Uncooled IR Detectors

Uncooled thermal imaging dates back to the mid-1960's with the pyroelectric vidicon. This product was used in the production of fire-fighting cameras.

Between that time and the mid-1990's, most thermal imaging has been of the cooled variety, meaning that the sensor elements are cooled to -200 C, or -321 F. During this time research into uncooled thermal imaging was pursued in order to reduce cost, weight, and increase reliability. In the mid-1990's cooled arrays were very common, and uncooled arrays were coming out of the research labs and moving into production cameras. The main types of
arrays today are known as microbolometer using vanadium oxide, microbolometer using amorphous silicon, and BST (barium strontium titanate). These three types have made the most significant market change of any uncooled technologies.

Vanadium Oxide Microbolometers

The detector works by measuring the temperature of each of its pixels. The material is designed to change its electrical resistivity as the temperature changes. The temperature of the scene is focused on the material by the lens system. The detector has a quite small mass, and is thermally isolated from its supports so that its temperature changes rapidly with the small amount of focused energy. The material varies in its reaction to temperature across the array, so precise calibration needs to be done by the processing electronics in order to generate a clear picture. So that the calibration data can be measured, it is common to have a shutter that can close off the optics. This calibration is done while the camera is operating every couple of minutes (between 5 and 60, depending on the design). The calibration is objectionable because the image is frozen for approximately 1-2 seconds, while it is being accomplished.

Microbolometers

Microbolometers are generally temperature stabilised by means of a thermo-electric (TE) device. The TE device heats or cools the detector in response to a particular voltage. Within the past 3 years, research has been successful in better understanding the detectors response to temperature. As a result, it is becoming more common that these detectors no longer need temperature stabilisation, and are being delivered to customers without TE devices.
These detectors are used in a variety of roles. There are several manufacturers, and they are spread across the United States and Europe. There are also several suppliers who sell the detector as a component, ready to be coupled with processing electronics, and then designed into a full camera system.
They are generally a little more expensive than BST, however the image quality is often better depending on how the system is designed.

Amorphous Silicon Microbolometers

The largest application for Amorphous Silicon (ASi) has been fire-fighting. Historically, they are lower resolution and lower frame rate (20 Hz) until recently when upgrades have been developed. ASi has been positioned as a lower cost alternative to Vanadium Oxide because it can be made with foundry machinery common for the manufacture of other electrical components, without many of the specialised processes of other detector technologies. To use the detector, the system designer must focus the infrared energy on the detector, which heats the detector elements, just like vanadium oxide detectors. When the elements change temperature, they also change resistivity. The processing electronics collect data about the resistance change to generate the picture of the scene. A shutter is often employed every couple of minutes, which freezes the image for 1-2 seconds, to collect calibration data.

ASi detectors also employ a TE device to stabilise their temperature. Similar to vanadium oxide, designers have developed algorithms to use the detector without a TE device, allowing a further reduction in cost. Typically, ASi detectors have a slight disadvantage to Vanadium Oxide in image quality, while a slight advantage in cost.

BST (Barium Strontium Titanate)

The detector outputs infrared data by looking at the change in temperature over time. Therefore, if the detector were held fixed, looking at a stable scene, it would eventually (over a couple of seconds) show nothing at all, just one shade of gray. To correct this problem, designers added a shutter wheel in front of the detector that spins continuously at a known rate. The wheel alternates between semi-transparent and transparent, which the readout electronics compare to determine the scene. A by-product of this moving disc is that there are fuzzy edges around everything in the scene. The fuzzy edge affects the image quality and the range performance of the system.
BST is an uncooled technology meaning that it does not require cryogenic cooling. It does however require temperature stabilisation. Typically, the detector is warmed slightly and then a regulator keeps the temperature constant.