Cathode tube

The cathode tube (CRT or Cathode Ray Tubes in English), was invented by Karl Ferdinand Braun. The device consists of a heated filament, Cathode S and Anode S in the shape of perforated lenses which subjected to a potential difference (tension) create an electric field accelerating the electrons. The latter come to strike one of the flat edges, the screen, on which one deposited a fluorescent layer reacting to the shock of the electron S by creating a luminous point.

This component is a display device used in the majority of the screens of computers, Télévision S and Oscilloscope S. the cathode tube was developed by work of Philo Farnsworth and was less used in televisions starting from the Années 2000 because replaced gradually by the screens plasmas and the screens LCD.

Origins

The first version of the cathode tube was a diode with Cold cathode, makes of it a modification of the tube of Crookes with a layer of Phosphore on the face, this tube is sometimes called Braun tube. The first version using a hot cathode was developed by J.B. Johnson and H.W. Weinhart of the company Western Electric. This product was marketed in 1922.

Operation

The cathode rays are flows of electron S at high-speed coming from the Cathode of the tube, this important speed is due to high the tension of the Anode. In a cathode tube, the electrons are focused, that is to say magnetically by a winds or électrostatiquement by a grid so as to obtain a thin ray, the density of the ray can possibly be controlled by a grid as it is the case in the tubes TV, the whole of the device is called “Electronic gun”. The ray which leaves the electron gun is then deviated, either magnetically by reels (as in a tube TV), or électrostatiquement by electrodes of deflections (in the majority of the oscilloscopes). This ray arrives then on the anode covered with a phosphorescent matter, often containing Rare earths. When the electrons strike this surface, of the Lumière is emitted, it is the “spot”.

Posting with sweeping

In the case of the Television S and of the screens of modern computers, all the face of the tube is scannée according to a well defined course, and the image is created while varying the intensity of the flow of electrons (the beam), and thus the luminous intensity of the spot, with the length of its course. Flow in all the TV modern is deviated by a magnetic field applied to the collar of the tube by a “magnetic yoke” ( magnetic yoke in English), which is composed of reels (often two) rolled up on Ferrite and controlled by an electronic circuit. It is a sweeping by deflection magnétique.
During sweeping the spot from left to right traverses lines which follow one another from top to bottom (as the lines of a book), the return to the following line and at the beginning of page is done extinct spot.

The Interlacing

Television is resulting from the cinema and posts 25 images a second in Europe (30 images a second for America and Japan) what is close to the 24 of films projected in room. But contrary to the cinema which projects a whole image each time the cathode tube shows only one luminous point with fast displacement, the spot, which is too little for the eye. To avoid a perception of flickering the 625 lines (in Europe) of the traditional image of television are swept in two times: initially the odd lines then the even lines, in this way one artificially obtains 50 images (60 in America and Japan) a second and the eye does not perceive more clignotement.
Dans the case of the data-processing monitors whose posting of the images is done at a higher frequency (60 to 70 per seconds) interlacing is not more necessary.

Vectorial posting

In the case of a Oscilloscope, the intensity of the beam is maintained constant, and the image is drawn by the way which the beam traverses. Normally, the horizontal deflection is proportional to time, and the vertical deflection is proportional to the signal. The tubes for this kind of use are long and narrow, moreover the deflection is ensured by the application of an electrostatic field in the tube using plates (of deflection) located at the collar of the tube. This type of deflection is faster than a magnetic deflection, because in the case of a magnetic deflection, the inductance of the reel prevents the fast variations of the magnetic field (because it prevents the fast variation of the current which creates the magnetic field).

Vectorial posting of the computers

The first graphic screens for Ordinateur S used tubes with vectorial order similar to those of the oscilloscopes. Here the beam traced lines between arbitrary points, by repeating that as quickly as possible. The vectorial monitors were used for the majority in the screens of computer of the end of the year 1970. Vectorial posting for computer does not suffer from Crènelage and pixelisation, but is limited, because it can only post contours of the forms, and a small quantity of text, preferably large (because the speed of posting is inversely proportional to the number of vectors to draw, “to fill” a zone while using full with vectors is impossible just like the writing of a great quantity of text). Certain vectorial screens are able to post several Couleur S, often by using two or three layers of Phosphore. In these screens, by controlling the speed of the beam of electrons, one controls the layer reached and thus the posted color which generally was the Vert, the orange or the red .

Other graphic screens used tubes of storage (storage tube). These cathode tubes stored the images and did not require periodic cooling…

Color screens

Principle

Each luminous point (Pixel) of a color screen consists of three matters, formerly three discs laid out in equilateral triangle, today three horizontally juxtaposed rectangles, the face of the tube is thus covered with tiny triple points. Each one of these matters produces a color if it is subjected to a flow of electrons, the colors are the suivantes : red, green and blue. There are three electronic guns, one by color, and each gun can light only the points of a color, a mask (metal plate opening of holes: one by Pixel) is laid out in the tube right before the face to prevent that a gun does not overflow on the other.

Protections

The Glass used for the face of the tube, allows the passage of the light produced by phosphorus towards outside, but in all the modern models it blocks the X-rays generated by the impact of the flow of electrons with high energy. For this reason glass of the face is charged in Plomb (it is thus Verre Cristal). It is thanks to that and with the other internal shieldings, that the tubes can satisfy the increasingly severe security standards as regards radiation.

Returned colors

The cathode tubes have a characteristic intensity of the flow of electrons, luminous intensity which is not linear, one calls that the Gamma. For the first televisions, gamma of the screen was an advantage, because by compressing the signal (a little with the manner of a pedal of compression for guitar) contrast is increased (note: one does not speak about numerical compression, but about compression of a signal, which can be defined by a reduction of what has a low level and an increase of what is higher). The modern tubes always have gamma (weaker), but this gamma can be corrected, so as to obtain a linear answer, making it possible to see the image under its true colors, which is very important in the Imprimerie inter alia.

Static electricity

Certain screens or televisions using of the cathode tubes can accumulate static electricity, inoffensive, on the face of the tube what can involve the accumulation of dust reducing the quality of the image, a cleaning is thus necessary (with a dry rag or an adapted product because certain products can damage the anti-reflecting layer if it exists).

Other technologies

The cathode tubes are likely to become obsolete because little by little the plasma display screens and the screens with liquid crystals replace the cathode tube screens. These new types of screens have as advantages small overall dimensions and a lower consumption. Moreover their price becomes increasingly close to that of the tube screens. Their returned colors is now identical to the tubes . The increasingly weak latency time allows (for certain models in 3 ms old lower part), to use plays of action such as the subjective Jeux of shooting, without having to undergo trails of posting at the time of fast movements. These trails were until now an important brake for a use in the screens of computers general public.

Applications

majority of the television sets and screens of computer
the Oscilloscope S
the Radar S
…

See too

Trinitron

Random links: