ABSTRACT

All NIR is presented in electromagnetic ®elds (EMFs), which propagate energy by timevarying electric and magnetic elds (Figure 31.2). Electromagnetic radiation is characterized by three related quantities: frequency, wavelength, and energy. Frequency is the number of complete cycles per second and is represented by the unit hertz (Hz) or (1/s; s−1). By convention, frequency is the primary parameter used to describe the part of the electromagnetic spectrum from sub-extremely low frequency (SELF) to supra-extremely high frequency (SEHF) as dened in Table 31.2. Wavelength is the primary parameter used to describe the part of the electromagnetic spectrum from infrared (IR) to ultra violet (UV) frequencies. Frequency and wavelength are related by the following equation:

λ = c

f (31.1)

where λ is the wavelength in meters (m) c is the velocity of light (about 300,000,000 m/s) f is the frequency in cycles per second (Hertz, or Hz)

Hazards potentially associated with exposure to EMFs in various bands may result in (a) currents produced in the body by contact with energized sources or without such contact (electrostimulation), (b) increased core-body temperature, or (c) increased body surface temperature (Table 31.2). How e€cient these elds interact with the body depends on several factors. For example, materials with high water content (muscles) absorb EMF energy at higher rates than dry materials. e absorption rate is higher when (a) the incident electric eld is parallel to the body and (b) the incident magnetic eld is perpendicular to a larger cross-sectional area. Sharp corners, edges and points concentrate electric elds. Depth of penetration of EMF energy decreases as conductivity or frequency increases, and as wavelengths decrease.