The rate at which electromagnetic radiation energy is emitted per unit-time, per unit-area of the surface spanning all wavelengths and in all directions. The emissive power is proportional to the absolute temperature to the fourth power, $E_{\text{b}}=\sigma T^4$, where subscript ‘b’ refers to black body radiation and σ is the Stefan–Boltzmann constant with a value of 5.67 × 10⁻⁸ Wm⁻² K⁻⁴.