CRT or Cathode ray tube, is the conventional "old fashioned" screen type that has been around for eighty years. This is basically a glass tube with a vacuum inside in which a cathode ray (light ray) is scanned (see scanning types above) across the inside of the screen which is coated with phosphor which glows when the beam hits it. This is the oldest, and essentially, an analogue technology. They are restricted in size and resolution, because of the weight and limited brightness with the biggest screens.