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Electromagnetic Shielding
Electromagnetic shielding is the process of limiting the flow of electromagnetic fields between two locations, by separating them with a barrier made of conductive material. Typically electromagnetic shields are applied to enclosures, separating electrical devices from the 'outside world', and to cables, separating wires from the environment through which the cable runs. Electromagnetic shielding used to block radio frequency electromagnetic radiation is also known as RF shielding.
When modern electrical devices operate they generate electromagnetic fields. Digital computers, radio equipment, typewriters, and so on generate massive amounts of electromagnetic signals which if properly intercepted and processed will allow certain amounts of information to be reconstructed based on these "compromising emanations". Basically anything with a microchip, diode, or transistor, gives off these fields.
Compromising emanations are those unintentional intelligence-bearing signals, which, if intercepted and analyzed, potentially disclose the national security information, transmitted, received, handled, or otherwise processed by any information-processing equipment.
These compromising emanation signals can then escape out of a controlled area by power line conduction, other fortuitous conduction paths such as the air conditioning duct work, or by simply radiating a signal into the air (like a radio station).
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Related topics - EMF magnetic shielding, Radiation shielding
