How does a cathode ray tube work? Operating principles and parameters of a cathode ray tube (CRT)

Electrostatic control

Let's consider a CRT device with electrostatic control (Fig. 2.12.) :

Figure 2.12. Electrostatically controlled cathode ray tube.

The simplest electron gun includes: a cathode, a control electrode, and the first and second anodes.

Cathode designed to create a flow of electrons. Typically, CRTs use an oxide heated cathode, made in the form of a small nickel cylinder with a heater inside. The active layer is applied to the bottom of the cylinder. Thus, the cathode has a flat emitting surface and electrons are emitted in a narrow beam towards the screen. The cathode lead is usually connected inside the container to one end of the filament.

Control electrode, or modulator, is designed to adjust the brightness of a luminous spot on the screen. The control electrode is made in the form of a nickel cylinder surrounding the cathode. The cylinder has a hole (diaphragm) through which electrons emitted by the cathode pass.

A small negative voltage relative to the cathode is applied to the control electrode. By changing this voltage, you can adjust the amount of beam current and, therefore, change the brightness of the luminous spot on the tube screen.

First anode It is a cylinder with two or three diaphragms.

The influence of the control electrode and the first anode on the beam current is similar to the influence of the control grid and the anode on the anode current in vacuum tubes.

Second anode also made in the form of a cylinder, but with a slightly larger diameter than the first. This anode usually has a single diaphragm.

A voltage of the order of magnitude is applied to the first anode 300-1000V(relative to the cathode). A higher voltage is applied to the second anode ( 1000-16000 V).

Let's consider the principle of operation of the tube. The heated cathode emits electrons. Under the influence of the electric field existing between the first anode and the cathode, the electrons are accelerated and fly through the diaphragms in the first anode. Electrons emerge from the first anode in the form of a narrow diverging beam.

The electric field between the first and second anodes is called focusing. It changes the trajectory of the electrons so that when leaving the second anode, the electrons move closer to the axis of the tube. In the space between the second anode and the screen, electrons move by inertia due to the energy acquired in the accelerating fields of the electron gun.

By changing the potential of the first anode, the strength of the focusing field can be adjusted so that the trajectories of all electrons intersect on the screen. When electrons fall on the screens, the kinetic energy is partially converted into light, resulting in a luminous point (spot) on the screen.

Electrons incident on the screen knock out secondary electrons from the screen material, which are captured by the conductive graphite layer ( aquadag), applied to the inner surface of the cylinder. In addition, the aquadag plays the role of an electrostatic screen and protects the electron flow of the tube from the effects of external electric fields, since it is connected to the second anode of the tube and grounded with it.

Diaphragms inside anodes contribute to narrowing the electron beam, since they intercept electrons that are strongly deviated from the axis of the tube.

Two pairs of deflection plates when control (modulating) voltages are applied to them, they ensure the occurrence between the corresponding plates X-X And Ooh potential differences that control the movement of a focused electron beam to the desired point on the screen to obtain the required image. When this flow is exposed to two modulating voltages simultaneously, it is possible to deflect the electron beam to any point on the working surface of the screen.

Conclusion: The advantage of electrostatically controlled CRTs is that they require little power to control the beam, and the electronic beam deflection control circuit is much simpler than in magnetically controlled CRTs. The amount of beam deflection in tubes of this type is practically independent of the frequency of the deflecting voltage.

§ 137. Cathode ray tube. Oscilloscope

Oscilloscopes are used to observe, record, measure and control various changing processes in automation devices, telemechanics and other fields of technology (Fig. 198). The main part of the oscilloscope is a cathode ray tube - an electric vacuum device, in its simplest form designed to convert electrical signals into light.

Let's consider how an electron and an electron beam are deflected in the electric field of a cathode ray tube of an oscilloscope.
If an electron is placed between two parallel plates (Fig. 199, a), having opposite electric charges, then under the influence of the electric field arising between the plates, the electron will deflect, since it is negatively charged. He pushes off the plate A, having a negative charge, and is attracted to the plate B having a positive electrical charge. The electron's movement will be directed along the field lines.

When a person moving at a speed enters the field between the plates V electron (Fig. 199, b), then it is acted upon not only by field forces F, but also strength F 1, directed along its movement. As a result of the action of these forces, the electron will deviate from its straight path and will move along the line OK. - diagonally.
If a narrow beam of moving electrons is passed between the plates - an electron beam (Fig. 199, c), it will deflect under the influence of an electric field. The angle of deflection of the electron beam depends on the speed of movement of the electrons that make up the beam and the magnitude of the voltage creating the electric field between the plates.
Each cathode ray tube (Fig. 200) is a cylinder from which air has been pumped out. The conical part of the inner surface of the cylinder is covered with graphite and is called aquadag. Inside the cylinder 3 fits electronic spotlight 8 - electron gun, deflection plates 4 And 6 , and screen 5 . The electron tube illuminator consists of a heated cathode, which emits electrons, and a system of electrodes that form the electron beam. This beam, emitted by the cathode of the tube, moves at high speed towards the screen and is essentially an electric current directed in the opposite direction to the movement of the electrons.

The cathode is a nickel cylinder, the end of which is coated with a layer of oxide. The cylinder is placed on a thin-walled ceramic tube, and a tungsten filament made in the form of a spiral is placed inside it to heat the cathode.
The cathode is located inside the control electrode 7 shaped like a cup. A small hole is made in the bottom of the cup through which electrons emitted from the cathode pass; this hole is called diaphragm. A small negative voltage (of the order of several tens of volts) is applied to the control electrode relative to the cathode. It creates an electric field that acts on the electrons emitted from the cathode so that they are collected into a narrow beam directed towards the tube screen. The point of intersection of electron flight trajectories is called first focus of the tube. By increasing the negative voltage on the control electrode, some electrons can be deflected so much that they will not pass through the hole and thus the number of electrons hitting the screen will decrease. By changing the voltage of the control electrode, you can regulate the number of electrons in it. This allows you to change the brightness of a luminous spot on the screen of a cathode ray tube, which is coated with a special composition that has the ability to glow under the influence of an electron beam falling on it.
The electron gun also includes two anodes that create an accelerating field: the first is focusing 1 and the second is the manager 2 . Each of the anodes is a cylinder with a diaphragm, which serves to limit the cross section of the electron beam.
The anodes are located along the axis of the tube at a certain distance from one another. A positive voltage of the order of several hundred volts is applied to the first anode, and the second anode, connected to the aquadag of the tube, has a positive potential several times greater than the potential of the first anode.
Electrons escaping from the hole of the control electrode, entering the electric field of the first anode, acquire high speed. Flying inside the first anode, the electron beam is compressed under the influence of the electric field forces and forms a thin electron beam. Next, the electrons fly through the second anode, acquire an even higher speed (several thousand kilometers per second), and fly through the diaphragm to the screen. On the latter, under the influence of electron impacts, a luminous spot with a diameter of less than one millimeter is formed. This spot is located second focus cathode ray tube.
To deflect the electron beam in two planes, the cathode ray tube is equipped with two pairs of plates 6 And 4 , located in different planes perpendicular to one another.
First pair of plates 6 , which is located closer in the electron gun, serves to deflect the beam in the vertical direction; these plates are called vertically deflecting. Second pair of plates 4 , located closer to the tube screen, serves to deflect the beam in the horizontal direction; these plates are called horizontally deflecting.
Let's consider the principle of operation of deflecting plates (Fig. 201).

Deflection plates IN 2 and G 2 connected to potentiometer sliders P in and P d. A constant voltage is applied to the ends of the potentiometers. Deflection plates IN 1 and G 1, like the midpoints of the potentiometers, are grounded and their potentials are zero.
When the potentiometer sliders are in the middle position, the potential on all plates is zero, and the electron beam creates a luminous spot in the center of the screen - a point ABOUT. When moving the potentiometer slider P g left onto the plate G 2, a negative voltage is applied and therefore the electron beam, repelling from this plate, will deviate and the luminous point on the screen will shift in the direction of the point A.
When moving the potentiometer slider P g right plate potential G 2 the electron beam will increase and, consequently, the luminous point on the screen will shift horizontally to the point B. Thus, with a continuous change in the potential on the plate G 2 the electron beam will draw a horizontal line on the screen AB.
Similarly when changing with a potentiometer P in the voltage on the vertical deflection plates, the beam will deflect vertically and draw a vertical line on the screen VG. By simultaneously changing the voltage on both pairs of deflection plates, the electron beam can be moved in any direction.
The screen of a cathode ray tube is coated with a special compound - a phosphor that can glow under the influence of impacts from rapidly flying electrons. Thus, when a focused beam hits one or another point on the screen, it begins to glow.
To cover the screens of cathode ray tubes, phosphors are used in the form of zinc oxide, beryllium zinc, a mixture of zinc sulfate with cadmium sulfate, etc. These materials have the property of continuing to glow for some time after the electron impacts have stopped. This means that they have afterglow.
It is known that the human eye, having received a visual impression, can hold it for about 1/16 of a second. In a cathode ray tube, the beam can move across the screen so quickly that a number of successive luminous points on the screen are perceived by the eye as a continuous luminous line.
The voltage to be studied (considered) using an oscilloscope is applied to the vertical deflection plates of the tube. A sawtooth voltage is applied to the horizontal deflection plates, the graph of which is shown in Fig. 202, a.

This voltage is supplied by an electronic sawtooth pulse generator, which is mounted inside the oscilloscope. Under the influence of a sawtooth voltage, the electron beam moves horizontally across the screen. During t 1 - t 8 the beam moves across the screen from left to right, and over time t 9 - t 10 quickly returns to its original position, then moves again from left to right, etc.
Let's find out how you can see on the screen of a cathode ray tube of an oscilloscope the shape of the curve of instantaneous voltage values ​​supplied to the vertical deflection plates. Let us assume that a sawtooth voltage with an amplitude of 60 is applied to the horizontal deflection tubes V and with a change period of 1/50 sec.
In Fig. Figure 202, b shows one period of sinusoidal voltage, the shape of the curve of which we want to see, and the circle (Fig. 202, c) shows the resulting movement of the electron beam on the screen of the oscilloscope tube.
Voltages at the same instants have the same designations on the top two graphs.
At a moment in time t 1 sawtooth voltage ( U d), deflecting the electron beam horizontally, is equal to 60 V, and the stress on the vertical plates U equals zero and a dot lights up on the screen O 1 . At a moment in time t 2 voltage U g = - 50 V, and the voltage U in = 45 V. In a time equal to t 2 - t 1, the electron beam will move to position O 2 on line O 1 - O 2. At a moment in time t 3 voltage U g = 35 V, and the voltage U in = 84.6 V. During t 3 - t 2 beam will move to the point O 3 on line O 2 - O 3, etc.
The process of action of the electric fields created by both pairs of deflection plates on the electron beam will continue, and the beam will be deflected further along the line O 3 - O 4 - o 6, etc.
During t 10 - t 9, the electron beam will quickly deviate to the left (the beam will reverse), and then the process will be repeated: The voltage being tested changes periodically, so the electron beam will move repeatedly along the same path, resulting in a fairly bright line, similar in shape to the shape of the voltage curve applied to the vertical deflection plates of the tube.
Since the period (and frequency) of the voltages of the sawtooth sweep pulses and the voltage under study are equal, the sinusoid on the screen will be motionless. If the frequency of these voltages is different and not a multiple of each other, then the image will move along the tube screen.
When two sinusoidal voltages of equal amplitudes and frequencies, but shifted in phase by 90°, are connected to both pairs of deflection plates, a circle will be visible on the tube screen. Thus, using an oscilloscope, you can observe and examine various processes occurring in electrical circuits. In addition to the sawtooth pulse generator, the oscilloscope has amplifiers to amplify the voltage applied to the vertical beam deflection plates and the sawtooth voltage applied to the horizontal deflection plates.

How does a cathode ray tube work?

Cathode ray tubes are electric vacuum devices in which an electron beam of small cross-section is formed, and the electron beam can be deflected in the desired direction and, hitting a luminescent screen, cause it to glow (Fig. 5.24). A cathode ray tube is an electron-optical converter that converts an electrical signal into its corresponding image in the form of a pulsed oscillation reproduced on the tube screen. The electron beam is formed in an electron spotlight (or electron gun), consisting of a cathode and focusing electrodes. The first focusing electrode, also called modulator, acts as a negative bias grid that directs electrons towards the axis of the tube. Changing the grid bias voltage affects the number of electrons, and therefore the brightness of the image obtained on the screen. Behind the modulator (toward the screen) are the following electrodes, whose task is to focus and accelerate the electrons. They operate on the principle of electronic lenses. Focusing-accelerating electrodes are called anodes and a positive voltage is applied to them. Depending on the type of tube, anode voltages range from several hundred volts to several tens of kilovolts.

Rice. 5.24. Schematic representation of a cathode ray tube:

1 - cathode; 2 - anode I: 3 - anode II; 4 - horizontal deflection plates; 5 - electron beam; 6 - screen; 7 - vertical deflection plates; 8 - modulator

In some tubes, the beam is focused using a magnetic field by using coils located outside the lamp, instead of electrodes located inside the tube, which create a focusing electric field. Beam deflection is also carried out by two methods: using an electric or magnetic field. In the first case, deflection plates are placed in the tube, in the second, deflection coils are mounted outside the tube. To deflect in both horizontal and vertical directions, plates (or coils) of vertical or horizontal beam deflection are used.

The tube screen is covered from the inside with a material - a phosphor, which glows under the influence of electron bombardment. Phosphors differ in different glow colors and different glow times after the excitation ceases, which is called afterglow time. Typically it ranges from a fraction of a second to several hours, depending on the purpose of the tube.

Cathode ray tubes(CRT) - electrovacuum devices designed to convert an electrical signal into a light image using a thin electron beam directed onto a special screen covered phosphor- a composition capable of glowing when bombarded with electrons.

In Fig. Figure 15 shows the device of a cathode ray tube with electrostatic focusing and electrostatic beam deflection. The tube contains an oxide heated cathode with an emitting surface facing the hole in the modulator. A small negative potential is established on the modulator relative to the cathode. Further along the axis of the tube (and along the beam) there is a focusing electrode, also called the first anode; its positive potential helps to draw electrons from the near-cathode space through the modulator hole and form a narrow beam from them. Further focusing and acceleration of electrons is carried out by the field of the second anode (accelerating electrode). Its potential in the tube is most positive and ranges from units to tens of kilovolts. The combination of the cathode, modulator and accelerating electrode forms an electron gun (electronic spotlight). The inhomogeneous electric field in the space between the electrodes acts on the electron beam as a collecting electrostatic lens. The electrons, under the influence of this lens, converge to a point on the inside of the screen. The inside of the screen is covered with a layer of phosphor - a substance that converts the energy of the electron flow into light. Outside, the place where the flow of electrons falls onto the screen glows.

To control the position of the luminous spot on the screen and thereby obtain an image, the electron beam is deflected along two coordinates using two pairs of flat electrodes - deflection plates X and Y. The angle of deflection of the beam depends on the voltage applied to the plates. Under the influence of variable deflecting voltages on the plates, the beam runs around different points on the screen. The brightness of the dot depends on the current strength of the beam. To control brightness, an alternating voltage is applied to the input of the modulator Z. To obtain a stable image of a periodic signal, it is periodically scanned on the screen, synchronizing the linearly varying horizontal scan voltage X with the signal under study, which is simultaneously supplied to the vertical deflection plates Y. In this way, images are formed on the screen CRT. The electron beam has low inertia.

In addition to electrostatic, it is also used magnetic focusing electron beam. It uses a direct current coil into which a CRT is inserted. The quality of magnetic focusing is higher (smaller spot size, less distortion), but magnetic focusing is bulky and continuously consumes power.

Magnetic beam deflection, carried out by two pairs of coils with currents, is widely used (in picture tubes). In a magnetic field, an electron is deflected along the radius of a circle, and the deflection angle can be significantly larger than in a CRT with electrostatic deflection. However, the performance of the magnetic deflection system is low due to the inertia of the current-carrying coils. Therefore, in oscillographic tubes, exclusively electrostatic beam deflection is used as it has less inertia.

The screen is the most important part of a CRT. As electroluminophores Various inorganic compounds and their mixtures are used, for example, zinc and zinc-cadmium sulfides, zinc silicate, calcium and cadmium tungstates, etc. with admixtures of activators (copper, manganese, bismuth, etc.). The main parameters of the phosphor: glow color, brightness, spot luminous intensity, luminous efficiency, afterglow. The color of the glow is determined by the composition of the phosphor. Luminescent brightness in cd/m2

B ~ (dn/dt)(U-U 0) m,

where dn/dt is the flow of electrons per second, that is, the beam current, A;

U 0 - phosphor glow potential, V;

U – accelerating voltage of the second anode, V;

The light intensity of the spot is proportional to the brightness. Luminous efficiency is the ratio of the luminous intensity of the spot to the beam power in cd/W.

Afterglow– this is the time during which the brightness of the spot after turning off the beam decreases to 1% of the original value. There are phosphors with very short (less than 10 μs) afterglow, short (from 10 μs to 10 ms), medium (from 10 to 100 ms), long (from 0.1 to 16 s) and very long (more than 16 s) afterglow. The choice of the afterglow value is determined by the field of application of the CRT. For kinescopes, phosphors with low afterglow are used, since the image on the kinescope screen is constantly changing. For oscilloscope tubes, phosphors with a medium to very long persistence are used, depending on the frequency range of the signals to be displayed.

An important issue that requires more detailed consideration is the potential of the CRT screen. When an electron hits the screen, it charges the screen with a negative potential. Each electron recharges the screen, and its potential becomes increasingly negative, so that a braking field very quickly arises, and the movement of electrons towards the screen stops. In real CRTs this does not happen, because each electron that hits the screen knocks out secondary electrons from it, that is, secondary electron emission occurs. Secondary electrons carry away a negative charge from the screen, and to remove them from the space in front of the screen, the inner walls of the CRT are covered with a carbon-based conductive layer, electrically connected to a second anode. In order for this mechanism to work, secondary emission factor, that is, the ratio of the number of secondary electrons to the number of primary ones must exceed one. However, for phosphors, the secondary emission coefficient Kve depends on the voltage at the second anode U a. An example of such a dependence is shown in Fig. 16, from which it follows that the screen potential should not exceed the value

U a max , otherwise the brightness of the image will not increase, but decrease. Depending on the phosphor material, the voltage U a max = 5...35 kV. To increase the limiting potential, the inside of the screen is covered with a thin film of metal (usually aluminum, permeable to electrons). aluminized screen) electrically connected to the second anode. In this case, the screen potential is determined not by the secondary emission coefficient of the phosphor, but by the voltage at the second anode. This allows you to use a higher voltage of the second anode and obtain a higher brightness of the screen. The brightness of the glow also increases due to the reflection of light emitted into the tube from the aluminum film. The latter is transparent only to sufficiently fast electrons, so the voltage of the second anode must exceed 7...10 kV.

The service life of cathode ray tubes is limited not only by the loss of emission from the cathode, as with other vacuum devices, but also by the destruction of the phosphor on the screen. Firstly, the power of the electron beam is used extremely inefficiently. No more than two percent of it turns into light, while more than 98% only heats up the phosphor, and its destruction occurs, which is expressed in the fact that the luminous efficiency of the screen gradually decreases. Burnout occurs faster with an increase in the power of the electron flow, with a decrease in the accelerating voltage, and also more intensely in places where the beam falls for a longer time. Another factor that reduces the life of a cathode ray tube is the bombardment of the screen by negative ions generated from the atoms of the cathode oxide coating. Accelerated by the accelerating field, these ions move towards the screen, passing through the deflection system. In electrostatic deflection tubes, ions are deflected just as efficiently as electrons, so they hit different areas of the screen more or less evenly. In tubes with magnetic deflection, ions are deflected weaker due to their many times greater mass than electrons, and fall mainly into the central part of the screen, over time forming a gradually darkening so-called “ion spot” on the screen. Tubes with an aluminized screen are much less sensitive to ion bombardment, since the aluminum film blocks the path of ions to the phosphor.

The two most widely used types of cathode ray tubes are: oscillographic And kinescopes. Oscilloscope tubes are designed to display a variety of processes represented by electrical signals. They have electrostatic beam deflection because it allows the oscilloscope to display higher frequency signals. The beam focusing is also electrostatic. Typically, an oscilloscope is used in periodic sweep mode: a sawtooth voltage with a constant frequency ( sweep voltage), an amplified voltage of the signal under study is applied to the vertical deflection plates. If the signal is periodic and its frequency is an integer number of times higher than the sweep frequency, a stationary graph of the signal over time appears on the screen ( oscillogram). Modern oscilloscope tubes are more complex in design than the one shown in Fig. 15, they have a larger number of electrodes, they are also used double beam oscillographic CRTs, which have a double set of all electrodes with one common screen and allow you to display two different signals synchronously.

CRTs are CRTs with brightness mark, that is, with control of the brightness of the beam by changing the modulator potential; they are used in household and industrial televisions, as well as monitors computers to convert an electrical signal into a two-dimensional image on a screen. CRTs differ from oscillographic CRTs in their larger screen sizes and the nature of the image ( halftone on the entire surface of the screen), the use of magnetic deflection of the beam along two coordinates, a relatively small size of the luminous spot, strict requirements for the stability of the spot size and the linearity of the scans. The most advanced are color picture tubes for computer monitors; they have high resolution (up to 2000 lines), minimal geometric raster distortion, and correct color rendition. At different times, picture tubes with a diagonal screen size from 6 to 90 cm were produced. The length of the picture tube along its axis is usually slightly less than the diagonal size, the maximum beam deflection angle is 110...116 0. The inside of a color picture tube screen is covered with many dots or narrow stripes of phosphors of different compositions, which convert the electric beam into one of three primary colors: red, green, blue. A color picture tube has three electron guns, one for each primary color. When scanned across the screen, the rays move in parallel and illuminate adjacent areas of the phosphor. The beam currents are different and depend on the color of the resulting image element. In addition to picture tubes for direct observation, there are projection picture tubes that, despite their small size, have a high brightness of the image on the screen. This bright image is then optically projected onto a flat white screen, creating a large image.

After the deflection system, the electrons fall on the CRT screen. The screen consists of a thin layer of phosphor applied to the inner surface of the end part of the balloon and capable of glowing intensely when bombarded with electrons.

In some cases, a conductive thin layer of aluminum is applied on top of the phosphor layer. Screen properties are determined by its

characteristics and parameters. The main screen parameters include: first And second critical screen potentials, glow brightness, luminous efficiency, afterglow duration.

Screen potential. When the screen is bombarded by a stream of electrons from its surface, secondary electron emission occurs. To remove secondary electrons, the tube walls near the screen are coated with a conductive graphite layer, which is connected to the second anode. If this is not done, then the secondary electrons, returning to the screen, together with the primary ones, will lower its potential. In this case, a braking electric field is created in the space between the screen and the second anode, which will reflect the electrons of the beam. Thus, to eliminate the braking field, it is necessary to remove the electric charge carried by the electron beam from the surface of the non-conducting screen. Almost the only way to compensate for the charge is to use secondary emission. When electrons fall on the screen, their kinetic energy is converted into the glow energy of the screen, heats it and causes secondary emission. The value of the secondary emission coefficient o determines the screen potential. The coefficient of secondary electron emission a = / in // l (/„ is the current of secondary electrons, / l is the beam current, or the current of primary electrons) from the screen surface in a wide range of changes in the energy of primary electrons exceeds unity (Fig. 12.8, O < 1 на участке O A curve at V < С/ кр1 и при 15 > S/cr2).

At And < (У кр1 число уходящих-от экрана вторичных электронов меньше числа первичных, что приводит к накоплению отрицательного заряда на экране, формированию тормозящего поля для электронов луча в пространстве между вторым анодом и экраном и их отражению; свечение экрана отсутствует. Потенциал and l2= Г/крР corresponding to point A in Fig. 12.8, called first critical potential.

At C/a2 = £/cr1 the screen potential is close to zero.

If the beam energy becomes greater than e£/cr1, then o > 1 and the screen begins to charge

Rice. 12.8

relative to the last anode of the spotlight. The process continues until the screen potential becomes approximately equal to the potential of the second anode. This means that the number of electrons leaving the screen is equal to the number of incident ones. In the range of beam energy changes from e£/cr1 to C/cr2 c > 1 and the screen potential is quite close to the potential of the projector anode. At and &2 > N cr2 secondary emission coefficient a< 1. Потенциал экрана вновь снижается, и у экрана начинает формироваться тормозящее для электронов луча поле. Потенциал And kr2 (corresponds to the point IN in Fig. 12.8) is called second critical potential or maximum potential.

At electron beam energies higher e11 kr2 The brightness of the screen does not increase. For various screens Г/кр1 = = 300...500 V, and kr2= 5...40 kV.

If it is necessary to obtain high brightness, the screen potential is forcibly maintained equal to the potential of the last electrode of the spotlight using a conductive coating. The conductive coating is electrically connected to this electrode.

Light output. This is a parameter that determines the ratio of light intensity J cv, emitted by the phosphor normal to the screen surface, to the power of the electron beam R el incident on the screen:

Light output μ determines the efficiency of the phosphor. Not all of the kinetic energy of primary electrons is converted into visible radiation energy; part of it goes to heating the screen, secondary electron emission and radiation in the infrared and ultraviolet spectral ranges. Light output is measured in candelas per watt: for different screens it varies within 0.1... 15 cd/W. At low electron velocities, glow occurs in the surface layer and part of the light is absorbed by the phosphor. As electron energy increases, light output increases. However, at very high speeds, many electrons penetrate the phosphor layer without producing excitation, and a decrease in light output occurs.

Brightness of the glow. This is a parameter that is determined by the intensity of light emitted in the direction of the observer by one square meter of a uniformly luminous surface. Brightness is measured in cd/m2. It depends on the properties of the phosphor (characterized by coefficient A), the current density of the electron beam y, the potential difference between the cathode and the screen II and minimum screen potential 11 0, at which luminescence of the screen is still observed. The brightness of the glow obeys the law

Exponent values p y potential £/ 0 for different phosphors vary within the limits of 1...2.5 and

30...300 V. In practice, the linear nature of the dependence of brightness on current density y is maintained up to approximately 100 μA/cm 2. At high current densities, the phosphor begins to heat up and burn out. The main way to increase brightness is to increase And.

Resolution. This important parameter is defined as the ability of a CRT to reproduce image detail. Resolution is estimated by the number of individually distinguishable luminous points or lines (rows), respectively, per 1 cm 2 of the surface or 1 cm of screen height, or the entire height of the working surface of the screen. Consequently, to increase the resolution it is necessary to reduce the diameter of the beam, i.e., a well-focused thin beam with a diameter of tenths of a mm is required. The lower the beam current and the higher the accelerating voltage, the higher the resolution. In this case, the best focusing is achieved. Resolution also depends on the quality of the phosphor (large phosphor grains scatter light) and the presence of halos resulting from total internal reflection in the glass part of the screen.

Duration of afterglow. The time during which the brightness decreases to 1% of the maximum value is called screen afterglow time. All screens are divided into screens with very short (less than 10 5 s), short (10“ 5 ...10“ 2 s), medium (10 2 ...10 1 s), long (10 Ch.Lb s) and very long (more than 16 s) afterglow. Tubes with short and very short persistence are widely used in oscillography, and those with medium persistence are widely used in television. Radar indicators typically use tubes with a long persistence.

In radar tubes, long-lasting screens with a two-layer coating are often used. The first layer of phosphor - with a short blue afterglow - is excited by an electron beam, and the second - with a yellow glow and a long afterglow - is excited by the light of the first layer. In such screens it is possible to obtain an afterglow of up to several minutes.

Types of screens. The glow color of the phosphor is very important. In oscillographic technology, when visually observing the screen, CRTs with a green glow are used, which is the least tiring for the eye. Zinc orthosilicate activated with manganese (willemite) has this glow color. For photography, screens with a blue emission color characteristic of calcium tungstate are preferred. In receiving television tubes with a black-and-white image, they try to obtain a white color, for which phosphors are used from two components: blue and yellow.

The following phosphors are also widely used for the manufacture of screen coatings: zinc and cadmium sulfides, zinc and magnesium silicates, oxides and oxysulfides of rare earth elements. Phosphors based on rare earth elements have a number of advantages: they are more resistant to various influences than sulfide ones, are quite efficient, have a narrower spectral band of emission, which is especially important in the production of color picture tubes, where high color purity is required, etc. As An example is the relatively widely used phosphor based on yttrium oxide activated by europium U 2 0 3: Ey. This phosphor has a narrow emission band in the red region of the spectrum. The phosphor, consisting of yttrium oxysulfide with an admixture of europium Y 2 0 3 8: Eu, which has a maximum emission intensity in the red-orange region of the visible spectrum and better chemical resistance than Y 2 0 3: Eu phosphor, also has good characteristics.

Aluminum is chemically inert when interacting with screen phosphors, is easily applied to the surface by evaporation in a vacuum and reflects light well. The disadvantages of aluminized screens include the fact that the aluminum film absorbs and scatters electrons with an energy of less than 6 keV, so in these cases the light output drops sharply. For example, the luminous efficiency of an aluminized screen at an electron energy of 10 keV is approximately 60% greater than at 5 keV. Tube screens have a rectangular or round shape.