A method for the determination of the electron density (ne) using the continuum radiation is presented. The radiation is calibrated with a standard tungsten ribbon lamp and thus expressed in absolute units. This method is applied to a microwave-induced argon plasma, created by a surfatron (2.45 GHz), for which the standard settings are: wavelength region at 648 nm, power of 60 W, pressure of 15 mbar, gas flow of 70 sccm and axial distance from the launcher of 3 cm. Due to the low degree of ionization, the influence of electron–ion interactions can be neglected; the radiation is predominantly generated by free–free interactions between electrons and atoms. The method provides the electron density values in the order of 1019 m−3 for different plasma settings. It is observed that the measured ne follows the well-known trends—it decreases in the direction of the propagating surface wave and increases with power.