black-body radiation

Black-body radiation (thermal emission or thermal radiation) is electromagnetic radiation within and surrounding a body in thermodynamic equilibrium at a constant, uniform temperature. This produces a continuous spectrum (a continuum emission, the black-body spectrum) whose shape is dependent only on the temperature of the body.

Planck's law (Planck function) describes the spectrum:

I(ν,T) = 2hν3/c2 × 1/(ehν/(kT)-1)

• I(ν,T) is power per unit area per solid angle in normal direction.
• h - Planck constant.
• c - speed of light in a vacuum.
• k - Boltzmann constant.
• ν - frequency of the electromagnetic radiation.
• T - temperature of the body in kelvins.

The above equation represents an ideal case, i.e., assuming the thermodynamic equilibrium and uniform temperature, but all materials produce radiation associated with their temperature, perhaps rightly called thermal emission or thermal radiation, and informally, black-body radiation. The ideal case and its equation represent a useful first approximation.