Charge-coupled device, abbreviated CCD, is an electronic device that can produce images. A CCD works by transmitting a signal as a succession of "packets" of electric charge from one part of an electric circuit to another. The best-known kinds of CCD's are used in cameras and other instruments that produce still images and motion pictures. CCD's have replaced film in applications ranging from handheld digital cameras to large cameras mounted in telescopes used by astronomers. Many television cameras use CCD's instead of vacuum tubes. Other applications include camcorders, photocopiers, facsimile machines, and medical imaging systems that use ultrasound.