Short answers, pulled from the story.
Macedonio Melloni created the first known thermocouple capable of detecting a person from ten meters away in 1833. He worked alongside Leopoldo Nobili to refine the Seebeck effect into a practical instrument. This early device did not produce a picture but rather a reading that proved invisible heat existed beyond the red end of the visible spectrum.
The first electronic television camera sensitive to infrared radiation was invented in 1929 by Hungarian physicist Kálmán Tihanyi. He developed the technology for anti-aircraft defense in Britain. While Tihanyi's work was theoretical, the first American thermographic camera was developed in 1947 by the US military and Texas Instruments.
The first commercial thermal imaging camera was sold in 1965 for high voltage power line inspections. This sale marked the transition of infrared technology from the battlefield to the industrial sector. The device allowed maintenance technicians to locate overheating joints and sections of power lines which are a sign of impending failure.
A material's emissivity can theoretically range from 0 to 1 representing its ability to emit thermal radiation. An example of a substance with low emissivity would be silver with an emissivity coefficient of 0.02 while an example of a substance with high emissivity would be asphalt with an emissivity coefficient of 0.98. This property makes accurate temperature measurement difficult because the camera integrates multiple sources of data in the areas surrounding the object to estimate its temperature.
Cooled detectors are typically contained in a vacuum-sealed case or Dewar and cryogenically cooled to temperatures ranging from 60 Kelvin to 100 Kelvin. Without cooling these sensors would be blinded or flooded by their own radiation requiring energy-intensive and time-consuming cooling systems like Stirling cryocoolers. The camera may need several minutes to cool down before it can begin working and the cooling apparatus may be comparatively bulky and expensive.
The first smartphone with an integrated thermal camera was the Cat S60 released in 2016. This release brought the technology to the general public for hundreds of US dollars. In 2013 DARPA announced a five-micron LWIR camera that uses a 1280×720 focal plane array pushing the boundaries of resolution and sensitivity.