR; 3 to 5 µm), long-wave IR (LWIR; 7 to 14 µm), and very-long-wave IR (30 µm and above). IR radiation suffers water absorption, which is why cameras are not designed to be sensitive in the so-called water window between 1.3 and 3 µm, where the effect is most pronounced.
Thermal imagers used in law enforcement and fire detection applications are designed primarily for the MWIR and LWIR spectral regions, driven primarily by application-specific requirements for performance, cost, compactness, and portability. hot thermal technology
In the 1990s, a few police agencies embraced the capability to see at night and to locate people with thermal imagers by installing systems on helicopters, and later on the top of police cruisers. Size, weight, and cost of available cameras limited the growth of this market segment.
Figure 1. Improvements to electronics and sensors have resulted in a recent evolution in package size for portable thermal cameras used in law enforcement. RAYTHEON COMMERCIAL INFRARED
Today, improvements to camera packaging, displays, optics, sensors, electronics, and power consumption have resulted in pocket-sized cameras that run on two AA batteries and weigh less than a pound. Prices have fallen dramatically, too: the first helicopter systems exceeded $100,000, but vehicle packages and portable units are now available for approximately $7000 (see figure 1). The Law Enforcement Thermographers Association now recognizes 11 separate application areas, including search and rescue, fugitive searches, vehicle pursuits, flight safety, marine and ground surveillance, perimeter surveillance, officer safety, structure (building) profiles, disturbed surface scenarios (tracks), environmental law enforcement, and hidden compartments in vehicles.
The biggest breakthrough in thermal imaging for law enforcement applications came with the advent of the uncooled thermal camera. The most mature IR-sensing technologies use expensive, cryogenically or Stirling-cooled semiconductor materials such as HgCdTe or InSb. Cooling the sensors stabilizes their IR sensitivity while increasing the contrast of the acquired images. These IR sensors generate signals based on photon absorption and are complex, expensive, and bulky. Even "uncooled" versions of these cameras often require stabilization of detector temperature to reduce pixel-to-pixel variation.
In contrast, newer uncooled focal plane array (FPA) sensors such as amorphous silicon and vanadium oxide (VOx) microbolometers and barium strontium titanate (BST) ferroelectric thermal imagers operate differently. Such sensors detect changes in capacitance or resistance within the semiconductor material caused by incoming IR radiation rather than converting electromagnetic radiation directly to electrons. The devices operate near room temperature, a characteristic that leads to reduced system complexity, size, and cost. As the sensors absorb incoming IR radiation, they detect minute changes in resistance, in the case of microbolometers made of VOx or amorphous silicon; or capacitance, as with the BST ferroelectric sensors.
Image quality is obviously an important attribute of a thermal camera and one that is very difficult to quantify. The ambient environment and the nature of objects being observed serve to complicate subjective evaluations of image quality. Although images from one camera may appear more pleasing than another in rarified conditions, the system may not perform as well in the intended application. So, what must we consider in evaluating image quality? quantifying quality
Much has been said in the industry about the differences between microbolometer sensors and ferroelectric sensors. The more technically inclined will wish to look beyond simple declarations of superiority to what makes a sensor effective for various situations.
A common parameter for camera specifications is the noise equivalent temperature difference (NETD), a measure of thermal sensitivity versus temporal noise. Most uncooled cameras offer NETD values of approximately 100 mK or 0.1°C. While percent differences in NETD from one camera to another may seem significant, other factors contribute to image quality. The level of steady-state or fixed-pattern noise introduced by the detector can degrade image quality, making an imager inappropriate for a specific application, for example. Because this noise is fixed or slowly varying, it has a much greater adverse impact on the image than the transient noise indicated by the value of the NETD.
Figure 2. When all factors including those due to DC coupling (left) and AC coupling (right) are combined, ferroelectric sensors often provide more useful images than those of microbolometers that may nominally have better NETD and MTF numbers.
The practice of operating ferroelectric sensors in an AC-coupled mode versus the DC-coupled mode common to microbolometers shows one means of alleviating image degradation due to fixed pattern noise. AC coupling refers to the removal of the fixed offset in an electrical signal so that the remaining signal is centered at zero (see figure 2). In comparison, a DC-coupled system must process the full amount of signal and differentiate between relatively small fluctuations (i.e., the AC portion of the signal). An AC-coupled thermal imaging system has to deal only with the smaller AC signal. The reduced signal-processing burden allows it to extract fixed-pattern noise with greater facility.
AC coupling also has the capability of handling substantially greater temperature variations in the scene. If the scene never changes, no AC component to the signal exists. AC-coupled systems therefore incorporate a chopper that continually alternates views between a reference scene and a focused scene. DC-coupled systems perform a similar comparative function by periodically shuttering the system at some time interval from a few seconds to a few minutes, usually resulting in a short image freeze upon shuttering. In general, AC-coupled systems simplify the processing of highly dynamic scenes and more robustly remove the fixed-pattern noise that is so deleterious to image quality.
The modulation transfer function (MTF) serves as another metric for thermal imagers. Within the IR sensor or detector, the MTF provides a measure of pixel-to-pixel isolation. At the camera level, the MTF also characterizes the quality of the optical system and gives an overall measure of image acuity.
Thermal-imaging engineers often attempt to combine multiple parameters into a composite figure of merit known as minimum resolvable temperature difference (MRTD). The MRTD essentially measures the level of thermal contrast needed for an observer to distinguish alternating hot and cold bars in a test target. The MRTD is a function of the target spatial frequency (the width and spacing of the bars in the target). For comparing different systems, we generally use a target or bar size that is exactly equal to the width of a detector pixel; the frequency of this target is defined as f0 (see table). Ultimately, we must judge image quality in a representative application environment and in conjunction with other system attributes that determine utility for law enforcement needs. through a lens, hotly
Optics and lens selection is another important consideration for IR cameras. When designing optical elements for MWIR systems, the material of choice is silicon, while LWIR systems require germanium. Dominant parameters for material choice include transmission and dispersion. With the appropriate spectral coatings applied to the optics, a typical single-element lens of silicon provides about 98% transmission in the 3 to 5 µm range. Likewise, a single-element germanium lens has approximately 95% transmission in the 7 to 14 µm range. Transmission numbers typically refer to average transmission values over the wavelength band, so the narrower band for MWIR cameras contributes significantly to the comparatively greater transmission.
Optical dispersion, like transmission, depends on wavelength. When a lens system optimally focuses an incoming ray bundle, the spread of passed wavelengths is in focus at a range of points in front of, on, and behind the sensor focal plane. A lens made of mildly dispersive material minimizes the axial depth of this range of points to create less blur and higher image acuity. Germanium offers both high transmission and low dispersion in the LWIR spectral range. The material also exhibits very good transmission in the MWIR, but its dispersion is much higher than at longer wavelengths, far exceeding that of silicon.
IR optical materials, particularly germanium, offer significant cost challenges to designers. In the quest for more economical LWIR cameras, engineers now focus their efforts on developing materials that perform similarly to germanium without the prohibitive material cost. One example has been the alloying of germanium with other proprietary materials to achieve comparable dispersion characteristics and transmission in excess of 90%. Resultant bulk material cost is less than half that of germanium. designing for the future
In addition to considering component tradeoffs, engineers developing thermal imagers for law enforcement should consider the systems-level design of the camera itself. With no viable thermoplastic optical materials available for the MWIR and LWIR bands, cameras must shield glass lenses to minimize vulnerability to shock damage. Electronics require protection from water and dust. Condensed moisture on lens elements can absorb long wavelength IR energy, which points toward sealed or vented designs. Minimizing control interfaces and entrances into camera bodies aids both the ruggedness and the simplicity of designs.
Such design concerns force engineering tradeoffs in the design process. Automatic contrast and brightness controls minimize the need for user adjustments but may sacrifice image optimization in some settings. A law enforcement officer's safety frequently hinges on quick access to visual information. Devices must provide at least one operating mode that requires little or no adjustment on the officer's part.
While thermal imaging technology has made great strides since cameras were first mounted to helicopters, law enforcement users will require further developments. New higher-resolution FPAs under development hold the promise of increases from the 160 x 120 and 320 x 240 arrays common today to 640 x 480 imaging arrays. At the same time, pixel pitch is decreasing from 50 to 25 µm. To allow users to take full advantage of the increased resolution, manufacturers must develop new optical elements with reduced transmission losses and distortion, improved MTF performance, and optimized bandpass for specific IR wavelengths. The true capabilities for thermal imaging technology in law enforcement are only beginning to emerge. oe
UV fights crime
Law enforcement personnel and crime scene investigators (CSI) have long used photonic tools such as low-light cameras and night vision devices for surveillance purposes. More recently, night vision devices have been taken into crime scenes after the fact to find and image latent evidence by spraying a chemiluminscent substance known as Luminol to create images from virtually anything, including trace amounts of blood. In many cases, however, the presence or absence of blood alone may not be strong enough for a conviction or an acquittal. Fingerprint evidence placing a suspect at a crime scene remains invaluable.
Reflected UV imaging systems (RUVIS) provide a detection method that can image latent fingerprints, body fluids, shoeprints, and even bite marks on human bodies that would otherwise be invisible to the naked eye; the UV nature of the system suppresses the background image.1 In RUVIS systems, UV illumination reflects off of residues in fingerprint ridges or other raised surfaces?the method has even detected shoe prints on a waxed floor. Numerous cases in law enforcement have been solved by using a RUVIS approach to find evidence at a crime scene or on a crime victim after the CSI team has finished examining it using other methods.
The method requires a source with high output in the short UV range (below 300 nm), preferably one that is easily portable, has a flexible light guide to aim the light, and provides uniform output. Of course, the user must take reasonable precautions to avoid eye and skin exposure to strong UV light.
One key requirement for such good image quality is a strongly solar blind, or at least well-filtered, intensifier tube that lacks visible light sensitivity, allowing personnel to operate the unit under normal interior or exterior lighting without detector saturation.
Another important consideration is minimizing distortion. Electrostatic intensifiers may introduce 5 to 7% of pin-cushion distortion at the edge of an image, which could be argued in court as having altered a suspect's fingerprints. Proximity-focused intensifiers offer improved image distortion.
The short-wavelength UV light source and solar blind intensifier that comprise a RUVIS can detect evidence on most non-porous surfaces and do not require the use of powders or chemicals; however, enhanced results have been obtained from objects suspected to hold latent evidence by fuming them with cyanoacrylate (Super Glue). The output of the intensifier can be imaged by eye, by digital camera, or fed via a relay lens to a TV camera recording system.
Using RUVIS technology, crime scene investigators can obtain high-quality images even on non-porous objects that may have printing on their surface, such as soda cans. In recent years, such systems have also been used in conjunction with Luminol to enhance blood detection. They are also being used to detect residue from explosives, which may open the door to various homeland security applications.
-David Fatlowitz, Hamamatsu Corp.
1. Michael West, Robert Barsley, et al., Journal of Forensic Identification, 40, 249-255 (1990).
Stan Kummer is the market manager for the Raytheon Commercial Infrared Industrial and Public Safety Division, Dallas, TX.