Thermal radiation involves energy transfer via electromagnetic waves, typically as infrared radiation.

All objects continually absorb and re-radiate thermal energy. This re-radiated energy is what is seen by night vision goggles.

In the steady state, the amount of energy received equals to the amount given off so that the temperature remains constant.

For an object at temperature T (Kelvin) and a surface area A, the net rate of radiated energy depends strongly on temperature:

P_net = P_rad - P_abs = σεA(T⁴ - T_env⁴).

where
T_env is the temperature of the surrounding environment,
σ = 5.67 x 10^-8 W/m² is the Stefan-Boltzmann constant,
ε is the emissivity of the object.

The emissivity measures how efficiently an object absorbs and emits radiated energy:

• Highly reflective objects have emissivities ε ≈ 0;
• Black objects have emissivities ε ≈ 1;
• An object with ε = 1 is a perfect blackbody,

The best absorbers are the best emitters. This condition arises so that a temperature difference is not generated between a black and a shiny object that are both illuminated by the same source.

Black objects heat up faster than shiny ones, but they cool down faster too.