Going thermal

As features are enhanced and prices decline, the technology is finding wider application


The increasing emphasis on homeland security — especially as it relates to ports and other large sections of critical infrastructure — has resulted in the development of technological tools that have enabled end-users to address security-related problems that were seemingly beyond their reach a decade ago. Examples of these technological advances are recognizable in any current security system design and include video analytics, IP-addressable security devices and Power over Ethernet enabled by IEEE Standard 802.3af.

However, thermal imaging cameras stand out as a particular class of technology that enables a new and effective approach to classic intrusion detection heat signature. Those of us familiar with traditional CCTV cameras that operate in the visible light spectrum expect the thermal imaging cameras (often referred to as infrared or IR cameras) to simply be an extension of the infrared sensitivity in modern digital cameras. At first glance, both cameras receive electromagnetic radiation from a source, detect and process it with a microelectronic chip, and pass the constructed video image to any of the current CCTV viewing and storage devices. However, the changes from the expected model are anything but subtle.

The essential characteristic of thermal imaging devices is that they operate by detecting the heat emitted by an object. This object could be a human body or a car whose engine is or has been running. The emitted heat signature by an object can be attenuated but normally not completely eliminated by fog, smoke, rain or other obscurants. This enables these devices to see the thermal image even when the visible light image is completely obscured. Examples of this are illustrated in the photos below where the visible and thermal images are compared side-by-side.

Light Spectrum

The chart on Page 32 illustrates the electromagnetic spectrum, ranging from the far infrared to the far ultraviolet. The world in which you and I as security practitioners tend to operate in is the visible light spectrum — with wavelengths ranging from approximately .4 to .7 micrometers (microns). Thermal imaging cameras work in the infrared range with wavelengths ranging from approximately 1 micrometer to more than 13.5 microns. Depending on the specific thermal imaging technology used, the infrared spectrum is further subdivided into medium wavelength, ranging from 3 to 5 microns, and long wavelength, ranging from 8 to approximately 15 microns. Most thermal imaging cameras used in security applications operate in the long wavelength portion of the spectrum. Thermal imaging devices used in military and in some long-range border detection applications operate in the medium wavelength range.

Lenses

Lenses on thermal imaging cameras are also different. Typical lenses for cameras operating in the visible light range of the spectrum are made of either glass or some plastics. However, glass is essentially opaque to infrared wavelength radiation; and many formulations of plastic used in CCTV lenses also offer poor thermal transmissivity in the infrared wavelength range. Therefore, lenses on thermal imaging cameras are usually made either of germanium or zinc selenide, which, interestingly enough, are opaque to visible light but highly transmissive in the infrared spectrum.

Detector

The CCD and CMOS sensors typically used in visible light cameras are sensitive to what is referred to as the near infrared (NIR .8 to 2.5 micrometers) portion of the infrared spectrum. This explains the nighttime performance of many current technology cameras in low light (not no light) at night with the IR cut filter removed.

Thermal imaging cameras operate in a different portion of the spectrum using different detection technologies. Thermal imaging cameras operating in the long wavelength range use a chip called a microbolometer. Infrared radiation strikes the detector material in the microbolometer, causing its temperature to rise, thus changing its electrical resistance. This change in electrical resistance can be correlated to the temperature of the object initially emitting the radiation and is therefore used to create a visual image. Microbolometers do not require cooling.

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