a fractal pattern. Optical feedback in science is often closely related to video feedback, so an understanding of video feedback can be useful for other applications of optical feedback. Video feedback has been used to explain the essence of fractal structure of unstable-cavity laser beams. Video feedback is also useful as an experimental-mathematics tool. Examples of its use include the making of Fractal patterns using multiple monitors, and multiple images produced using mirrors. Optical feedback is also found in the image intensifier tube and its variants. Here the feedback is usually an undesirable phenomenon, where the light generated by the phosphor screen "feeds back" to the photocathode, causing the tube to oscillate, and ruining the image. This is typically suppressed by an aluminum reflective screen deposited on the back of the phosphor screen, or by incorporating a microchannel plate detector. Optical feedback has been used experimentally in these tubes to amplify an image, in the manner of the cavity laser, but this technique has had limited use. Optical feedback has also been experimented with as an electron source, since a photocathode-phosphor cell will 'latch' when triggered, providing a steady stream of electrons. See US Patent 4,531,122 for a typical application. In philosophy Douglas Hofstadter discusses video feedback in his book I Am a Strange Loop about the human mind and consciousness. He devotes a chapter to describing his experiments with video feedback. See also Audio/Acoustic feedback Computer graphics Cymatics Droste effect Feedback Real-time computer graphics Recursion Self-reference Strange loop Video art References Audiovisual ephemera Feedback Artistic techniques