Cathode ray tube

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. Anode (collector)
6. Mask for separating beams for red, green, and blue part of displayed image
7. Phosphor layer (screen)with red, green, and blue zones
8. Close-up of the phosphor-coated inner side of the screen

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. [1] 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 Since the late 2000s, CRTs have largely been superseded by newer display technologies such as LCD, plasma display, and OLED screens, which have lower manufacturing costs and power consumption, and significantly less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 [2] to 133 nPa. [3]

In television sets and computer monitors, the entire front area of the tube is scanned repetitively and systematically in a fixed pattern called a raster. An image is produced by controlling the intensity of each of the three electron beams, one for each additive primary color (red, green, and blue) with a video signal as a reference. [4] In all modern CRT monitors and televisions, the beams are bent by magnetic deflection, a varying magnetic field generated by coils and driven by electronic circuits around the neck of the tube, although electrostatic deflection is commonly used in oscilloscopes, a type of electronic test instrument. [4]

A 14-inch cathode ray tube showing its deflection coils and electron guns
Typical 1950s United States television set
A flat CRT assembly inside a 1984 Sinclair FTV1 pocket TV
Electron gun

History

Braun's original cold-cathode CRT, 1897

Cathode rays were discovered by Johann Hittorf in 1869 in primitive Crookes tubes. He observed that some unknown rays were emitted from the cathode (negative electrode) which could cast shadows on the glowing wall of the tube, indicating the rays were traveling in straight lines. In 1890, Arthur Schuster demonstrated cathode rays could be deflected by electric fields, and William Crookes showed they could be deflected by magnetic fields. In 1897, J. J. Thomson succeeded in measuring the mass of cathode rays, showing that they consisted of negatively charged particles smaller than atoms, the first " subatomic particles", which were later named electrons. The earliest version of the CRT was known as the "Braun tube", invented by the German physicist Ferdinand Braun in 1897. [5] [6] It was a cold-cathode diode, a modification of the Crookes tube with a phosphor-coated screen.

In 1907, Russian scientist Boris Rosing used a CRT in the receiving end of an experimental video signal to form a picture. He managed to display simple geometric shapes onto the screen, which marked the first time that CRT technology was used for what is now known as television. [1]

The first cathode ray tube to use a hot cathode was developed by John B. Johnson (who gave his name to the term Johnson noise) and Harry Weiner Weinhart of Western Electric, and became a commercial product in 1922. [7]

It was named by inventor Vladimir K. Zworykin in 1929. [8] RCA was granted a trademark for the term (for its cathode ray tube) in 1932; it voluntarily released the term to the public domain in 1950. [9]

The first commercially made electronic television sets with cathode ray tubes were manufactured by Telefunken in Germany in 1934. [10] [11]