Visibility barrier generation

摘要

Generating a visibility barrier in at least a range of wavelengths comprises the use of an artificial fog (2) produced by an aerosol- or gas-cloud. It is possible to see through the artificial fog (2) without special equipment, but by irradiating the artificial fog with electromagnetic waves (5), causing the fog (2) particles to emit or diffusely reflect particular wavelengths, so that a visibility barrier is generated. Also claimed are: (i) an apparatus for the above process; and (ii) an artificial fog which is opaque to wavelengths in the range of the irradiating waves, and is transparent to wavelengths outside the range, for use with the above process.