Cathode ray tube (CRT)

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Cathode ray tube (CRT)

Cutaway rendering of a color CRT:

1. Electron guns

2. Electron beams

3. Focusing coils

4. Deflection coils

5. Anode connection

6. Mask for separating beams for red, green, and blue part of displayed image

7. Phosphor layer 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 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 form 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.

The single electron beam can be processed in such a way as to display moving pictures in natural colors.

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 key stoning.

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 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.

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 out gassing 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 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 aluminum 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 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 light emitted towards the back of the tube, and protecting the phosphor from ion bombardment.

Oscilloscope tubes

For use of an oscilloscope, the design is somewhat different. Rather than tracing out a raster, the electron beam is directly steered along an arbitrary path, while its intensity is kept constant. Usually the beam is deflected horizontally (X) by a varying potential difference between a pair of plates to its left and right, and vertically (Y) by plates above and below, although magnetic deflection is possible. The instantaneous position of the beam will depend upon the X and Y voltages. It is most useful for the horizontal voltage, repeatedly, to increase linearly with time until the beam reaches the edge of the screen, then jump back to its starting value (saw tooth waveform, generated by a time base). This causes the display to trace out the Y voltage as a function of time. Many oscilloscopes only function in this mode. However it can be useful to display, say, the voltage versus the current in an inductive component with an oscilloscope that allows X-Y input, without using the time base.

The electron gun is always centered in the tube neck; the problem of ion production is either ignored or mitigated by using an aluminized screen.

The beam can be moved much more rapidly, and it is easier to make the beam deflection accurately proportional to the applied signal, by using electrostatic deflection as described above instead of magnetic deflection. Magnetic deflection is achieved by passing currents through coils external to the tube; it allows the construction of much shorter tubes for a given screen size. Circuit arrangements are required to approximately linearize the beam position as a function of signal current and the very wide deflection angles require arrangements to keep the beam focused (dynamic focusing).

In principle either type of deflection can be used for any purpose; but electrostatic deflection is best for oscilloscopes with relatively small screens and high performance requirements, while a television receiver with a large screen and electrostatic deflection would be many meters deep.

Some issues must be resolved when using electrostatic deflection. Simple deflection plates appear as a fairly large capacitive load to the deflection amplifiers, requiring large current flows to charge and discharge this capacitance rapidly.

Another, more subtle, problem is that when the electrostatic charge switches, electrons which are already part of the way through the deflection plate region will only be partially deflected. This results in the trace on the screen lagging behind a rapid change in signal.

Extremely high performance oscilloscopes avoid these problems by subdividing the vertical (and sometimes horizontal) deflection plates into a series of plates along the length of the "deflection" region of the CRT, and electrically joined by a delay line terminated in its characteristic impedance; the timing of the delay line is set to match the velocity of the electrons through the deflection region. In this way, a change of charge "flows along" the deflection plate along with the electrons that it should affect, almost negating its effect on those electrons which are already partially through the region. Consequently the beam as seen on the screen slews almost instantly from the old point to the new point. In addition, because the entire deflection system operates as a matched-impedance load, the problem of driving a large capacitive load is mitigated.

It is very common for oscilloscopes to have amplifiers which rapidly chop or swap the beam, blanking the display while switching. This allows the single beam to show as two or more traces, each representing a different input signal. These are properly called multiple-trace (dual trace, quadruple trace, etc.) oscilloscopes.

Much rarer is the true dual beam oscilloscope, whose tube contains an electron gun that produces two independent electron beams. Usually, but not always, both beams are deflected horizontally by a single shared pair of plates, while each beam has its own vertical deflection plates. This allows a time-domain display to show two signals simultaneously.

Many modern oscilloscope tubes pass the electron beam through an expansion mesh. This mesh acts like a lens for electrons and has the effect of roughly doubling the deflection of the electron beam, allowing the use of a larger faceplate for the same length of tube envelope. The expansion mesh also tends to increase the "spot size" on the screen, but this trade off is usually acceptable.

When displaying one-shot fast events the electron beam must deflect very quickly, with few electrons impinging on the screen, leading to a faint or invisible display.

A simple improvement can be attained by fitting a hood on the screen against which the observer presses his face, excluding extraneous light, but oscilloscope CRTs designed for very fast signals give a brighter display by passing the electron beam through a micro-channel plate just before it reaches the screen. Through the phenomenon of secondary emission this plate multiplies the number of electrons reaching the phosphor screen, giving a brighter display, possibly with a slightly larger spot.

The phosphors used in the screens of oscilloscope tubes are different from those used in the screens of other display tubes. Phosphors used for displaying moving pictures should produce an image which fades very rapidly to avoid smearing of new information by the remains of the previous picture; i.e., they should have short persistence. An oscilloscope will often display a trace which repeats unchanged, so longer persistence is not a problem; but it is a definite advantage when viewing a single-shot event, so longer-persistence phosphors are used.

An oscilloscope trace can be any color without loss of information, so a phosphor with maximum effective luminosity is usually used. The eye is most sensitive to green: for visual and general-purpose use the P31 phosphor gives a visually bright trace, and also photographs well and is reasonably resistant to burning by the electron beam. For displays meant to be photographed rather than viewed, the blue trace of P11 phosphor gives higher photographic brightness; for extremely slow displays, very-long-persistence phosphors such as P7, which produce a blue trace followed by a longer-lasting amber or yellow afterimage, are used.

The phosphor screen of most oscilloscope tubes contains a permanently-marked internal graticule, dividing the screen using Cartesian coordinates. This internal graticule allows for the easy measurement of signals with no worries about parallax error.

Less expensive oscilloscope tubes may instead have an external graticule of glass or acrylic plastic. Most graticule can be side-illuminated for use in a darkened room.

Oscilloscope tubes almost never contain integrated implosion protection (see below). External implosion protection must always be provided, either in the form of an external graticule or, for tubes with an internal graticule, a plain sheet of glass or plastic. The implosion protection shield is often colored to match the light emitted by the phosphor screen; this improves the contrast as seen by the user.

Graphical displays for early computers used vector monitors, a type of CRT similar to the oscilloscope but typically using magnetic, rather than electrostatic, deflection. Magnetic deflection allows the construction of much shorter tubes for a given viewable image size.

Here, the beam traces straight lines between arbitrary points, repeatedly refreshing the display as quickly as possible. Vector monitors were also used by some late-1970s to mid-1980s arcade games such as Asteroids. Vector displays for computers did not noticeably suffer from the display artifacts of Aliasing and pixilation, but were limited in that they could display only a shape's outline (advanced vector systems could provide a limited amount of shading), and only a limited amount of crudely-drawn text (the number of shapes and/or textual characters drawn was severely limited, because the speed of refresh was roughly inversely proportional to how many vectors needed to be drawn). Some vector monitors are capable of displaying multiple colors, using either a typical tri-color CRT, or two phosphor layers (so-called "penetration color").

In these dual-layer tubes, by controlling the strength of the electron beam, electrons could be made to reach (and illuminate) either or both phosphor layers, typically producing a choice of green, orange, or red.

Other graphical displays used 'storage tubes', including Direct View Bistable Storage Tubes (DVBSTs). These CRTs inherently stored the image, and did not require periodic refreshing.

Some displays for early computers (those that needed to display more text than was practical using vectors, or that required high speed for photographic output) used Charactron CRTs. These incorporate a perforated metal character mask (stencil), which shapes a wide electron beam to form a character on the screen. The system selects a character on the mask using one set of deflection circuits, and selects the position to draw the character at using a second set. The beam is activated briefly to draw the character at that position. Graphics could be drawn by selecting the position on the mask corresponding to the code for a space (in practice, they were simply not drawn), which had a small round hole in the center; this effectively disabled the character mask, and the system reverted to regular vector behavior.

Many of the early computer displays used "slow", or long-persistence, phosphors to reduce flicker for the operator. While it reduces eyestrain for relatively static displays, the drawback of long-persistence phosphor is that when the display is changed, it produces a visible afterimage that can take up to several seconds to fade. This makes it inappropriate for animation, or for real-time dynamic information displays.

Color tubes use three different phosphors which emit red, green, and blue light respectively. They are packed together in strips (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 shape of the Greek letter delta.

Dot pitch defines the "native resolution" of the display. On delta-gun CRTs, as the scanned resolution approaches the dot pitch resolution, moiré (a kind of soft-edged banding) appears, due to interference patterns between the mask structure and the grid-like pattern of pixels drawn. Aperture grille monitors do not suffer from vertical moiré, however, because the phosphor strips have no vertical detail.

The glass envelope

The outer glass allows the light generated by the phosphor out of the monitor, but (for color tubes) it must block dangerous X-rays generated by high energy electrons impacting the inside of the CRT face. For this reason, the glass is leaded. Color tubes require significantly higher anode voltages than monochrome tubes (as high as 32,000 volts in large tubes), partly to compensate for the blockage of some electrons by the aperture mask or grille; the amount of X-rays produced increases with voltage. Because of leaded glass, other shielding, and protective circuits designed to prevent the anode voltage from rising too high in case of malfunction, the X-ray emission of modern CRTs is well within approved safety limits.

CRTs have a pronounced triode characteristic, which results in significant gamma (a nonlinear relationship between beam current and light intensity). In early televisions, screen gamma was an advantage because it acted to compress the screen contrast. However in systems where linear response is required (such as when desktop publishing), gamma correction is applied. The gamma characteristic exists today in all digital video systems.

CRT displays accumulate a static electrical charge on the screen, unless preventive measures are taken. This charge does not pose a safety hazard, but can lead to significant degradation of image quality through attraction of dust particles to the surface of the screen. Unless the display is regularly cleaned with a dry cloth or special cleaning tissue (using ordinary household cleaners may damage anti-glare protective layer on the screen), after a few months the brightness and clarity of the image drops significantly.

The high voltage (EHT) used for accelerating the electrons is provided by a transformer. For CRTs used in televisions, this is usually a fly back transformer that steps up the line (horizontal) deflection supply to as much as 32,000 volts for a color tube, although monochrome tubes and specialty CRTs may operate at much lower voltages. The output of the transformer is rectified and the pulsating output voltage is smoothed by a capacitor formed by the tube itself (the accelerating anode being one plate, the glass being the dielectric, and the grounded (earthed) Aquadag coating on the outside of the tube being the other plate). Before all-glass tubes, the structure between the screen and the electron gun was made from a heavy metal cone which served as the accelerating anode. Smoothing of the EHT was then done with a high voltage capacitor, external to the tube itself. In the earliest televisions, before the invention of the fly back transformer design, a linear high-voltage supply was used; because these supplies were capable of delivering much more current at their high voltage than fly back high voltage systems – in the case of an accident they proved extremely dangerous. The fly back circuit design addressed this: in the case of a fault, the fly back system delivers relatively little current, improving a person's chance of surviving a direct shock from the high voltage anode.