The cathode ray tube (CRT) is a vacuum tube that contains one or more electron guns and a phosphorescent screen, and is used to display images. It modulates, accelerates, and deflects electron beam(s) onto the screen to create the images. The images may represent electrical waveforms (oscilloscope), pictures (television, computer monitor), radar targets, or others. CRTs have also been used as memory devices, in which case the visible light emitted from the fluorescent material (if any) is not intended to have significant meaning to a visual observer (though the visible pattern on the tube face may cryptically represent the stored data).
The CRT uses an evacuated glass envelope which is large, deep (i.e. long from front screen face to rear end), fairly heavy, and relatively fragile. As a matter of safety, the face is typically made of thick lead glass so as to be highly shatter-resistant and to block most X-ray emissions, particularly if the CRT is used in a consumer product. Since the late 2000s, CRTs have largely been superseded by newer display technologies such as LCD, plasma display, and OLED screens, which have lower manufacturing costs and power consumption, and significantly less weight and bulk. Newer display technologies can also be made in larger sizes; whereas 38" to 40" was about the largest size of a CRT TV, new display technologies are available in 50" to 60" and even larger sizes. The vacuum level inside the tube is high vacuum on the order of 0.01 Pa to 133 nPa.