As the name suggests, thermal imaging makes use of the difference in temperature between an object and its surroundings. Thermal radiation occurs in wavelengths of about 900 to 1,400 nanometers, above that of visible light. This band is called the infrared (pronounced infra-RED, or IR) spectrum. There are many applications for thermal imaging outside of law enforcement and military operations, such as finding leaky areas in the insulation of a house or detecting overheating and failure-prone connections on high voltage power lines. Thermal imagers are also called thermographic cameras, bolometers, or microbolometers.
It's easy to confuse thermal imaging with IR cameras and photography, as they share similar terminology. Some consumer still and movie cameras come equipped with IR filters and illuminators for low-light photography, and surveillance cameras use IR light to monitor dimly lit areas. This type of infrared technology uses a shorter wavelength band below the thermal zone, called near-infrared. Near-infrared light occurs around 700 nanometers, just above visible red light.
IR cameras don't "see" temperature differences-they see IR light reflected back to the lens. In order for IR imaging to work, there has to be a source of IR light. Most IR cameras have IR emitters arranged around the lens to produce this light. People can usually see active emitters in the dark if they look directly at them. Seen with bare eyes, the emitters show a dull red glow. If you happen to be wearing IR goggles, they'll show up like car headlights. For this reason, IR night vision has limited effectiveness. It's great if you're the only one using it, but when the other guys have it too, you might as well turn on the overhead lights and sirens.
Thermal imaging used in law enforcement applications is passive, in that there is no IR source at the viewing end. A man in dark clothing hiding in a tree at night may be all but invisible to the naked eye, and even difficult to spot if a flashlight is turned on him. To a thermal imager, the body heat coming off his skin and through his clothing is as bright as a road flare in a cave.
High-end thermal imagers have color displays showing temperature gradients across the entire observed area, and are very expensive. Law enforcement is usually more concerned with "warm" vs. "not warm," so the equipment is simpler and not nearly as costly.
Traditionally, thermal imaging gear has been large, heavy, and expensive. The most sensitive equipment required mechanically cooled housings with Stirling engines or liquid nitrogen to keep their sensors chilled, and that added to weight, bulk, and reliability. The newest equipment doesn't require cooling and brings the size and weight down comparable to a handheld telescope or binocular.
At last year's IACP show, FLIR debuted its H-Series Compact Tactical Thermal Night Vision Camera. The handheld monocular device, about the size of a soda can, runs on rechargeable AA batteries that provide about four hours of use. An included AC charger restores the batteries in two hours. The standard Patrol model has a 24-degree field of view, and an optional 2X extender provides extra magnification. There is also a long-range model with a 7-degree field of view. Resolution on all models is 320x240 pixels-about one-quarter of a standard TV display.
The upgraded Command model incorporates still and video image recording capability. Data is saved to a standard SD card, with all images encoded with the date and time captured. Incorporated into the Command model is a radio transceiver that will send the camera output to a receiver or monitor up to 30 feet away.
List price for the H-Series Patrol model is $6,248, with a street price around $5,000.