where ω is the angular frequency of the exciting radiation, ε r ″ is the imaginary part of the complex relative permittivity of the absorbing material, ε 0 is the permittivity of free space and E the electric field strength. The imaginary part of the (frequency-dependent) relative permittivity is a measure for the ability of a dielectric material to convert electromagnetic field energy into heat, also called dielectric loss. (The real part of the permittivity is the normal effect of capacitance and results in non-dissipative reactive power.)