Appendix I Formulae for calculating non-ionizing radiation levels Non-ionizing radiation is measured in a number of ways, but in practice for microwave equipment, the safety limits are often expressed as power density levels, in W/m 2 or mW/cm 2 as these can be easily measured. The near-field region is the region in general proximity to an antenna in which the electric (E) and magnetic (H) fields do not have a substantially plane-wave character, but vary considerably from point to point. In other words, as the signals are not necessarily in phase at any single point, the sum of the signals do not add up in-phase. The far-field region is that region of the field of an antenna where the angular field distribution is essentially independent of the distance from the antenna. In this region (the free space region), the field has a predominantly plane-wave character, i.e. a locally uniform distribution of E and H field strengths in planes transverse to the direction of propagation. The signals are essentially in-phase and therefore add up at any particular point.