8.4 QUANTITIES USED IN RF SAFETY ASSESSMENT
RF safety assessments use several different physical quantities. Some describe the incident electromagnetic field outside the body and can generally be measured or calculated directly. Others describe fields induced or energy absorbed within or at the surface of the body and must normally be determined through standardized laboratory measurements, analytical calculations, or computational dosimetry. The applicable exposure standard, operating frequency, field region, exposure duration, and assessment objective determine which quantities are required.
The principal external quantities are incident electric-field strength, incident magnetic-field strength, incident power density, incident energy density, and, where applicable, induced limb current. The principal dosimetric quantities are induced electric field, specific absorption rate, specific energy absorption, absorbed power density, and absorbed energy density. These quantities are not interchangeable and apply over different frequency ranges, exposure durations, and spatial averaging areas or volumes.
8.4.1 Electric-Field Strength
The electric component of an electromagnetic field is described by electric-field strength, denoted by E and expressed in volts per meter (V m⁻¹). For exposure assessment, the relevant external quantity is generally incident electric-field strength, Einc, determined at the location where a person may be exposed.
Electric-field strength can be measured directly using a calibrated electric-field probe or measurement antenna. In a verified plane-wave field, it is related to magnetic-field strength and incident power density through the free-space impedance. In the reactive near field, however, that relationship does not apply reliably, and electric-field strength must be assessed independently.