Spectrum
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Introduction
The electromagnetic spectrum is the range of frequencies used by both light (visible or not) and radio transmissions. Portions of this spectrum are regulated by government bodies in order to ensure that it is effectively used. The following chart gives a general overview of how the spectrum is laid out. See the next section for a more detailed look at a key region of this spectrum.
Wireless Technology
Most of the wireless technology mentioned on this site surrounds the RF and Microwave bands. The following chart provides a detailed plot of a key region of this spectrum:
Key Technologies
Note that the above charts have been plotted logarithmically to better illustrate the spectrum. As such, the width of various bands on these chart is not directly indicative of their relative bandwidth.
Bands
As shown in the figure above, the electromagnetic spectrum has been divided into a number of discrete bands that serve different purposes. The following sections quickly describe each of the significant bands, as well as the functions that they serve.
Radio Frequency/Microwave
The bands of primary interest for electronic transmission, these wavelengths allow devices to wirelessly communicate with one another. Thanks to their longer wavelengths, these signals can travel through many solid objects - allowing them to operate in conditions where line of light is not possible. With sufficient power, signals transmitted in this band can cover extremely large areas.
Infrared Light
The band directly above Microwaves on the spectrum, Infrared light spans the range between 3THz and 400THz. This light is not visible to human eyes, so it is commonly used for wireless communication. Unfortunately, most bands of Infrared light are readilly absorbed by most solid objects and thus typically require clear line of sight. As such, electronic communication using Infrared typically is only useful for short range tasks.
As most digital cameras are sensitive to light in the near infrared region (100-300THz), this band is also important for photographic purposes. As this is typically not a desirable characteristic, most digital sensors are equipped with filters that block infrared light. Those filters are not perfect, however, and some IR can get into the image. For some disciplines, however, this may be a desirable effect so this sensitivity may also be a benefit.
Visible Light
Directly above infrared is the portion of the spectrum that human eyes can detect. Naturally, this is the region of the spectrum most important to conventional photography, as it represents the range of wavelengths that we interpret as light.
Ultraviolet Light
The region of spectrum above visible light and below X-Rays, Ultraviolet light can also be a useful tool for photography. As many plants and animals reflect or emit light in this band, it can be used to generate creative images of these objects. Using proper filtration, some of this light can be captured by digital cameras (although sensitivity is much lower than Infrared).
