The concept of blackbody radiation was introduced in Chapter 3. It was shown that the total power emitted by an object is a function of the temperature, and the emissivity of the material is proportional to T4 as described by the Stefan-Boltzmann law. It was also shown that if the power is measured in a region far from the emission peak, the source brightness, Bf (W/m2/Hz/ sr), is directly proportional to the temperature, T (K), according to the relationship described by the Rayleigh-Jean law (Currie and Brown 1987): Bf = 2kT/λ2 , (4.1) where k is Boltzmann's constant (1.3804 x 10-23 J/K), T is the source temperature (K), and l is the wavelength (m). This approximation is accurate to within 1% for frequencies below 100 GHz, and to within 3% for frequencies below 300 GHz. It can therefore be applied to both microwave and millimeter wave systems (Ulaby 1987).