Van Eck Phreaking
According to Wikipedia, Van Eck phreaking is the process of eavesdropping on the contents of a CRT or LCD display by detecting its electromagnetic emissions. It is named after Dutch computer researcher Wim van Eck, who in 1985 published the first paper on it, including proof of concept. Phreaking is the process of exploiting telephone networks, used here because of its connection to eavesdropping.
The electro magnetic radiations emitted by the computer's display can be captured with a high gain antenna. Using DSP hardware the picture can be reconstructed from the captured noise. Later Van Eck devices were proved to be effective even against laptop computers.
Wim Van Eck outlined the possibility in Computers & Security in 1985. He concluded: "If no preventive measures are taken, eavesdropping on a video-display unit is possible at several hundred meters distance, using only a normal black-and-white TV receiver, a directional antenna, and an antenna amplifier." To prove his point he took BBC crew around London in a van containing the required equipments and showed them what was on the computer screens at various companies.
There is an open source implementation for Van Eck Phreaking named EckBox.
During cold war KGB and CIA agents is said to have conducted Van Eck attacks against each other. The US Government concerned about EM spectrum security introduced a special class of secure environment called "Tempest Security."
TEMPEST (Transient EMission of PulsE Surveillance Technology) is a U.S. government codename for a set of standards for limiting electric or electromagnetic radiation emissions from electronic equipments. It is a counter-intelligence measure aimed at the prevention of radiation espionage such as Van Eck Phreaking.