6.5 RF DOSIMETRY
The preceding section described exposure in terms of the electric and magnetic fields, incident power density, and related quantities present outside the body. These external quantities are essential for assessing exposure, but they do not directly reveal the fields produced or the power or energy absorbed within the body. When an RF field encounters the body, some of the incident energy is reflected, some may pass through the body, and some is absorbed. The resulting internal fields and absorption patterns are generally complex and non-uniform.
RF dosimetry is the determination, by measurement or calculation, of the electromagnetic fields induced within the body and the power or energy absorbed by its tissues. Different dosimetric quantities are required for different frequencies, exposure durations, and spatial distributions. There is therefore no single quantity that can be regarded as a universal RF dose. This section first considers how external exposure gives rise to internal fields and energy absorption, then introduces the principal dosimetric quantities and the methods used to determine them.
6.5.1 From External Exposure To Internal Fields And Energy Absorption
The relationship between an external RF field and the resulting internal fields depends on both the characteristics of the field and the characteristics of the exposed body. Important field characteristics include frequency, polarization, direction of incidence, field strength, spatial distribution, and whether the exposure occurs in the reactive near field, radiating near field, or far field. Relevant body characteristics include size, shape, posture, orientation, and the electrical properties of the tissues.
The human body is neither electrically uniform nor geometrically simple. Different tissues have different permittivities and conductivities, and these properties vary with frequency. Boundaries between tissues can therefore alter the direction and magnitude of internal fields, while variations in body dimensions and posture can change the way in which the body couples to an external field. Internal fields and energy absorption may consequently be distributed unevenly, with some tissues or regions receiving substantially more energy than others.
Proximity to the source is particularly important. In the far field, the incident electric and magnetic fields have a predictable relationship, and exposure can often be characterized using incident power density. In the near field, the electric and magnetic fields may have different spatial distributions and must sometimes be considered separately. Where a transmitter is used very close to or against the body, the body may also alter the operating characteristics of the antenna. The source and body may then need to be treated as a coupled system rather than assessed independently.
Consequently, two people exposed to the same external field strength may not experience identical internal fields or energy absorption, and two transmitters with the same output power may produce very different internal quantities. RF dosimetry quantifies these internal physical conditions; it does not, by itself, determine whether they produce a biological response or whether that response is harmful. Those questions are considered in Chapter 7.