Most people misunderstand the concept of infrared imaging, in particular, laypersons due to movies where an infrared camera was used to see through walls.
Anything above absolute zero (-273.15°C) emits heat (infrared energy) as a function of its temperature. Thermal imager measures temperature by assessing the heat radiated by objects based on Stefan-Boltzmann’s 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. It can 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. Thermal imager can see through the wall, concrete, or rock? No
But it can detect something inside the wall that causes a notable temperature difference. Thermal imagers can be used in building maintenance inspections such as detections 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 the geometric wall structure.
3. It can 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 the reflection of the surrounding objects.
4. It can see through plastic? Mostly, it depends.
Most thin plastics allow infrared radiation to pass through, but thicker ones will block the IR.
5. 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.