Countermeasures
Countermeasures are detailed in the article on TEMPEST, the NSA's standard on spy-proofing digital equipment. One countermeasure involves shielding the equipment to minimize electromagnetic emissions. Another method, specifically for video information, scrambles the signals such that the image is perceptually undisturbed, but the emissions are harder to reverse engineer into images. Examples of this include low pass filtering fonts and randomizing the least significant bit of the video data information.
Read more about this topic: Van Eck Phreaking