Computer screens emit signals that can be detected from a distance. In fact, any components, including printers, disk drives, and processors, can emit information. Tempest is a U.S. government program under which computer equipment is certified as emission free (that is, no detectable emissions). There are two approaches for preparing a device for Tempest certification: enclosing the device and modifying the emanations.
The obvious solution to preventing emanations is to trap the signals before they can be picked up. Enclosing a device in a conductive case, such as copper, diffuses all the waves by conducting them throughout the case. Copper is a good conductor, and the waves travel much better through copper than through the air outside the case, so the emissions are rendered harmless.
This solution works very well with cable, which is then enclosed in a solid, emanation-proof shield. Typically, the shielded cable is left exposed so that anyone can inspect visually for signs of tapping or other tampering. The shielding must be complete. That is, it does little good to shield a length of cable but not also shield the junction box at which that cable is connected to a component. The line to the component and the component itself must be shielded, too.