A superlattice of GaAs/Ga1-xAlxAs quantum wells forms a Stark ladder under the influence of a perpendicular electric field. A two level incoherent emitter system, formed by radiative intersubband transitions between adjacent wells, is investigated as a tunable far-infrared radiation source. Intersubband transition rates are calculated at 4, 77, and 300 K for applied fields from 0 to 40 kV cm-1. It is shown that the quantum efficiency of the radiative emission reaches a maximum at low temperatures for a field of 32 kV cm-1. Under these conditions the emission wavelength is 38 μm with an estimated power output of 1.1 mW.