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The cathode ray tube (CRT) is a vacuum tube containing an electron gun (a source of electrons) and a fluorescent screen, with internal or external means to accelerate and deflect the electron beam, used to create images in the form of light emitted from the fluorescent screen. The image may represent electrical waveforms (oscilloscope), pictures (television, computer monitor), radar targets and others.
Color CRTs have three separate electron guns (shadow mask) or electron guns that share some electrodes for all three beams (Sony Trinitron, and licensed versions)
The CRT uses an evacuated glass envelope which is large, deep, heavy, and relatively fragile. Display technologies without these disadvantages, such as flat plasma screens, liquid crystal displays, DLP, OLED displays have replaced CRTs in many applications and are becoming increasingly common as costs decline.
An exception to the typical bowl-shaped CRT would be the flat CRTs used by Sony in their Watchman series (the FD-210 was introduced in 1982). One of the last flat-CRT models was the FD-120A. The CRT in these units was flat with the electron gun located roughly at right angles below the display surface thus requiring sophisticated electronics to create an undistorted picture free from effects such as keystoning.
General description
The earliest version of the CRT was invented by the German physicist Ferdinand Braun in 1897 and is also known as the 'Braun tube'. 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 crystalline phosphorescent coating (doped with transition metals or rare earth elements), which emits visible light when excited by high-energy electrons. The beam (or beams, in color CRTs) is deflected either by a magnetic or an electric field to move the bright dot(s) to the required position on the screen. External electromagnets deflect the beams magnetically, while internal plates placed near to and alongside the beam deflect it electrostatically. (Electrostatic deflection is used only for single-beam tubes.)
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. A raster is a rectangular array of closely-spaced parallel lines, scanned one at a time, from left to right (and, ever so slightly, "downhill", because the beam is moving steadily down while drawing the image frame). An image is produced by modulating the intensity of each of the three electron beams, one for each primary color (red, green, and blue) with a received video signal (or another signal derived from it). In all CRT TV receivers except some very early models (The earliest commercial TV receivers used electrostatic deflection, even by the end of the 1940s, many of them relying on the famous 7JP4), the beam is deflected by magnetic deflection, a varying magnetic field generated by coils (the deflection yoke), driven by electronic circuits, around the neck of the tube.
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. Earlier, black-and-white TV CRTs used magnetic focusing, but electrostatic focus has totally superseded focus coils. 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 inside of the CRT neck 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 aluminum coating used in later CRTs eliminated the need for ion traps; they are no longer used.
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 aluminum 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, the light emitted towards the back of the tube. The aluminum layer also protects the phosphors from ion bombardment.
CRT details
Oscilloscope CRTs
For use of an oscilloscope, the design is somewhat different from that in a TV or monitor. Rather than tracing out a raster, the electron beam is directly steered along an arbitrary path, while its intensity is kept constant. So electrostatic deflection is used. Usually the beam is deflected horizontally (X) by a varying potential difference between a pair of plates (inside the neck, of course, part of the electron gun) to its left and right, and vertically (Y) by plates (also inside, part of the gun) above and below, although magnetic deflection is possible (but never seen in modern oscilloscopes). The instantaneous position of the beam will depend upon the X and Y voltages. The first commercial CRT, the WE224, was an electrostatic deflection tube, with a soft vacuum, made by Western Electric intended to be used in the first oscilloscopes.
Color CRTs
Color tubes use three different phosphors which emit red, green, and blue light respectively. They are packed together in stripes (as in aperture grille designs) or clusters called "triads" (as in shadow mask CRTs). Color CRTs have three electron guns, one for each primary color, arranged either in a straight line or in a triangular configuration (the guns are usually constructed as a single unit). Each gun's beam reaches the dots of exactly one color; a grille or mask absorbs those electrons that would otherwise hit the wrong phosphor. Since each beam starts at a slightly different location within the tube, and all three beams are perturbed in essentially the same way, a particular deflection charge will cause the beams to hit a slightly different location on the screen (called a 'sub pixel'). Color CRTs with the guns arranged in a triangular configuration are known as delta-gun CRTs, because the triangular formation resembles the triangular shape of the Greek letter Δ (delta). While having deep color reproduction, CRTs can often exaggerate red.
Convergence in color CRTs
The three beams in color CRTs would not, of themselves, strike the screen at the same place at the same time. Uncorrected, the three colors in a typical image would be unacceptably misaligned. Elaborate measures are needed to make the beams converge acceptably over the entire screen. For one, static convergence brings the beams together, while dynamic convergence (such as from auxiliary coils near the deflection yoke) maintains convergence over the whole screen.
Delta-gun CRTs required the electonically driven convergence coils placed on a "triangle" device on the deflection yoke, powered from the deflection oscillators through a complex set of tunable coils, capacitors and resistors. This set had around 15 points of manual adjustment to align the color and was usually loc
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Few changes for CTV's fall schedule, new U.S. shows slated for A Channel
With so many high-profile U.S. programs already in its lineup, CTV announced Tuesday that it would largely stay the course on this fall, although programmers have packed sister station A Channel with a host of anticipated new U.S. programs.
