text: For many years the photocathode was the only practical method for converting light to an electron current. As such it tends to function as a form of electric film and shared many characteristics of photography. It was therefore the key element in opto-electronic devices, such as TV camera tubes like the orthicon and vidicon, and in image tubes such as intensifiers, converters, and dissectors. Simple phototubes were used for motion detectors and counters.
  Phototubes have been used for years in movie projectors to read the sound tracks on the edge of movie film.
  The more recent development of solid state optical devices such as photodiodes has reduced the use of photocathodes to cases where they still remain superior to semiconductor devices.