Most people misunderstand the concept of infrared imaging, in particular, laypersons due to movies where an infrared camera is used to see through walls.
Anything above absolute zero (-273.15°C) emits heat (infrared energy) as a function of its temperature. A thermal imager measures temperature by assessing the heat radiated by objects based on the Stefan-Boltzmann law: L=εσT^4
L - Radiation; σ - Stefan-Boltzmann’s constant; ε - Emissivity of the object's surface.
The radiation gets collected selectively through the imager’s IR window, arrives at the IR detector, and gets transformed into an electrical signal that is processed into visible images on the screen. The thermal imager can detect even minute differences in temperature depending on its detector’s sensitivity.
1. Can it see in the dark? Yes.
A thermal imager can detect the heat from objects whether there is light or not. Visible light wavelengths are short, but a thermal imager can detect the long wavelength of the emitted energy. Thermal imagers are sensitive to heat radiation coming from objects that are warmer than the surrounding environment.
A thermal image captured in the dark
2. Can a thermal imager see through walls, concrete, or rocks? No.
But it can detect something inside walls that causes a notable temperature difference. Thermal imagers can be used in building maintenance inspections such as the detection of moisture penetration and missing insulation without breaking open the wall to spot the problem source.
A thermal imager cannot see the inside of the building but the geometric wall structure.
3. Can it see through glass? No.
If you point a thermal imager at a window, only the reflected temperature of the surrounding objects can be seen instead of seeing through it since glass is highly reflective like a mirror for infrared radiation, which also applies to polished metal.
The fingers behind the glass cannot be seen by the thermal imager but by the reflection of the surrounding objects.
4. Can it see through plastic? Mostly, it depends.
Most thin plastics allow infrared radiation to pass through, but thicker ones will block the IR.
5. Can it see through metal? It can see through metal? No.
Like glass, smooth metals will reflect the temperature of the surrounding objects, so it’s not easy to monitor pipes or machinery for overheating parts unless you paint some materials with low reflectivity and high thermal conductivity on the metal surface.
However, thermal imagers can reveal hot spots, cold spots, and the liquid level inside a metal container due to its high conductivity.
It can detect the hot water level in the iron pot.
6. Can it see ghosts? No, never be misled by movies.
[1] Boltzmann, Ludwig (1884). "Ableitung des Stefan'schen Gesetzes, betreffend die Abhängigkeit der Wärmestrahlung von der Temperatur aus der electromagnetischen Lichttheorie" [Derivation of Stefan's law, concerning the dependency of heat radiation on temperature, from the electromagnetic theory of light]. Annalen der Physik und Chemie (in German). 258 (6): 291–294.