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GLOSSARY* CONTINUOUS WAVE (CW): Refers to an unmodulated electromagnetic wave. When a wave is abruptly turned "on" and "off," the resulting burst is referred to as a pulsed wave. ELECTRIC FIELD; An electric field is said to exist in a region if charged objects in the region experience a force. It is described in terms of the voltage gradient that exists over a given distance, volts per meter. ELECTROMAGNETIC RADIATION: Radiation by which energy is propagated by interrelated electric and magnetic fields. Electromagnetic radiation travels at the speed of light in free space. ENERGY DOSE; The quantity of electromagnetic energy in joules that is absorbed per unit of mass of a biological body. The unit of mass is kilogram (2.2 Ib) and the dose is stated as joules per kilogram (J/kg). For convenience in working with small animals er with small samples of tissue, many researchers prefer to use joules or millijoules per gram (J/g or mJ/g) as working units. Dose is synonymous with "Specific Absorption" (SA), q.v. [ENERGY] DOSE RATE; The time rate at which electromagnetic energy is imparted per unit of mass to a biological body, i.e., watts per kilogram (W/kg) or watts or milliwatts per gram (W/g or mW/g). Synonymous with "Specific Absorption Rate" (SAR), q.v. FAR AND NEAR FIELDS; The E and H fields of an electromagnetic wave that is propagating in a uniform medium are always at right angles to each other and to the line of propagation in the "far field"; however, waves near an emitting source are complex in the sense that relative magnitudes and orientations of E and H fields can each vary greatly. Beyond the near field, i.e., in the far field, the relative maximum strengths of E and H fields do not change (although the total strength diminishes as distance from a point source increases). FREE FIELD; A free (or open) field is generally considered to be an electromagnetic wave that is propagating in a vacuum or in air without interacting bodies. *Reprinted from A Technical Review of the Biological Effects of Non- Ionizing Radiation, with the addition of definitions of "peak power density" and "time-averaged power density." -82-
GIGA: Prefix denoting billion(s) in the U.S.A., i.e., 109. HERTZ (Hz); The cyclic rate at which a wave of energy changes; equivalent to frequency in cycles per second. HOTSPOT; Electromagnetic waves are seldom, if ever, absorbed uniformly by a biological body. Concentrations of energy will occur that are called electrical hotspots. If the field is sufficiently intense to overcome local cooling by flow of blood in affected tissues, thermal hotspots may also occur. JOULE; Under the International System, the basic unit of all forms of energy. As a thermal unit, one joule equals 0.239 calories. Since the calorie is defined as the energy required to heat one gram of water from 4 to 5°C, 4.184 joules is the equivalent of one calorie. KILO; Prefix denoting thousand(s), i.e., 10. MAGNETIC FIELD: Exists in a region if magnetic objects in the region experience a force. An electrical current passing through a wire creates a magnetic field. The intensity of a magnetic field is measured in teslas. (1 tesla = 10,000 gauss) MEGA; Prefix denoting million(s), i.e., 106. MICRO: Prefix denoting millionth (microwatt is one millionth of a watt). MILLI; Prefix denoting thousandth (milliwatt is one thousand of a watt). MODULATION; When a continuous series of waves of electromagnetic energy is modified by pulsing, or by varying its amplitude, frequency, or phase, the waves are said, respectively, to be pulse-, amplitude-, frequency-, or phase-modulated. In order to convey information by radiating electro- magnetic energy, it must be modulated. MULTIPATH RADIATION; In contrast with a so-called plane wave, which moves in a straight line through space, an area or volume where electro- magnetic waves arrive from different directions because of reflection or multiple sources is said to be the site of multipath radiation. The cavity of a microwave oven exemplifies the raultipath case. PEAK POWER DENSITY; The maximal value of the rate of energy transmission of a time-varying electromagnetic field. PLANE WAVE; The wave emanating from a point source is an expanding sphere. A segment of the wave at a great distance (with respect to wavelength) from the source may have little curvature relative to the dimensions of a small target and can therefore be treated as a moving plane. -83-
POINT SOURCE; A source of radiation that is small in size with respect to its distance from a radiated target. The "inverse-square law" holds for a point-source radiation, i.e., the intensity of the field decreases rapidlyâas a function of the square of the distance from the source. POLARIZATION: The E and H fields that comprise a propagating electro- magnetic wave may be fixed or they may rotate. If the E vector is perpendicular to the ground, the wave is said to be vertically polarized; if parallel, horizontally polarized. When the E and H fields are con- tinuously rotating with respect to the direction of propagation the wave is said to be circularly polarized. POWER: The time rate at which energy is generated, transferred, or dissipated. The unit of power, the watt (W), is defined as one joule per second (J/s). POWER DENSITY: The time rate per unit of area at which electromagnetic energy flows through some medium. The quantity of energy is complexly related to the strengths of the E and the H fields. Power density is specified in watts per square meter (W/m ), but it is often expressed in milliwatts per square centimeter (mW/cm ). In common usage, power density is taken to mean the time rate at which electromagnetic energy is incident on a body per unit of surface area. RESONANCE [ELECTRICAL]; A conductor or absorbing body of a given length in the free field will react maximally to an electromagnetic wave that is about twice the body's length. The maximal reaction is technically known as "resonance." SCATTER; When an electromagnetic wave is incident on a body, the energy it carries will be absorbed and scattered. "Scatter" refers collectively to reflection, refraction, and diffraction of unabsorbed energy. SPECIFIC ABSORPTION (SA); The quantity of electromagnetic energy in joules that is absorbed per unit of mass of an absorbing body is specified is joules per kilogram (J/kg); but often expressed as millijoules or joules per gram (mJ/g or J/g). Synonymous with (energy) dose q.v. SPECIFIC ABSORPTION RATE (SAR); The time rate at which electromagnetic energy is absorbed per unit of mass of an absorbing body is specified in watts per kilogram (W/kg); but is often expressed as milliwatts or watts per gram (mW/g or W/g). TIME-AVERAGED POWER DENSITY; The integral of the product of the instantaneous power density of pulsed electromagnetic radiation and the pulse duration taken over a characteristic sampling interval and divided by the duration of the sampling interval. -84-
