Unlike optical imaging, where sunlight reflected from objects is observed, or SAR imaging, which relies on objects reflecting radio signals transmitted from the sensor, thermal imaging observes the heat emitted by the objects themselves.
Heat emissions by objects are visible in the thermal infrared (TIR) bands – either mid-wave infrared, at wavelengths from 3000 to 8000 nm, or long-wave infrared (LWIR) from 8000 to 14000 nm.
Generally, the higher the temperature of an object, the shorter the wavelength of the thermal radiation it emits. This means surfaces hotter than their surrounding environment would appear “brighter”, allowing the sensor to discern areas with higher temperatures from those with lower ones.
Mid-wave infrared is at shorter wavelengths, which makes it possible to achieve higher imaging resolutions – although imaging remains susceptible to clouds, which absorb TIR wavelengths.