The Cathode Ray Tube (CRT) is an evacuated glass envelope containing an electron gun (a source of electrons) and a fluorescent screen, usually with internal or external means to accelerate and deflect the electrons. When electrons strike the fluorescent screen, light is emitted. The electron beam is deflected and modulated in a way which causes it to display an image on the screen. The image may represent electrical waveforms (oscilloscope), pictures (television, computer monitor), echoes of aircraft detected by radar, etc., The single electron beam can be processed in such a way as to display moving pictures in natural colors. The generation of an image on a CRT by deflecting an electron beam requires the use of an evacuated glass envelope which is large, deep, heavy, and relatively fragile (this has earned it the nickname 'Fishbowl'). The development of imaging technologies without these disadvantages has caused CRTs to be largely displaced by flat plasma screens, liquid crystal displays, DLP, OLED displays, and other technologies.