Monitors - CRT

The earliest version of the CRT was invented by the German physicist Ferdinand Braun in 1897 and is also known as the 'Braun tube'.^[3] It was a cold-cathode diode, a modification of the Crookes tube with a phosphor-coated screen. The first version 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. The cathode rays are now known to be a beam of electrons emitted from a heated cathode inside a vacuum tube and accelerated by a potential difference between this cathode and an anode. The screen is covered with a phosphorescent coating (often transition metals or rare earth elements), which emits visible light when excited by high-energy electrons. The beam is deflected either by a magnetic or an electric field to move the bright dot to the required position on the screen.

In television sets and computer monitors the entire front area of the tube is scanned systematically in a fixed pattern called a raster. An image is produced by modulating the intensity of the electron beam with a received video signal (or another signal derived from it). In all CRT TV receivers except some very early models, the beam is deflected by magnetic deflection, a varying magnetic field generated by coils (the magnetic yoke), driven by electronic circuits, around the neck of the tube.

Electron gun

The source of the electron beam is the electron gun, which produces a stream of electrons through thermionic emission, and focuses it into a thin beam. The gun is located in the narrow, cylindrical neck at the extreme rear of a CRT and has electrical connecting pins, usually arranged in a circular configuration, extending from its end. These pins provide external connections to the cathode, to various grid elements in the gun used to focus and modulate the beam, and, in electrostatic deflection CRTs, to the deflection plates. Since the CRT is a hot-cathode device, these pins also provide connections to one or more filament heaters within the electron gun. When a CRT is operating, the heaters can often be seen glowing orange through the glass walls of the CRT neck. The need for these heaters to 'warm up' causes a delay between the time that a CRT is first turned on, and the time that a display becomes visible. In older tubes, this could take fifteen seconds or more; modern CRT displays have fast-starting circuits which produce an image within about two seconds, using either briefly increased heater current or elevated cathode voltage. Once the CRT has warmed up, the heaters stay on continuously. The electrodes are often covered with a black layer, a patented process used by all major CRT manufacturers to improve electron density.

The electron gun accelerates not only electrons but also ions present in the imperfect vacuum (some of which result from outgassing of the internal tube components). The ions, being much heavier than electrons, are deflected much less by the magnetic or electrostatic fields used to position the electron beam. Ions striking the screen damage it; to prevent this the electron gun can be positioned slightly off the axis of the tube so that the ions strike the side of the CRT instead of the screen. Permanent magnets (the ion trap) deflect the lighter electrons so that they strike the screen. Some very old TV sets without an ion trap show browning of the center of the screen, known as ion burn. The aluminium coating used in later CRTs reduced the need for an ion trap.

When electrons strike the poorly-conductive phosphor layer on the glass CRT, it becomes electrically charged, and tends to repel electrons, reducing brightness (this effect is known as "sticking"). To prevent this the interior side of the phosphor layer can be covered with a layer of aluminium connected to the conductive layer inside the tube, which disposes of this charge. It has the additional advantages of increasing brightness by reflecting towards the viewer light emitted towards the back of the tube, and protecting the phosphor from ion bombardment.