Summary In this paper we study the spectral brightness of the radiation emitted by electrons moving in longitudinal wiggler and solenoidal
magnetic field. The analysis is based on the evaluation of the Lienard-Wiechert potential retaining the condition of far-field
limit. Some features of a longitudinal wiggler are presented and compared with the results relevant to the brightness of the
helical wiggler and the solenoidal magnetic field employed in gyrotron-type devices.