QUANTITIES AND SI UNITS
Absorbed dose or dose. The quotient of the mean energy, dĒ, absorbed in a mass dm (i.e., dĒ/dm). The term is used regularly in the case of ionizing radiations but less frequently for non-ionizing radiation. The SI unit is joules per kilogram (J/kg), where 1 J/kg is a Gray.
Average (temporal) power. The time-averaged rate of energy transfer. The SI unit is joules per second (J/s).1
Average (temporal) power density. The instantaneous power density integrated (averaged) over a specific time duration. The time duration could be source related (e.g., the source repetition period) or use related (e.g., the averaging time specified in exposure guidelines). The SI unit is watts per square meter (W/m2).2
Averaging time. The appropriate time period in second(s) over which exposure is averaged for purposes of determining compliance with radio frequency exposure limits.3
1 IEEE Std C95.1™-2005.
2 IEEE Std C95.1™-2005.
3 IEEE Std C95.1™-2005.
Averaging volume. The volume over which the peak spatial-average specific absorption rate is averaged when determining compliance with the basic restrictions. The SI unit is meters squared (m2).4
Continuous exposure. Exposure in seconds (s) for durations exceeding the corresponding averaging time.5
Decibel (dB). Ten times the logarithm to the base 10 of the ratio of two power levels.6
Duty factor. The ratio of pulse duration to the pulse period of a periodic pulse train. Also, may be a measure of the temporal transmission characteristic of an intermittently transmitting radio frequency source, such as a paging antenna, by dividing average transmission duration by the average period for transmissions. A duty factor of 1.0 corresponds to continuous operation.7
Energy density (electromagnetic field). The electromagnetic energy contained in an infinitesimal volume. The SI unit is joules per meter squared (J/m2).8
Exposure. The condition by which a person is subjected to electric, magnetic, or electromagnetic fields other than those originating from physiological processes in the body and/or other natural phenomena.9 See also whole-body-exposure.
Exposure, partial-body. The condition in which radio frequency fields are substantially nonuniform over a body. Fields that are nonuniform over volumes comparable to the human body may occur due to highly directional sources, standing waves, re-radiating sources, or sources in the near field.10
Far-field equivalent. A commonly used term associated with any electromagnetic wave equal in magnitude to the power density of a plane wave having the same electric (E) or magnetic (H) field strength.11 See also plane-wave equivalent and power density.
4 IEEE Std C95.1™-2005.
5 IEEE Std C95.1™-2005.
6 IEEE Std C95.1™-2005.
7 IEEE Std C95.1™-2005.
8 FCC OET Bulletin 65, 1997.
9 FCC OET Bulletin 65, 1997.
10 FCC OET Bulletin 65, 1997.
11 FCC OET Bulletin 65, 1997.
Far-field region. 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 (also called the free space region), the field has a predominantly plane-wave character—that is, locally uniform distribution of electric field strength and magnetic field strength in planes transverse to the direction of propagation.12
Heat. The internal or intrinsic energy in an object that can transfer between two regions of different termperatures. The SI unit is joules (J).
Intensity. The power per unit area, which is typically expressed in units of milliwatts (10−3 W) per square centimeter (mW/cm2).
Maximum permissible exposure (MPE). The Root mean square and peak electric and magnetic field strength, their squares, or the plane-wave equivalent power densities (see below) associated with these fields to which a person may be exposed without harmful effect and with an acceptable safety factor.13
Near-field region. A region generally in proximity to an antenna or other radiating structure in which the electric and magnetic fields do not have a substantially plane-wave character, but vary considerably from point to point. The near-field region is further subdivided into the reactive near-field region, which is closest to the radiating structure and contains most or nearly all of the stored energy, and the radiating near-field region where the radiation field predominates over the reactive field but lacks substantial plane-wave character and is complicated in structure. For most antennas, the outer boundary of the reactive near-field region is commonly taken to exist at a distance of one-half wavelength from the antenna surface.14
Plane-wave equivalent. A commonly used term associated with any electromagnetic wave equal in magnitude to the power density of a plane wave having the same electric (E) or magnetic (H) field strength.15 See also far-field equivalent and power density.
Power. Work per unit time (1 joule per second = 1 watt). For electrical circuits, this is equivalent to the current times the voltage drop.
12 FCC OET Bulletin 65, 1997.
13 FCC OET Bulletin 65, 1997.
14 FCC OET Bulletin 65, 1997.
15 FCC OET Bulletin 65, 1997.
Power density (S). Power per unit area normal to the direction of propagation, usually expressed in SI units of watts per square meter (W/m2). For plane waves, power density, electric field strength (E), and magnetic field strength (H) are related by the impedance of free space (i.e., 377 ohms), as discussed in Section 1 of FCC OET Bulletin 65. Although many survey instruments indicate power density units (“far-field equivalent” power density), the actual quantities measured are E or E2 or H or H2.16
Power density, peak. The maximum instantaneous power density occurring when power is transmitted.17
Power density. A commonly used term associated with any electromagnetic wave equal in magnitude to the power density of a plane wave having the same electric (E) or magnetic (H) field strength.18 See also plane-wave equivalent and far-field equivalent.
Specific absorption rate (SAR). A measure of the rate of energy absorbed by (dissipated in) an incremental mass contained in a volume element of dielectric materials, such as biological tissues. SAR is usually expressed in terms of SI units of watts per kilogram (W/kg). Guidelines for human exposure to radio frequency (RF) fields are based on SAR thresholds where adverse biological effects may occur. When the human body is exposed to an RF field, the SAR experienced is proportional to the squared value of the electric field strength induced in the body.19
Specific absorption rate, peak spatial average. The maximum local specific absorption rate averaged over a specified volume or mass (e.g., any 10 g of tissue in the shape of a cube). The SI unit is watts per kilogram (W/kg).20
Whole-body-exposure. The case in which the entire body is exposed to the incident fields.21
16 FCC OET Bulletin 65, 1997.
17 FCC OET Bulletin 65, 1997.
18 FCC OET Bulletin 65, 1997.
19 FCC OET Bulletin 65, 1997.
20 IEEE Std C95.1™-2005.
21 IEEE Std C95.1™-2005.