Cathode ray tube

A cathode ray tube (CRT) is a vacuum tube containing one or more electron guns, which emit electron beams, which are directed and controlled to display images on a phosphorescent screen. The images may represent electrical waveforms on an oscilloscope, a frame of video on an analogtelevision set (TV), digitalraster graphics on a computer monitor, or other phenomena like radar targets. A CRT in a TV is commonly called a picture tube. CRTs have also been used as memory devices, in which case the screen is not intended to be visible to an observer. The term cathode ray was used to describe electron beams when they were first discovered, before it was understood that what was emitted from the cathode was a beam of electrons.

A CRT works by electrically heating a tungsten coil which in turn heats a cathode in the rear of the CRT, causing it to emit electrons which are modulated and focused by electrodes. The electrons are steered by deflection coils or plates, and an anode accelerates them towards the phosphor-coated screen, which generates light when hit by the electrons. The entire front area of the tube is scanned repeatedly and systematically in a fixed pattern called a raster. In color devices, an image is produced by controlling the intensity of each of three electron beams, one for each additive primary color (red, green, and blue). In modern CRT monitors and TVs, the beams are bent by magnetic deflection, using a deflection yoke. Electrostatic deflection is commonly used in oscilloscopes.

The tube is a glass envelope that is heavy, fragile, and long from the front screen face to its rear end. Its interior must be a partial vacuum to prevent the emitted electrons from colliding with air molecules and scattering before they hit the tube's face. Thus, the interior is evacuated to less than a millionth of atmospheric pressure. As such, handling a CRT carries the risk of violent implosion that can hurl glass at great velocity. The face is typically made of thick lead glass or special barium-strontium glass to be shatter-resistant and to block most X-ray emissions. The CRT makes up most of the weight of CRT TVs and computer monitors.

Since the late 2000s, CRTs have been superseded by flat-panel display technologies such as liquid-crystal display (LCD), plasma display, and OLED, which are cheaper to manufacture and operate and are also significantly lighter and thinner. Flat-panel displays can also be made in very large sizes, whereas 40–45 inches (100–110 cm) was about the largest size of a CRT.