Performance metrics & physics
Blackbody Radiation
Blackbody radiation is the thermal radiation emitted by any object due to its temperature; warm interior surfaces in a sun-baked car emit blackbody radiation in the mid-to-far infrared.
Once heated by sunlight, interior surfaces re-emit energy as blackbody radiation. The wavelength distribution depends on temperature — a 60 °C dashboard emits primarily in the 8–12 μm band (far infrared).
This re-radiated thermal energy is what keeps cabins hot long after the sun moves off the car. Window film doesn't reflect this re-emitted heat outward (the glass doesn't transmit it), but by reducing the initial heating it cuts the magnitude of re-radiation.
For architectural applications, low-e films can be tuned to reflect blackbody radiation back into a room (winter heating) or out (summer cooling).