The brightness temperature of a body is defined according to
(1)
where is the specific intensity and is given by the Planck Law. In radio astronomy, the Rayleigh-Jeans law can be used to approximate the Planck Law, so
(2)
where has MKS units of , is the frequency, k is Boltzmann's constant, T is the physical temperature, and c is the speed of light. Solving for then gives
(3)
(4)
where is the wavelength (Rybicki and Lightman 1979, p. 26; Thompson et al. 1986, p. 8; Janssen 1993, pp. 7-12). This can also be written in terms of the wavelength specific intensity by using
of a body is defined according to
is the specific intensity and 
is given by the Planck Law. In radio astronomy, the Rayleigh-Jeans law can be used to approximate the Planck Law, so
,
is the frequency, k is Boltzmann's constant, T is the physical temperature, and c is the speed of light. Solving for 
is the wavelength (Rybicki and Lightman 1979, p. 26; Thompson et al. 1986, p. 8; Janssen 1993, pp. 7-12). This can also be written in terms of the wavelength specific intensity by using 
