Electromagnetic Dosimetry

Electromagnetic dosimetry is the science of determining how much electric field (dose) exists for specific sources and environments. It predicts the dose of the electromagnetic field present at any point inside or outside of the body. Dosimetry consists of two main parts. The first is the determination of the incident fields, which are produced by some kind of source. These incident fields are either measured (without object present) or calculated from a knowledge of the source.

The second part is the determination of the E and B fields inside an object exposed to the incident fields. The fields inside an object are called the internal fields. The internal fields are also either measured or calculated

The relationship between the incident EM fields and the internal EM fields is a strong function of the frequency, the size and shape of the body, and the electromagnetic properties of the body. Different techniques are used to calculate and measure internal fields in each of the ranges described in Table. In all cases, the relationship between the incident fields and the internal fields is very complicated.

Device or System Size L and Wavelength λ Summary of EM Techniques and Characteristics as a Function of the Relationship Between Device or System Size L and Wavelength λ Device or System Size L and Wavelength λ EM Techniques and Characteristics When λ >> L (low frequency for most typical devices) (figure 1) Electric circuit theory and quasi-static EM field theory are used. E and B are uncoupled. Energy is transmitted by wires and cables, but not in beams through the air. When λ ≈ L(medium frequency for most typical devices) (figure 2) Microwave theory is used. E and H are strongly coupled. Energy is transmitted through cables, hollow waveguides, and beamed through the air. When λ << L (high frequency for most typical devices) (figure 3) Optics and ray theory are used. Energy is beamed through the air and is not transmitted through metallic cables or along metallic wires, but can be transmitted through optical fibers

Fig:1:The wavelength is large compared to the size of the device Fig:3: The size of the device is large compared to the wavelength.. Fig:2: The wavelength and the size of the device are comparable.

In general, the penetration of incident fields into biological bodies decreases as frequency increases. This effect is illustrated by the graph in Figure, which shows the skin depth

The skin depth is defined as the depth at which the EM fields have decreased to 1/e (i.e., 0.37) of their value at the surface of the body. While the graph is for a dielectric half space, a similar effect occurs in humans and other animals. As the frequency is increased, the penetration generally becomes less and less.

At optical frequencies, the penetration is very slight, and whatever effects the EM fields have on the body are primarily surface effects. Even at microwave frequencies, the penetration is relatively shallow (a few centimeters).

Electromagnetic Fields Affect the Body Bioelectromagnetics is the study of how electromagnetic fields affect the body. This includes therapeutic applications as well as natural effects and safety concerns. Visual Phosphenes (Flashing Lights) Peripheral Nerve Stimulation (Sensation) Direct Muscle Cell Excitation Electroporation Thermal Effects (Heating) Audio Effects