Cathode-ray tube

A cathode-ray tube (CRT) is a vacuum tube containing one or more electron guns, which emit electron beams that are manipulated to display images on a phosphorescent screen.[2] The images may represent electrical waveforms (oscilloscope), pictures (television set, computer monitor), radar targets, or other phenomena. A CRT on a television set 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.

Cathode-ray tube using electromagnetic focus and deflection. Parts shown are not to scale.
A cathode-ray tube as found in an oscilloscope
Cutaway rendering of a color CRT:
1. Three electron emitters (for red, green, and blue phosphor dots)
2. Electron beams
3. Focusing coils
4. Deflection coils
5. Connection for final anodes (referred to as the "ultor"[1] in some receiving tube manuals)
6. Mask for separating beams for red, green, and blue part of the displayed image
7. Phosphor layer (screen)with red, green, and blue zones
8. Close-up of the phosphor-coated inner side of the screen
Cutaway rendering of a monochrome CRT:
1. Deflection coils
2. Electron beam
3. Focusing coil
4. Phosphor layer on the inner side of the screen; emits light when struck by the electron beam
5. Filament for heating the cathode
6. Graphite layer on the inner side of the tube
7. Rubber or silicone gasket where the anode voltage wire enters the tube (anode cup)
8. Cathode
9. Air-tight glass body of the tube
10. Screen
11. Coils in yoke
12. Control electrode regulating the intensity of the electron beam and thereby the light emitted from the phosphor
13. Contact pins for cathode, filament and control electrode
14. Wire for anode high voltage.
The only visible differences are the single electron gun, the uniform white phosphor coating, and the lack of a shadow mask.

In CRT television sets and computer monitors, 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) with a video signal as a reference.[3] In modern CRT monitors and televisions the beams are bent by magnetic deflection, using a deflection yoke. Electrostatic deflection is commonly used in oscilloscopes.[3]

The rear of a 14-inch color cathode-ray tube showing its deflection coils and electron guns
Typical 1950s United States monochrome television set
Snapshot of a CRT television showing the line of light being drawn from left to right in a raster pattern
Animation of the image construction with interlacing method
Color computer monitor electron gun

A CRT is a glass envelope which is deep (i.e., long from front screen face to rear end), heavy, and fragile. The interior is evacuated to approximately 0.01 pascals (9.9×10−8 atm)[4] to 133 nanopascals (1.31×10−12 atm),[5] to facilitate the free flight of electrons from the gun(s) to the tube's face without scattering due to collisions with air molecules. 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. CRTs make up most of the weight of CRT TVs and computer monitors.[6][7]

Since the early 2010s, CRTs have been superseded by flat-panel display technologies such as LCD, plasma display, and OLED displays which are cheaper to manufacture and run, as well as significantly lighter and less bulky. Flat-panel displays can also be made in very large sizes whereas 40 in (100 cm) to 45 in (110 cm)[8] was about the largest size of a CRT.[9]

A CRT works by electrically heating a tungsten coil[10] 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.[11][12][13]


Share this article:

This article uses material from the Wikipedia article Cathode-ray tube, and is written by contributors. Text is available under a CC BY-SA 4.0 International License; additional terms may apply. Images, videos and audio are available under their respective licenses.