Cryogenically Cooled Infrared Cameras vs Non-cryogenically Cooled Infrared Cameras

One of the major differences between cryogenically cooled infrared cameras and non-cryogenically cooled infrared cameras is that the materials that detect photons directly, in both photovoltaic infrared cameras and photoconductive infrared cameras, only exhibit this “photon detection” capability when they are cooled to very low temperatures. These have to be cryogenic temperatures, which is around 77 degrees Kelvin. When these materials are warmer than cryogenic temperatures, they don’t work.

On the other hand, microbolometers do not have to be cooled in order to work. They can operate at or near room temperature. While it is useful to maintain a stable temperature on the microbolometer array, they are considered “uncooled” devices because they do not use cryogenic coolers.

As a general rule, LWIR devices are uncooled and MWIR devices require cryogenic cooling.

Sensitivity and Response Time

As a general rule, photo-detectors (cooled systems) are more sensitive than microbolometer devices. A microbolometer takes slightly longer to respond to changes since it needs to warm up or cool down, depending on the scene. With a photodetector, the signal will appear much faster; this is because the imaging elements (called ‘pixels’) convert photons directly to an electrical signal. It can form an image much faster, since it does not have to allow time for the imaging element to heat up or cool down, as is the case with microbolometers. For some specialty uses such as some research applications, this difference in response time can matter. For most other situations, the difference in imaging rate shouldn’t matter. For general surveillance, either type is often suitable.

The greater sensitivity provided by cooled detectors can be utilized by using smaller, “slower” optics (ie., lenses with higher F/#’s) than those which are used with uncooled detectors, which usually require F/1 optics (or something fairly close). Higher F/# lenses are physically smaller, but allow less energy to reach the imaging array (compared to a larger, low F/# lens of the same focal length). This allows the cooled system to be more compact, which can be a big benefit for a long focal-length system (used for long range imaging). The downside of this, though, is that the cooled detectors are much more expensive and complicated than uncooled devices. In general, cooled detectors (and high F/# lenses) are used in long-range applications, while uncooled detectors (with lenses closer to F/1) are used in short to moderate range situations.

Power, Cost and Reliability

There are additional reasons for choosing between cryogenically cooled infrared cameras and non-cryogenically cooled infrared cameras. Power, cost and reliability are all factors when considering infrared cameras. Cooling a thermal imaging detector requires a closed-cycle cooling system, which can add cost and complexity. In contrast, a non-cryogenically cooled camera has no moving parts and is in a completely solid state. This means less cost, less power, and typically a longer life span.

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