A blackbody is a body or gas volume that

• absorbs radiation perfectly, i.e. all incoming radiation at all wavelengths, and
• also has the maximum possible emission in all directions and at all wavelengths at a particular temperature.

Blackbody emission is described by Planck's Law:

where

Eλ	monochromatic radiance [W·m-2·m-1]
λ	wavelength [m]
h	Planck's constant, 6.6260755(40) 6•10–34 [J·s]*
k	Boltzmann's constant, 1.380 658(12)•10–23 [J·K–1]*
c	speed of light, 2.99792458·108 [m·s-1]
T	temperature [K]
	* Source 'Handbook of Chemistry and Physics', CRC, 1993-94 editions.

Two approximations of blackbody emission were developed before Planck's Law:

• Rayleigh-Jeans Law is valid for long wavelengths,
• Wien's Law is valid for short wavelengths.

Blackbody radiation has the following characteristics:

• There is a large dependence of radiated energy on wavelength.
• Emitted power increases rapidly with temperature.
• Curves from different temperatures do not cross each other.
• Total emitted power is the integral over all wavelengths.