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.

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, by 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 properly 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 electronically 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 type had around 15 points of manual adjustment to align the color and was usually located on a board from the side of the TV case, which could be accessed without dismounting the rear cover of the TV, and easily accessed while viewing a test grid picture on the screen.
Flat-gun modern CRTs require no power for the convergence magnets.

Beam-landing trim magnets
Modern shadow-mask color CRTs have a set of six weak ring magnets around their necks, behind the deflection yoke. These magnets have tabs that permit them to be rotated around the neck to correct beam-landing errors that would affect color purity and cannot be corrected otherwise. (Each of the three beams must excite only the color of phosphor it is intended to; this is color purity.)
In the magnet sets, each ring of one pair has N and S poles diametrically opposite, creating a dipolar field. Rotating these together orients the field, while relative rotation between the pair causes the individual ring fields to aid, to have no effect on, or cancel each other, depending upon relative position.
Similarly, a second pair has four poles, alternating N and S around each ring. This pair provides a quadrupolar field, and its magnitude and orientation are adjusted as with the dipole-field magnets.
Finally, the third pair has six poles, again N and S alternating around the ring, and provides a hexapolar field.
When adjusted, the field inside the neck is a composite of the three types of field, and ensures that each beam lands only on the color of phosphor that it is intended to excite.

Fast events
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.

Phosphor persistence
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 rapidly to avoid smearing of new information by the remains of the previous picture; i.e., they should have relatively-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.
There were moving-film oscilloscope cameras in which film movement provided the time base; they functioned much like strip-chart recorders.

Phosphor colors and designations
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. Earlier, the green P1 phosphor was used for the same purposes, but it burned more easily and had a somewhat shorter persistence. For displays meant to be photographed rather than viewed, the blue trace of the P11 phosphor gives higher photographic brightness; it has a very short persistence. For extremely slow displays, such as radar PPIs, very-long-persistence phosphors such as P7, which produce a blue trace followed by a longer-lasting amber or yellow afterimage, are used. P7 is a dual-layer phosphor, and the long-persistence layer is excited by the light from the blue phosphor rather than from the electron beam. Color TV CRT phosphors are collectively designated P22

Graticules
The phosphor screen of most oscilloscope tubes contains a permanently-marked internal graticule, a simple array of squares that serves as a reference grid. It divides 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 graticules can be side-illuminated for use in a darkened room. The markings disperse the light, appearing bright, while the smooth surface acts like a light pipe.

Implosion protection

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.