An RF electromagnetic wave or RF field has both an electric and a magnetic component (electric field and magnetic field), and it is often convenient to express the intensity of the RF environment at a given location in terms of units specific for each component. For example, the unit volts per meter (V/m) is used to measure the strength of the electric field (electric field strength), and the unit amperes per meter (A/m) is used to express the strength of the magnetic field (magnetic field strength). Another commonly used unit for characterizing an RF electromagnetic field is power density. Power density is most accurately used when the point of measurement is far enough away from an antenna to be located in what is commonly referred to as the far-field zone of the antenna. Power density is defined as power per unit area. For example, power density can be expressed in terms of milliwatts per square centimeter (mW/cm2) or microwatts per square centimeter (µW/cm2). One mW equals 0.001 watt of power, and one µW equals 0.000001 watt. With respect to frequencies in the microwave range and higher, power density is usually used to express intensity.