Thermal Cameras: Fundamentals And Applications
THERE’S no doubt the greatest strength of thermal cameras is their ability to see great distances at night. It’s this 24-hour operability without the support of expensive lighting solutions, giving the ability to actually confirm intrusion that makes thermal cameras such a great choice.
These cameras are manufactured by quality makers like Opgal, FLIR, Bosch and Axis, to name a few and they’re particularly useful on large sites which have security officers conducting patrols and responding to alarm events. In concert with response, thermal cameras allow the establishment of a virtual perimeter beyond which intrusions elicit an immediate response. Thermal cameras can also push beyond site boundaries to facilitate pre-intrusion response.
We’ve spoken in recent issues about the importance of thermal cameras in applications like mining, where sites are simply too large to be defended by physical perimeter defences. The same rule applies on medium sized sites where the budget does not extend to extensive installations of artificial light.
Thermals are also useful on sites where heavy tree cover exits right on the boundary or within it. There’s no technology that can detect human presence in shrubbery day or night as effectively as thermal cameras and they are effective in rain and in fog.
Thermal camera technology
The heart of any modern thermal imaging camera is the un-cooled microbolometer – essentially that’s a bolometer fitted as an image sensor. This sensor is responsive to IRE in the 8-13um range and when IRE reaches the microbolometer, it generates a cascade of electrical signals based on resistance which can be analysed to construct an image based on heat signals. When you’re selecting a thermal camera you want coverage of that long 8um to 13um spectral band – that’s the spot you’ll get best penetration of smoke, fog and rain.
The uncooled microbolometer was developed by Honeywell’s boffins in the early 1980s as part of a U.S. defense contract and then declassified and licensed for manufacture in 1992. The microbolometer functions without expensive customized cooling systems employing things like liquid nitrogen or the Stirling cycle.
In very general terms, all microbolometers have certain similar characteristics, though each company licensed to build the technology interprets it independently. Typically, there’s a bottom layer of silicon, a readout integrated circuit and a reflector beneath the sensing layer to reabsorb spilled IRE. There’s also a sacrificial layer thermally isolating the IR material from the integrated circuit and an absorbing layer for the placement of electrical contacts.
Building a microbolometer is a fiddley and expensive businesss with some or all of the fabrication process undertaken with the sensor encapsulated under a vacuum. This means even the most affordable thermal imaging cameras aren’t cheap but they are getting more and more affordable all the time.
In terms of image quality, the more expensive versions deliver 320 x 240 pixels while the cheaper arrays are 160 x 120. Recent releases incorporate 640 x 480 pixels using 25um pixel sizes. At 640 x 480 pixels you are beginning to get serious detail.
The latest thermal cameras have the ability to penetrate fog, smoke and mist, as well as darkness; their small, lightweight size – especially when scene depths are moderate, modest power consumption and their ability to function on power up. Like most passive sensing devices, mean time between failure is lengthy. A thermal camera installed in a protective housing might last decades.
Another strong feature of thermal cameras is the fact they give both analog (mini-BNC) and digital outputs and that means the unit will stream video on a LAN with viewing and control handled by a Windows software application. Basic control features include Freeze/Live, Horizontal and vertical flip, Black-Hot/White-Hot, Color palettes control, Video recorder, NUC control, Histogram display, Automatic Contrast Enhancement, Gamma correct ion and digital zoom.
From a security management point of view, the great strength of thermal video cameras is there ability to view things that are not visible to the human eye without expensive lighting solutions. Better still, the image streams from these thermal cameras can be viewed on any CRT of VGA monitor just like an ordinary surveillance camera.
According to David Lee, applications engineer at FLIR Systems, thermal cameras don’t suffer from the basic limitations of visible light imaging.
“First, thermal cameras make pictures from heat, having nothing to do with reflected light energy. They see the heat given off by everything under the sun,” he explains.
“Everything encountered in daily life creates a heat signature that a thermal imager can see.”
Lee says that not only does everything have a heat signature, but these heat signatures create their own contrast.
“What’s more, the thermal energy seen by thermal cameras generally creates a better image at night than during the day. They work just fine during the day — as long as there is the tiniest bit of temperature contrast between an object and its background, you can see it — but they work best at night. And night time, as we all know, is when security professionals need the most help to see.”
Lee says thermal cameras offer security managers not just protection but safety, too.
“The first thing security professionals need to do when contemplating indoor thermal dome cameras is to change their mindset about the cameras’ use,” he says. “Thermal dome cameras play as big a role in safety as they do in security.
“When the lights go out, thermal dome cameras let security and safety professionals survey buildings, quickly and safely ensuring that everyone is out of the area.”
Lee says thermal cameras see clearly through smoke — especially long wave cameras that use inexpensive VOx detectors — and so they are perfect tools for finding people in smoky rooms and buildings.
“Another indoor use for thermal cameras is door access control. Few conditions are more challenging for a visible light camera than to be inside a building and looking out an external access door,” Lee says.
“When someone opens that door, the visible light camera quickly becomes oversaturated with light until its automatic gain controls can adjust. Then the door closes, and the camera is plunged into darkness.
“Because thermal cameras see heat, not light, they don’t have any of these limitations. Coupled with a simple video analytics tripwire alarm, a thermal dome camera can give instant and reliable feedback of door access.”
Lee says some people are still stuck in the mindset that thermal security cameras are only used for long-range applications and are so expensive that only governments can afford them.
“Thermal cameras see during the day, at night, and in conditions of limited visibility like smoke,” Lee says. “And, they’re inexpensive enough to be used anywhere.”
According to Axis Communications’ Phil Doyle (regional manager, Europe), the integration of thermal cameras into the conventional video surveillance market is not free from challenges – technical, legal and others.
“Regardless of these restrictions, resolutions are generally much lower for thermal cameras than for conventional network cameras,” he explains. “This is primarily due to the more expensive sensor technology involved in thermal imaging.
“However, lower frame rate is less of a problem in most surveillance applications since thermal cameras are first and foremost used for detection and not for identification.
“More troublesome, at least from an economic point of view, is that there are no standard optics for thermal cameras. Any adaptation of focal length or a special required field of view must be done at the factory. The reason why regular optics and lenses, such as a standard CS-mount or C-mount, cannot be used is that ordinary glass efficiently blocks thermal radiation.”
According to Doyle, manufacturers have to rely on other materials. Presently, Germanium is most used for thermal camera optics. He says this very expensive metalloid, chemically similar to tin and silicon, blocks visible light while letting thermal radiation through.
“Naturally, the same requirements apply for housings, making it impossible to use standard camera housings for outdoor installations. Like lenses, housings must thus be specially adapted for thermal cameras,” he says.
“Although the investment costs are high, thermal cameras are not unknown within security and surveillance and are used in high security buildings and areas, such as, nuclear power plants, prisons, airports, pipelines, and sensitive railway sections.
“So far, however, incorporating thermal cameras into a conventional video surveillance system has not been a straightforward operation. With the development of thermal network cameras, compatibility will of course, be far less of an issue. New devices will more easily integrate with existing video management systems,” Doyle explains.