quency (EHF) at 300 GHz and extends to just below the red end of the visible light spectrum at around 0.76 um wavelength. Unlike radio frequency radiation, which is transmitted from an antenna when excited by an oscillating electrical signal, infrared radiation is generated by the rotational and vibrational oscillations of molecules. The infrared spectrum is usually divided into three regions, near, middle and far, where "near" means nearest to visible light (Table 5-1). Although all infrared radiation is invisible to the human eye, far infrared is experienced as thermal, or heat, radiation. Rather than using frequency as an alternative to wavelength, as is commonly done in the RF region, the wavenumber is used instead in the infrared region. This is the reciprocal of the wavelength and is usually expressed as the number of wavelengths per centimetre. One aspect of wireless communication that becomes simpler outside the RF region is spectrum regulation, since the remit of the FCC and equivalent international agencies runs out at 300 GHz or 1 mm wavelength. Infrared Propagation and Reception The near infrared is the region used in data communications, largely as a result of the cheap availability of infrared emitting LEDs and optodetectors, Chapter Five Table 5-1: Subdivision of the Infrared Spectrum Infrared region Wavelength (um) Wavenumber (/cm) 0.78-2.5 12,800-4000 Middle 2.5-50 4000-200 50-1000 200-10 solid state devices that convert an electrical current directly into infrared radiation and vice versa. Infrared LEDs emit at discrete wavelengths in the range from 0.78 to 1.0 um, the specific wavelength of the LED being determined by the particular molecular oscillation that is used to generate the radiation. Transmitted Power Density - Radiant Intensity Ir propagation is generally a simpler topic than RF propagation, although the same principles, such as the concept of a link budget, still apply. As described in Chapter 4, the link budget predicts how much transmitter power is required to enable the received 