Optical time-domain eavesdropping risks of CRT displays

A new eavesdropping technique can be used to read cathode-ray tube (CRT) displays at a distance. The intensity of the light emitted by a raster-scan screen as a function of time corresponds to the video signal convolved with the impulse response of the phosphors. Experiments with a typical personal computer color monitor show that enough high-frequency content remains in the emitted light to permit the reconstruction of readable text by deconvolving the signal received with a fast photosensor These optical compromising emanations can be received even after diffuse reflection from a wall. Shot noise from background light is the critical performance factor In a sufficiently dark environment and with a large enough sensor aperture, practically significant reception distances are possible. This information security risk should be considered in applications with high confidentiality requirements, especially in those that already require "TEMPEST"-shielded equipment designed to minimize radio-frequency emission-security concerns.