Security fence

A security fence which comprises at least one section of fence, wherein said at least one section of fence consists essentially of a single weft-knitted interlooped optical wire and a frame, said frame having two upright posts, an upper tension wire spanning the distance between said upright posts, and a lower tension wire spanning the distance between said upright posts, said upright posts and tension wires being arranged so as to define a parallelogram, said single weft-knitted interlooped optical wire being connected to said frame by being looped around said posts and said tension wires, said single weft-knitted interlooped optical wire being under tension in both the vertical and horizontal directions so as to maintain the geometrical configuration without any auxiliary means, one end of said single weft-knitted interlooped optical wire being connected to a light transmitter, and the other end of said single weft-knitted interlooped optical wire being connected to a light receiver, which light receiver is connected to an alarm so that an abrupt change in the intensity of light passing through the single weft-knitted interlooped optical wire actuates said alarm.