8.4.2 Magnetic-Field Strength
The magnetic component is described by magnetic-field strength, denoted by H and expressed in amperes per meter (A m⁻¹). Magnetic flux density, B, expressed in teslas (T), may also be used where appropriate. In air or free space, B and H are related through the magnetic permeability of the medium.
Magnetic-field measurements are particularly important at lower RF frequencies and near sources such as loop antennas, induction heaters, wireless power-transfer systems, and the RF components of magnetic-resonance systems. In the reactive near field, magnetic-field strength cannot generally be predicted from electric-field strength, so the two components may require separate measurement.
8.4.3 Incident Power Density And Energy Density
Incident power density, Sinc, describes the rate at which electromagnetic energy flows through a unit area and is expressed in watts per square meter (W m⁻²). It should not be confused with transmitter power, which describes energy produced by a source rather than the energy arriving at an exposed location.
Under verified plane-wave conditions, incident power density can be determined from either electric-field strength or magnetic-field strength using the relationships given in Chapter 5. A value obtained from one field component in this way is a plane-wave-equivalent incident power density. That conversion is not automatically valid in the reactive near field, where electric and magnetic fields are not related by the free-space impedance.
At higher microwave and millimeter-wave frequencies, incident power density is an important external assessment quantity because RF energy is absorbed increasingly close to the body surface. It remains distinct from absorbed power density, which is a dosimetric quantity describing power absorbed at or near that surface.
For brief or pulsed exposures, the relevant external quantity may be incident energy density, Uinc, expressed in joules per square meter (J m⁻²). Incident energy density represents incident power density integrated over a specified time interval and must be assessed using the temporal and spatial averaging requirements of the applicable standard.