Uncooled infrared detectors

Uncooled thermal cameras use a sensor operating at ambient temperature, or a sensor stabilized at a temperature close to ambient using small temperature control elements. Modern uncooled detectors all use sensors that work by the change of resistance, voltage or current when heated by infrared radiation. These changes are then measured and compared to the values at the operating temperature of the sensor. Uncooled infrared sensors can be stabilized to an operating temperature to reduce image noise, but they are not cooled to low temperatures and do not require bulky, expensive cryogenic coolers. This makes infrared cameras smaller and less costly. However, their resolution and image quality tend to be lower than cooled detectors. This is due to difference in their fabrication processes, limited by currently available technology.

Uncooled detectors are mostly based on pyroelectric and ferroelectric materials [1] or microbolometer technology. The material are used to form pixels with highly temperature-dependent properties, which are thermally insulated from the environment and read electronically.

Ferroelectric detectors operate close to phase transition temperature of the sensor material; the pixel temperature is read as the highly temperature-dependent polarization charge. The achieved NETD of ferroelectric detectors with f/1 optics and 320x240 sensors is 70-80 mK. A possible sensor assembly consists of barium strontium titanate bump-bonded by polyimide thermally insulated connection.

Silicon microbolometers can reach NETD down to 20 mK. They consist of a thin film vanadium pentoxide sensing element suspended on silicon nitride bridge above the silicon-based scanning electronics. The electric resistance of the sensing element is measured once per frame.

Current improvements of uncooled focal plane arrays (UFPA) are focused primarily on higher sensitivity and pixel density.

Some of the materials used for the sensor arrays are eg.: [2]

vanadium(V) oxide (metal insulator phase change material, for microbolometer arrays)
lanthanum barium manganite (LBMO, metal insulator phase change material)
amorphous silicon
lead zirconate titanate (PZT)
lanthanum doped lead zirconate titanate (PLZT)
lead scandium tantalate (PST)
lead lanthanum titanate (PLT)
lead titanate (PT)
lead zinc niobate (PZN)
lead strontium titanate (PSrT)
barium strontium titanate (BST)
barium titanate (BT)
antimony sulfoiodide (SbSI)
polyvinylidene difluoride (PVDF)