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Thermal cameras: Cooled and uncooled sensors

Thermal cameras: Cooled and uncooled sensors

The mainstream impression of thermal imaging usually conjures up simple images of black backgrounds with white or colored silhouettes of people, but this advanced technology is much more than that. With the ability to read heat signatures of humans and objects, the benefits of thermal technology have been long utilized by the military sector for its accuracy and range. In a security setting, the ability of thermal imaging to accurately detect intruders, whether they are human or inanimate objects, makes thermal cameras a valuable asset in securing a location. Unfortunately, high prices have kept the thermal security market on ice. However, as prices for thermal sensors and equipment begin to melt, the thermal security market is expected to see substantial growth.

Cooled Versus Uncooled
Thermal imaging cameras can be broadly divided into two categories based on the sensor used: cooled and uncooled.

Cooled infrared sensors typically operate slightly under room temperature. In the event that the sensor does not cool off, the sensor risks being flooded by its own thermal radiation, causing the sensors to be blinded. Because of this need to cool off, cooled thermal cameras are integrated with a device called a cryocooler. This device lowers the sensor temperature to cryogenic temperatures (-150 °C, -238 °F, or 123 K), which reduces the level of thermally-induced noise to below that of the signal from the scene that is being surveyed. Thermal cameras that use cooled image sensors are more expensive to manufacture, require more maintenance, and consume more energy for ventilation. Furthermore, when starting up the camera, a wait time of up to several minutes may be needed for it to cool down before it can be used. Even though cooled equipment may be bulky and expensive, they can produce crisper, higher resolution images than uncooled cameras.

Unlike cooled thermal cameras, uncooled cameras do not require expensive and bulky cryocoolers. The sensor in an uncooled thermal camera is stabilized at or close to room temperature, using less complicated temperature control elements. These sensors are able to stabilize the changes of resistors, voltage, and power when infrared radiation causes the temperature to rise. In this case, uncooled thermal cameras can reduce thermally-induced noise in a stable working temperature even though they are not cool, like their cooled counterpart. Despite their lower resolution and image quality in comparison to cooled cameras, uncooled thermal cameras are smaller and more economical.

With improvements in the manufacturing process, most uncooled thermal cameras can now work with CCD sensors and CMOS focal plane arrays (FPAs). As a result, inexpensive uncooled FPA micro-bolometers have become more widely adopted. Although their resolution is generally lower than conventional visible cameras, that are approximately at 160 x120 or 320 x 240 pixels, enhanced versions of 17μm (640 x 480 pixel option), 25μm (384 x 288 pixel option), and sensitivity capability of less than 50mk noise equivalent temperature difference (NETD) thermal imagers have been introduced.

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