Today, the most common type of monitors are CRT (Cathode Ray Tube) monitors. As the name implies, all such monitors are based on a cathode ray tube - a cathode ray tube (CRT). CRT stands for Cathode Ray Terminal, which no longer corresponds to a handset, but to a device based on it.
The technology used in this type of monitor was developed by the German scientist Ferdinand Braun in 1897. and was originally created as a special tool for measuring alternating current, that is, for an oscilloscope.
The design of the CRT - monitor.
The most important element of the monitor is a kinescope, also called a cathode ray tube (see Appendix A, Figure 1.). The kinescope consists of a sealed glass tube, inside of which there is a vacuum, that is, all the air is removed. One of the ends of the tube is narrow and long - this is the neck, and the other - wide and rather flat - is the screen. On the front side, the inner part of the tube glass is coated with a phosphor. Quite complex compositions based on rare earth metals - yttrium, erbium, etc. are used as phosphors for color CRTs. A phosphor is a substance that emits light when bombarded with charged particles. Note that sometimes the phosphor is called phosphorus, but this is not true, because. The phosphor used in the CRT coating has nothing to do with phosphorus. Moreover, phosphorus "glows" as a result of interaction with atmospheric oxygen when oxidized to P 2 O 5 and "glow" occurs for a small amount of time.
To create an image in a CRT monitor, an electron gun is used, from where a stream of electrons comes from under the action of a strong electrostatic field. Through a metal mask or grate, they fall on the inner surface of the glass screen of the monitor, which is covered with multi-colored phosphor dots. The electron flow (beam) can be deflected in the vertical and horizontal planes, which ensures that it consistently hits the entire screen field. The beam is deflected by means of a deflecting system (see Appendix A, Fig. 2.). Deflecting systems are subdivided into saddle-toroidal and saddle-shaped. The latter are preferable because they create a reduced level of radiation.
The deflecting system consists of several inductors located at the neck of the kinescope. With the help of an alternating magnetic field, two coils create a deflection of the electron beam in the horizontal plane, and the other two - in the vertical plane.
A change in the magnetic field occurs under the action of an alternating current flowing through the coils and changing according to a certain law (this is usually a sawtooth change in voltage over time), while the coils give the beam the desired direction. The path of the electron beam on the screen is shown schematically in Appendix B, fig. 3. The solid lines are the active path of the beam, the dotted line is the reverse.
The frequency of transition to a new line is called the horizontal (or horizontal) scanning frequency. The frequency of the transition from the bottom right corner to the top left corner is called the vertical (or vertical) scan frequency. The amplitude of the overvoltage pulses on the horizontal scanning coils increases with the horizontal frequency, so this node is one of the most stressed places in the structure and one of the main sources of interference in a wide frequency range. The power consumed by the horizontal scanning nodes is also one of the serious factors taken into account when designing monitors.
After the deflecting system, the electron flow on its way to the front of the tube passes through the intensity modulator and the accelerating system, which operate on the principle of a potential difference. As a result, the electrons acquire more energy, some of which is spent on the glow of the phosphor.
The electrons hit the phosphor layer, after which the energy of the electrons is converted into light, i.e. the flow of electrons causes the dots of the phosphor to glow. These glowing dots of phosphor form the image you see on your monitor. As a rule, three electron guns are used in a color CRT monitor, as opposed to a single gun used in monochrome monitors, which are now practically not produced.
It is known that human eyes respond to the primary colors: red (Red), green (Green) and blue (Blue) and their combinations, which create an infinite number of colors. The phosphor layer covering the front of the cathode ray tube consists of very small elements (so small that the human eye cannot always distinguish them). These phosphor elements reproduce the primary colors, in fact there are three types of multi-colored particles whose colors correspond to the RGB primary colors (hence the name of the group of phosphor elements - triads). The phosphor begins to glow, as mentioned above, under the influence of accelerated electrons, which are created by three electron guns. Each of the three guns corresponds to one of the primary colors and sends a beam of electrons to different phosphor particles, whose glow of the primary colors with different intensities is combined and as a result an image with the required color is formed. For example, if you activate red, green and blue phosphor particles, then their combination will form a white color (see Appendix B, Fig. 4).
To control a cathode ray tube, control electronics is also needed, the quality of which largely determines the quality of the monitor. By the way, it is the difference in the quality of control electronics created by different manufacturers that is one of the criteria that determine the difference between monitors with the same cathode ray tube.
Each gun emits an electron beam (or stream or beam) that affects phosphor elements of different colors (green, red or blue). An electron beam intended for red phosphor elements must not affect a green or blue phosphor. To achieve such an action, a special mask is used, the structure of which depends on the type of kinescopes from different manufacturers, which ensures the discreteness (raster) of the image. CRTs can be divided into two classes - three-beam with a delta-shaped arrangement of electron guns and with a planar arrangement of electron guns. These tubes use slit and shadow masks, although it is more correct to say that they are all shadow masks. At the same time, tubes with a planar arrangement of electron guns are also called kinescopes with self-convergence of beams, since the effect of the Earth's magnetic field on three planar beams is almost the same, and when changing the position of the tube relative to the Earth's field, no additional adjustments are required.
The most common types of masks are shadow masks, and they come in two types: "shadow mask" (shadow mask) and "slit mask" (slot mask).
The shadow mask is the most common type of mask and has been used since the invention of the first color picture tubes. The surface of kinescopes with a shadow mask is usually spherical (convex). This is done so that the electron beam in the center of the screen and along the edges has the same thickness.
The shadow mask consists of a metal plate with round holes that occupy approximately 25% of the area (see Appendix B, Fig. 5). There is a mask in front of a glass tube with a phosphor layer. As a rule, most modern shadow masks are made from invar. Invar (InVar) - a magnetic alloy of iron (64%) with nickel (36%). This material has an extremely low coefficient of thermal expansion, so even though the electron beams heat up the mask, it does not adversely affect the color purity of the image. The holes in the metal grid work like a sight (albeit not an accurate sight), this is what ensures that the electron beam hits only the required phosphor elements and only in certain areas. The shadow mask creates a lattice of uniform dots (also called triads), where each such dot consists of three phosphor elements of primary colors - green, red and blue - that glow at different intensities when exposed to beams from electron guns. By changing the current of each of the three electron beams, it is possible to achieve an arbitrary color of an image element formed by a triad of dots.
One of the "weak" points of monitors with a shadow mask is its thermal deformation. Part of the rays from the electron beam gun hits the shadow mask, resulting in heating and subsequent deformation of the shadow mask. The resulting displacement of the shadow mask holes leads to the appearance of a variegated screen effect (shifting RGB colors). The material of the shadow mask has a significant impact on the quality of the monitor. The preferred mask material is Invar.
The disadvantages of the shadow mask are well known: firstly, this is a small ratio of electrons transmitted and retained by the mask (only about 20-30% passes through the mask), which requires the use of phosphors with high light output, and this, in turn, worsens the monochrome glow, reducing the color rendering range , and secondly, it is rather difficult to ensure the exact coincidence of three rays that do not lie in the same plane when they are deflected at large angles.
The shadow mask is used in most modern monitors - Hitachi, Panasonic, Samsung, Daewoo, LG, Nokia, ViewSonic.
The minimum distance between phosphor elements of the same color in adjacent rows is called dot pitch and is an index of image quality (see Appendix B, Fig. 6). Dot pitch is usually measured in millimeters. The smaller the dot pitch value, the higher the quality of the image displayed on the monitor. The horizontal distance between two adjacent points is equal to the step of the points multiplied by 0.866.
The slit mask is a technology widely adopted by NEC under the name "CromaClear". This solution in practice is a combination of a shadow mask and an aperture grille. In this case, the phosphor elements are located in vertical elliptical cells, and the mask is made of vertical lines. In fact, the vertical stripes are divided into elliptical cells, which contain groups of three phosphor elements in three primary colors. The slit mask is used, in addition to monitors from NEC (where the cells are elliptical), in Panasonic monitors with a PureFlat tube (formerly called PanaFlat). Note that it is not possible to directly compare the pitch size for tubes of different types: the pitch of dots (or triads) of a shadow mask tube is measured diagonally, while the pitch of the aperture grille, otherwise known as the horizontal dot pitch, is measured horizontally. Therefore, for the same dot pitch, a tube with a shadow mask has a higher dot density than a tube with an aperture grating. For example, a stripe pitch of 0.25 mm is approximately equivalent to a dot pitch of 0.27 mm.
Also in 1997 Hitachi, the largest designer and manufacturer of CRTs, has developed EDP, the latest shadow mask technology. In a typical shadow mask, the triads are placed more or less equilaterally, creating triangular groups that are evenly distributed across the inner surface of the tube. Hitachi reduced the horizontal distance between the triad elements, thereby creating triads that are closer in shape to an isosceles triangle. To avoid gaps between the triads, the dots themselves have been lengthened, and are more ovals than circles.
There is another type of tube that uses an "aperture grille". These tubes became known as the Trinitron and were first introduced to the market by Sony in 1982. Aperture grating tubes use an original technology, where there are three beam guns, three cathodes and three modulators, but there is one common focus (see Appendix B, Fig. 7).
An aperture grille is a type of mask used by different manufacturers in their technologies to produce kinescopes that have different names but are essentially the same, such as Sony's Trinitron technology, Mitsubishi's DiamondTron, and ViewSonic's SonicTron. This solution does not include a metal grid with holes, as in the case of the shadow mask, but a grid of vertical lines. Instead of dots with phosphor elements of the three primary colors, the aperture grille contains a series of filaments consisting of phosphor elements arranged in vertical stripes of the three primary colors. This system provides high image contrast and good color saturation, which together provide high quality monitors with tubes based on this technology. The mask used in Sony (Mitsubishi, ViewSonic) tubes is a thin foil on which thin vertical lines are scratched. It rests on a horizontal wire (one in 15", two in 17", three or more in 21") wire, the shadow of which is visible on the screen. This wire is used to dampen vibrations and is called a damper wire. It is clearly visible, especially with a light background images on the monitor.Some users fundamentally do not like these lines, while others, on the contrary, are satisfied and use them as a horizontal ruler.
The minimum distance between phosphor strips of the same color is called the strip pitch and is measured in millimeters. The smaller the stripe pitch value, the higher the image quality on the monitor. With an aperture grille, only the horizontal size of the dot makes sense. Since the vertical is determined by the focusing of the electron beam and the deflecting system. The aperture grille is used in monitors from ViewSonic, Radius, Nokia, LG, CTX, Mitsubishi, all monitors from SONY.
It should be noted that one cannot directly compare the pitch size for tubes of different types: the pitch of dots (or triads) of a tube with a shadow mask is measured diagonally, while the pitch of the aperture grille, otherwise called the horizontal dot pitch, is measured horizontally. Therefore, for the same dot pitch, a tube with a shadow mask has a higher dot density than a tube with an aperture grating. For example: 0.25 mm strip pitch is approximately equivalent to 0.27 mm dot pitch.
Both types of tubes have their advantages and their supporters. Shadow-mask tubes produce a more accurate and detailed image because light passes through the sharp-edged holes in the mask. Therefore, monitors with such CRTs are good for intensive and long-term work with texts and small graphics elements, for example, in CAD/CAM applications. Tubes with an aperture grille have a more openwork mask, it obscures the screen less, and allows you to get a brighter, more contrasting image in saturated colors. Monitors with these tubes are well suited for desktop publishing and other color oriented applications. In CAD systems, monitors with a tube that uses an aperture grille are disliked, not because they reproduce fine details worse than shadow mask tubes, but because the screen of a Trinitron type monitor is flat vertically and convex horizontally, i.e. . has a dedicated direction.
As already mentioned, in addition to the cathode ray tube, there is also control electronics inside the monitor that processes the signal coming directly from the video card of your PC. This electronics must optimize the signal amplification and control the operation of the electron guns, which initiate the glow of the phosphor that creates the image on the screen. The image displayed on the monitor screen looks stable, although, in fact, it is not. The image on the screen is reproduced as a result of a process in which the glow of the phosphor elements is initiated by an electron beam passing sequentially through the lines in the following order: from left to right and from top to bottom on the monitor screen. This process happens very quickly, so it seems to us that the screen is constantly lit. The image is stored in the retina of our eyes for about 1/20 of a second. This means that if the electron beam moves slowly across the screen, we can see this movement as a separate moving bright dot, but when the beam starts moving, quickly drawing a line on the screen at least 20 times per second, our eyes will not see a moving dot, but see only a uniform line on the screen. If we now make the beam sequentially run over many horizontal lines from top to bottom in less than 1/25 of a second, we will see a uniformly lit screen with little flicker. The movement of the beam itself will be so fast that our eye will not be able to see it. The faster the electron beam passes over the entire screen, the less flickering of the picture will be noticeable. It is believed that such flicker becomes almost imperceptible at a frame repetition rate (beam passes through all image elements) of about 75 per second. However, this value is somewhat dependent on the size of the monitor. The fact is that the peripheral areas of the retina contain light-sensitive elements with less inertia. Therefore, the flickering of monitors with large viewing angles becomes noticeable at high frame rates. The ability of the control electronics to form small image elements on the screen depends on the bandwidth (bandwidth). The bandwidth of a monitor is proportional to the number of pixels used by the computer's video card to form an image.
Some parameters that determine the quality of the CRT monitor:
Tube diagonal and apparent diagonal
One of the main parameters of a CRT monitor is the diagonal size. tubes. Distinguish directly between the size of the diagonal of the tube and the visible size, which is usually about 1 inch smaller than the diagonal of the tube, partially covered by the monitor case.
Light transmission coefficient
The light transmission coefficient is defined as the ratio of the useful light energy emitted to the outside to the energy emitted by the inner phosphorescent layer. Typically, this ratio is in the range of 50-60%. The higher the light transmission coefficient, the lower the video signal level required to provide the required brightness. However, this reduces the contrast of the image due to the decrease in the difference between the radiating and non-radiating areas of the screen surface. With a low light transmission coefficient, the focus of the image is improved, however, a more powerful video signal is required and, accordingly, the monitor circuit becomes more complicated. The specific value of the light transmission coefficient can be found in the manufacturer's documentation. Typically, 15-inch monitors have a light transmission coefficient in the range of 56-58%, and 17-inch monitors - 52-53%.
Horizontal scan
The horizontal sweep period is the time it takes the beam to travel from the left to the right edge of the screen. Accordingly, the reciprocal of this is called the horizontal frequency and is measured in kilohertz. As the frame rate increases, the horizontal refresh rate must also be increased.
Vertical scan
Vertical scan is the number of image updates on the screen per second, this parameter is also called the frame rate.
The higher the vertical scan value, the correspondingly less noticeable to the eye is the effect of a frame change, which manifests itself in the flickering of the screen. It is believed that at a frequency of 75 Hz, flicker is almost imperceptible to the eye, but the VESA standard recommends operation at a frequency of 85 Hz.
Resolution
Resolution is characterized by the number of pixels and the number of lines. For example, a monitor resolution of 1024 x 768 indicates the number of dots per line is 1024 and the number of lines is 768.
Uniformity
Uniformity is determined by the constancy of brightness over the entire surface monitor screen. A distinction is made between "brightness uniformity" and "white uniformity". Usually monitors have different brightness in different parts of the screen. The ratio of brightness in areas with a maximum and minimum value of brightness is called the uniformity of the distribution of brightness. White uniformity is defined as the difference in brightness of white color (when displaying a white image).
Non-convergence of rays
The term "non-convergence of the rays" means the deviation of red and blue from the centering green. Such a deviation prevents obtaining pure colors and a clear image. Distinguish between static and dynamic non-reduction. The first refers to the non-convergence of three colors over the entire surface of the screen, which is usually associated with errors in the assembly of the cathode ray tube. Dynamic non-convergence is characterized by errors at the edges with a clear image in the center.
Image purity and clarity
Optimum image clarity and clarity can be achieved when each of the RGB rays hits the surface at exactly the right point, which is ensured by a strict relationship between the electron gun, shadow mask holes and phosphor dots. Beam displacement, forward or backward displacement of the gun center, and beam deflection caused by external magnetic fields can all affect image clarity and clarity.
Moire- this is a type of defect that is perceived by the eye as undulating image stains associated with incorrect interaction between the shadow mask and the scanning beam. Focus and moiré are related settings for CRT monitors, so some moiré is acceptable with good focus.
jitter
Jitter is usually understood as oscillatory changes in the image. with a frequency above 30 Hz. They can be caused by vibration of the monitor mask holes, which, in particular, may be due to improper grounding. At frequencies less than 30 Hz, the term "swimming" is used, and below 1 Hz - "drift". Slight jitter is inherent in all monitors. In accordance with the ISO standard, a diagonal point deviation of no more than 0.1 mm is allowed.
Mask deformation
All shadow mask monitors are subject to some degree of distortion due to thermal distortion of the mask. Thermal expansion of the material from which the mask is made leads to its deformation and, accordingly, to displacement of the mask holes.
The preferred mask material is Invar, an alloy having a low coefficient of linear expansion.
Screen coating
While the monitor is in use, the surface of the monitor is exposed to intense electron bombardment, which can build up a charge of static electricity. This leads to the fact that the screen surface “attracts” a large amount of dust, and in addition, when a user touches a charged screen with a hand, a weak electric discharge can unpleasantly “click”. To reduce the potential of the screen surface, special conductive antistatic coatings are applied to it, which in the documentation are denoted by the abbreviation AS - anti-static.
The next purpose of coating is to eliminate reflections of surrounding objects in the glass of the screen, which interfere with operation. These are the so-called anti-reflective coatings (anti-reflection, AR). To reduce the reflection effect, the screen surface should be matte. One way to obtain such a surface is to etch glass to obtain not a specular, but a diffuse reflection (Diffuse is a reflection in which the incident light is reflected not at an angle of incidence, but in all directions). However, in this case, the light from the phosphor elements is also diffusely scattered, the image becomes blurry and loses its brightness. Recently, to obtain anti-reflective coatings, a thin layer of silicon dioxide is used, on which profiled horizontal grooves are etched, preventing reflections of external objects from entering the user's field of view (at its normal position near the monitor). In this case, such a profile of the grooves is selected so that the attenuation and dispersion of the useful signal is maximum.
Another unfavorable factor that is dealt with by processing the screen is glare from external light sources. To reduce these effects, a dielectric layer with a low refractive index, which has a low reflectivity, is applied to the surface of the monitor. Such coatings are called anti-glare or anti-halation (anti-glare, AG). Usually, combined multilayer coatings are used that combine protection against several interfering factors. Panasonic has developed a coating that uses all the described types of coatings, and it has the name AGRAS (anti-glare, anti-reflection, anti-static). To increase the intensity of the transmitted useful light between the screen glass and the layer with a low reflection coefficient, a transition layer is applied, which has a refractive index that is average between the glass and the outer layer (enlightenment effect), which also has conductive properties to remove static charge.
Sometimes other combinations of coatings are used - ARAG (anti-reflection, anti-glare) or ARAS (anti-reflection, anti-static). In any case, the coatings somewhat reduce the brightness and contrast of the image and affect the color reproduction, but the convenience of working with the monitor, obtained from the use of coatings, pays for these shortcomings. You can check the presence of an anti-reflective coating visually by examining the reflection from an external light source when the monitor is off and comparing it with the reflection from ordinary glass.
The presence of anti-glare and anti-static coatings has become the norm for modern monitors, and some differences in the quality of coatings that determine their effectiveness and the degree of image distortion associated with technological features have little effect on the choice of model.
A personal computer monitor is a truly important component for every type of computer.
Without a monitor, there is no way to fully evaluate the characteristics, as well as the functions and capabilities of the software provided, because no type of information will be displayed visually. Only through the monitor used can receive up to 100% of information.
Currently, cathode ray tube monitors are no longer common and common. This technique can be seen only in rare users. CRTs have successfully replaced liquid crystal monitors.
Despite this situation, there is a need to understand all the important advantages and nuances of the manufactured equipment, because only in this case it becomes possible to appreciate the old products and understand why they have lost their relevance. Is it really only because of the large size and excessive weight, high power consumption and potentially harmful radiation for users? 
What were the old CRT monitors like?
All CRT monitors can be divided into three types.
- CRT monitors with a shadow mask. This option turned out to be one of the most popular and truly worthy of manufacturers. The technique had a convex monitor.
- LT with an aperture grille that includes several vertical lines.
- Monitors with a slit mask.
What technical characteristics of CRT monitors should be taken into account? How to understand how worthy the technique of its application?
- Screen diagonal. This parameter is usually considered from opposite corners from the upper and lower parts: the lower right corner is the upper left. The value must be measured in inches. In most cases, the models had a diagonal of 15 and 17 inches.
- Monitor Screen Grain Size a. In this case, it is supposed to consider special holes located in the monitor's color separation mask at certain distances. If this distance is less, you can count on improving the image quality. The grain size should indicate the distance between the nearest holes. For this reason, you can focus on the following indicator: a smaller characteristic is proof of the high quality of a computer display.
- Power consumption b, measured in watts.
- Display cover type.
- The presence or absence of a protective screen. Scientific researchers have managed to prove that the generated radiation is harmful to human health. For this reason, CRT monitors began to be offered with special protection, which can be glass, film, mesh. The main task was the desire to reduce the level of radiation.
Advantages of CRT monitors
Despite the features and specifics of CRT monitors, it remains possible to appreciate the advantages of the proposed old products:
- CRT models can work with switching (shutter) stereo glasses. At the same time, even the most advanced LCD displays have not acquired such a skill. If a person wants to note how versatile and perfect a full-fledged 3D stereo video can be, it is best to give preference to a CRT model, which will be 17-inch. With this approach, you can allocate 1,500 - 4,500 rubles for a purchase, but get the opportunity to enjoy 3D in switching stereo glasses. The most important thing is to check, focusing on the passport data of the released equipment, its characteristics: the resolution should be 1024x768. Frame rate - from 100 Hz. If these data are not observed, there is a risk of flickering of the stereo image.
- A CRT monitor with a modern video card can successfully display images of various resolutions, including thin lines and italic letters. This characteristic depends on the resolution of the phosphor. The LCD display will correctly and accurately reproduce the text only if the resolution is set equal to the number of rows and columns of the LCD monitor itself, standard resolution, because other versions will be interpolated by the electronics of the technology used.
- High-quality CRT monitors can please with dynamic (transient) characteristics, allowing you to enjoy watching dynamic scenes in games and movies. It is expected to be able to successfully and easily eliminate unwanted blur from image details that change rapidly. This can be explained by the following nuance: the transient response time of a CRT phosphor cannot exceed 1–2 ms according to the criterion for the decrease in full brightness to several percent. LCD displays have a transient response of 12 - 15 ms, with 2, 6, 8 ms being a purely publicity stunt, as a result of which fast-changing parts can be lubricated in dynamic scenes.
- CRT monitors that meet these high criteria and are properly color-adjusted can guarantee correct color reproduction of the observed scenes. This characteristic is appreciated by artists and designers. LCD monitors cannot please with perfect color reproduction.
Disadvantages of CRT monitors
- Large dimensions.
- High level of energy consumption.
- The presence of harmful electromagnetic radiation.
It is possible that LCD displays will catch up with CRT in terms of their technical characteristics, because modern manufacturers are trying to combine convenience and practicality, functionality in the products offered.
Somehow, imperceptibly, the time has come when televisions and monitors based on cathode-ray technology have almost completely disappeared from store shelves. Recall that these are the very bulky devices that occupied almost half of the computer desk. Now their thickness rarely exceeds 10 cm, and then only taking into account the lamp illumination.
It is not surprising that many have safely forgotten what a CRT monitor is. But in vain! If only because in some respects it is ahead of even the most modern liquid crystal counterparts.
How CRT monitors work
First of all, let's give an explanation of the abbreviation. So, the term "CRT" means a cathode beam or, as we indicated earlier, a cathode tube (from the English CRT - Cathode Ray-Tube). As a rule, with the word "tube" most people imagine a cylinder with no walls at the ends. Speaking about the CRT monitor, it should be mentioned that in this case such a representation is erroneous. Because the shape of the tube in it is far from cylindrical and expands to a plane on one side. This surface is the front glass part, the one on which the images are formed. The inner side of this area is covered with a special substance - a phosphor. Its unique property is that when a stream of charged particles hits it, they naturally transform into a glow.
Thus, a CRT monitor is a device in which beams of electron beams draw a picture on the inside of the screen. A person sees it, thanks to the glow of the phosphor.
On the other side of the flask there is a block of electrodes called guns. It is they who create the flow of particles.
In other words, a CRT monitor consists of a glass tube, gun electrodes, and a control circuit.
Principle of operation
As you know, by mixing three green, red and blue in a certain ratio, you can get all the others, including shades. In color monitors, the entire inner surface of the screen consists of dots conditionally grouped into triads (blocks of 3 each). Each of them is capable of glowing in one of the primary colors. There are also three electrodes, each of which illuminates “its own” points. In a certain order, lighting and passing them on the screen, it is possible to form a color picture. By the way, in black-and-white image devices, there is only one gun.
To control the flow of particles, electromagnetic deflection is used, and the initial direction of their movement is created due to the potential difference.
Since it is technically quite difficult to ensure the accuracy of the beam hitting its point, a special solution is used - a mask. Relatively speaking, this is a perforated mesh between the screen and the guns. There are different types of masks. In part, they are responsible for the display features (clearness, the shape of dots-pixels).
Since the glow of the phosphor decreases very quickly after the impact of the particle, it is necessary to constantly recreate the picture. Both static and dynamic. Therefore, the rays draw an image dozens of times per second. This is the famous frame scan hertz. The higher the frequency, the less noticeable flicker.
Currently, the repair of CRT monitors for subsequent use as part of a computer system is impractical, since modern LCD technology is more promising. The exception is specific usage.
Since 1902, Boris Lvovich Rosing has been working with Brown's pipe. On July 25, 1907, he applied for the invention "Method of electrical transmission of images over distances." The beam was scanned in the tube by magnetic fields, and the signal was modulated (brightness changed) using a capacitor that could deflect the beam vertically, thereby changing the number of electrons passing to the screen through the diaphragm. On May 9, 1911, at a meeting of the Russian Technical Society, Rosing demonstrated the transmission of television images of simple geometric shapes and their reception with playback on a CRT screen.
At the beginning and middle of the 20th century, Vladimir Zworykin, Allen Dumont and others played a significant role in the development of CRT.
Device and principle of operation
General principles
Black and white kinescope device
in a balloon 9 a deep vacuum is created - first the air is pumped out, then all the metal parts of the kinescope are heated by an inductor to release the absorbed gases, a getter is used to gradually absorb the remaining air.
To create an electron beam 2 , a device called an electron gun is used. Cathode 8 heated by a filament 5 , emits electrons. To increase the emission of electrons, the cathode is coated with a substance having a low work function (the largest manufacturers of CRTs use their own patented technologies for this). By changing the voltage at the control electrode ( modulator) 12 you can change the intensity of the electron beam and, accordingly, the brightness of the image (there are also models with cathode control). In addition to the control electrode, the gun of modern CRTs contains a focusing electrode (until 1961, electromagnetic focusing was used in domestic kinescopes using a focusing coil 3 core 11 ), designed to focus a spot on the kinescope screen to a point, an accelerating electrode for additional acceleration of electrons within the gun and the anode. After leaving the gun, the electrons are accelerated by the anode 14 , which is a metallized coating of the inner surface of the kinescope cone, connected to the gun electrode of the same name. In color kinescopes with an internal electrostatic screen, it is connected to the anode. In a number of kinescopes of early models, such as 43LK3B, the cone was made of metal and represented the anode itself. The voltage at the anode is in the range from 7 to 30 kilovolts. In a number of small-sized oscillographic CRTs, the anode is only one of the electron gun electrodes and is powered by voltages up to several hundred volts.
Next, the beam passes through the deflecting system 1 , which can change the direction of the beam (the figure shows a magnetic deflection system). In television CRTs, a magnetic deflection system is used as it provides large deflection angles. In oscilloscope CRTs, an electrostatic deflection system is used as it provides faster response.
The electron beam hits the screen 10 coated with phosphor 4 . From bombardment by electrons, the phosphor glows and a rapidly moving spot of variable brightness creates an image on the screen.
The phosphor acquires a negative charge from the electrons, and secondary emission begins - the phosphor itself begins to emit electrons. As a result, the entire tube acquires a negative charge. To prevent this from happening, over the entire surface of the tube there is a layer of aquadag connected to a common wire - a graphite-based conductive mixture ( 6 ).
The kinescope is connected through the leads 13 and high voltage socket 7 .
In black and white TVs, the composition of the phosphor is selected so that it glows in a neutral gray color. In video terminals, radars, etc., the phosphor is often made yellow or green to reduce eye fatigue.
Beam Deflection Angle
The deflection angle of the CRT beam is the maximum angle between two possible positions of the electron beam inside the bulb, at which a luminous spot is still visible on the screen. The ratio of the diagonal (diameter) of the screen to the length of the CRT depends on the angle. For oscillographic CRTs, it is usually up to 40 degrees, which is associated with the need to increase the sensitivity of the beam to the effects of deflecting plates. For the first Soviet television kinescopes with a round screen, the deflection angle was 50 degrees, for black-and-white kinescopes of later releases it was 70 degrees, starting from the 60s it increased to 110 degrees (one of the first such kinescopes was 43LK9B). Domestic color kinescopes have 90 degrees.
With an increase in the beam deflection angle, the dimensions and mass of the kinescope decrease, however, the power consumed by the scanning nodes increases. Currently, the use of 70-degree kinescopes has been revived in some areas: in color VGA monitors of most diagonals. Also, an angle of 70 degrees continues to be used in small-sized black-and-white kinescopes (for example, 16LK1B), where the length does not play such a significant role.
Ion trap
Since it is impossible to create a perfect vacuum inside a CRT, some of the air molecules remain inside. When colliding with electrons, ions are formed from them, which, having a mass many times greater than the mass of electrons, practically do not deviate, gradually burning out the phosphor in the center of the screen and forming the so-called ion spot. To combat this until the mid-60s. an ion trap was used, which has a major drawback: its correct installation is a rather painstaking operation, and if it is installed incorrectly, the image is absent. At the beginning of the 60s. A new way to protect the phosphor was developed: aluminizing the screen, which also made it possible to double the maximum brightness of the kinescope, and the need for an ion trap disappeared.
Delay in applying voltage to the anode or modulator
In a TV, the horizontal scanning of which is made on lamps, the voltage at the anode of the kinescope appears only after the horizontal scanning output lamp and the damper diode have warmed up. The glow of the kinescope by this moment has time to warm up.
The introduction of all-semiconductor circuitry into horizontal scanning nodes has created the problem of accelerated wear of the cathodes of the kinescope due to the voltage being applied to the anode of the kinescope simultaneously with switching on. To combat this phenomenon, amateur nodes have been developed that provide a delay in the supply of voltage to the anode or kinescope modulator. Interestingly, in some of them, despite the fact that they are intended for installation in all-semiconductor TVs, a radio tube is used as a delay element. Later, industrial TVs began to be produced, in which such a delay was provided initially.
Scan
To create an image on the screen, the electron beam must constantly pass over the screen at a high frequency - at least 25 times per second. This process is called sweep. There are several ways to scan an image.
Raster scanning
The electron beam traverses the entire screen in rows. There are two options:
- 1-2-3-4-5-… (progressive scanning);
- 1-3-5-7-… then 2-4-6-8-… (interlaced).
Vector unwrapping
The electron beam travels along the lines of the image.
Color kinescopes
Color kinescope device. 1 - Electron guns. 2 - Electron beams. 3 - Focusing coil. 4 - Deflecting coils. 5 - Anode. 6 - Mask, due to which the red beam hits the red phosphor, etc. 7 - Red, green and blue grains of the phosphor. 8 - Mask and phosphor grains (enlarged).
A color kinescope differs from a black-and-white one in that it has three guns - “red”, “green” and “blue” ( 1 ). Accordingly, on the screen 7 three types of phosphor are applied in some order - red, green and blue ( 8 ).
Only the beam from the red gun hits the red phosphor, only the beam from the green one hits the green phosphor, etc. This is achieved by the fact that a metal grate is installed between the guns and the screen, called mask (6 ). In modern kinescopes, the mask is made of Invar, a steel grade with a small coefficient of thermal expansion.
Types of masks
There are two types of masks:
- the actual shadow mask, which exists in two forms:
- Shadow mask for kinescopes with a delta-shaped arrangement of electron guns. Often, especially in translated literature, it is referred to as a shadow grid. Currently used in most monitor kinescopes. Television kinescopes with a mask of this type are not currently being produced, however, such kinescopes can be found in TVs of past years (59LK3Ts, 61LK3Ts, 61LK4Ts);
- Shadow mask for kinescopes with a planar arrangement of electron guns. Also known as slotted grating. Currently, it is used in the vast majority of television kinescopes (25LK2Ts, 32LK1Ts, 32LK2Ts, 51LK2Ts, 61LK5Ts, foreign models). It is almost never found in monitor kinescopes, with the exception of Flatron models;
- aperture grille (Mitsubishi Diamondtron). This mask, unlike other types, consists of a large number of wires stretched vertically. The fundamental difference between this type of mask is that it does not restrict the electron beam, but focuses it. The transparency of the aperture grille is about 85% versus 20% for the shadow mask. Kinescopes with such a mask are used in both monitors and televisions. Attempts were made to create such kinescopes in the 70s in the USSR (for example, 47LK3Ts).
- color kinescopes of a special type stand apart - single-beam chromoscopes, in particular, 25LK1Ts. According to the device and principle of operation, they are strikingly different from other types of color kinescopes. Despite the obvious advantages, including reduced power consumption, comparable to that of a black-and-white kinescope with a diagonal of the same size, such kinescopes have not received wide distribution.
There is no clear leader among these masks: the shadow mask provides high quality lines, the aperture mask provides more saturated colors and high efficiency. Slotted combines the advantages of shadow and aperture, but is prone to moire.
Types of gratings, ways to measure the step on them
The smaller the phosphor elements, the higher the image quality the tube is capable of producing. An indicator of image quality is mask step.
- For a shadow grating, the mask pitch is the distance between two nearest mask holes (respectively, the distance between two nearest phosphor elements of the same color).
- For aperture and slit gratings, the mask pitch is defined as the horizontal distance between the mask slits (respectively, the horizontal distance between the vertical stripes of a phosphor of the same color).
In modern monitor CRTs, the mask pitch is at the level of 0.25 mm. Television kinescopes, which are viewed from a greater distance, use steps of the order of 0.8 mm.
convergence of rays
Since the radius of curvature of the screen is much greater than the distance from it to the electron-optical system up to infinity in flat kinescopes, and without the use of special measures, the point of intersection of the rays of a color kinescope is at a constant distance from the electron guns, it is necessary to ensure that this point is exactly at surface of the shadow mask, otherwise misregistration of the three color components of the image is formed, increasing from the center of the screen to the edges. To prevent this from happening, it is necessary to properly shift the electron beams. In kinescopes with a delta-shaped arrangement of guns, this is done by a special electromagnetic system controlled separately by a device that, in old TVs, was placed in a separate unit - the mixing unit - for periodic adjustments. In kinescopes with a planar arrangement of guns, adjustment is made using special magnets located on the neck of the kinescope. Over time, especially for kinescopes with a delta-shaped arrangement of electron guns, convergence is disturbed and needs additional adjustment. Most computer repair companies offer a monitor beam refacing service.
Demagnetization
It is necessary in color kinescopes to remove the residual or accidental magnetization of the shadow mask and the electrostatic screen that affects the image quality. Demagnetization occurs due to the appearance in the so-called demagnetization loop - an annular flexible coil of large diameter located on the surface of the kinescope - a pulse of a rapidly changing damped magnetic field. In order for this current to gradually decrease after turning on the TV, thermistors are used. Many monitors, in addition to thermistors, contain a relay that, at the end of the kinescope demagnetization process, turns off the power to this circuit so that the thermistor cools down. After that, you can use a special key, or, more often, a special command in the monitor menu, to trigger this relay and re-demagnetize at any time without resorting to turning off and turning on the monitor's power.
Trinescope
A trinescope is a design consisting of three black-and-white kinescopes, light filters and translucent mirrors (or dichroic mirrors that combine the functions of translucent mirrors and filters) used to obtain a color image.
Application
Kinescopes are used in raster imaging systems: various kinds of televisions, monitors, video systems. Oscillographic CRTs are most often used in functional dependency display systems: oscilloscopes, wobblescopes, also as a display device at radar stations, in special-purpose devices; in the Soviet years they were also used as visual aids in the study of the design of electron beam devices in general. Character-printing CRTs are used in various special-purpose equipment.
Designation and marking
The designation of domestic CRTs consists of four elements:
- First element: a number indicating the diagonal of a rectangular or round screen in centimeters;
- The second element: the purpose of the CRT, in particular, LK - television kinescope, LM - monitor kinescope, LO - oscilloscope tube;
- Third element: a number indicating the model number of a given tube with a given diagonal;
- Fourth element: a letter indicating the color of the screen glow, in particular, C - color, B - white glow, I - green glow.
In special cases, a fifth element may be added to the designation, carrying additional information.
Example: 50LK2B - a black-and-white kinescope with a screen diagonal of 50 cm, the second model, 3LO1I - an oscilloscope tube with a green glow screen diameter of 3 cm, the first model.
Health impact
Electromagnetic radiation
This radiation is not created by the kinescope itself, but by a deflecting system. Tubes with electrostatic deflection, in particular oscilloscope tubes, do not radiate it.
In monitor kinescopes, to suppress this radiation, the deflecting system is often covered with ferrite cups. Television kinescopes do not require such shielding, since the viewer usually sits at a much greater distance from the TV than from the monitor.
ionizing radiation
There are two types of ionizing radiation in kinescopes.
The first of these is the electron beam itself, which is, in fact, a stream of low-energy beta particles (25 keV). This radiation does not go outside, and does not pose a danger to the user.
The second is X-ray bremsstrahlung, which occurs when the screen is bombarded with electrons. To reduce the output of this radiation to the outside to completely safe values, the glass is doped with lead (see below). However, in the event of a malfunction of the TV or monitor, leading to a significant increase in the anode voltage, the level of this radiation may increase to noticeable values. To prevent such situations, horizontal scanning units are equipped with protection nodes.
In domestic and foreign color televisions produced before the mid-1970s, there may be additional sources of X-ray radiation - stabilizing triodes connected in parallel to the kinescope and serving to stabilize the anode voltage, and hence the image size. 6S20S triodes are used in Raduga-5 and Rubin-401-1 TVs, and GP-5 in early ULPCT models. Since the glass of the cylinder of such a triode is much thinner than that of a kinescope and is not alloyed with lead, it is a much more intense source of x-rays than the kinescope itself, so it is placed in a special steel screen. Later models of ULPCT TVs use other methods of high voltage stabilization, and this X-ray source is excluded.
flicker
Monitor Mitsubishi Diamond Pro 750SB (1024x768, 100 Hz) shot at 1/1000 s. The brightness is artificially high; shows the actual brightness of the image at different points on the screen.
The beam of a CRT monitor, forming an image on the screen, causes the particles of the phosphor to glow. Before the formation of the next frame, these particles have time to go out, so you can observe the "flickering of the screen." The higher the frame rate, the less noticeable flicker. Low frequency leads to eye fatigue and is harmful to health.
Most cathode ray tube televisions have 25 frames per second, which, with interlacing, is 50 fields (half frames) per second (Hz). In modern TV models, this frequency is artificially increased to 100 hertz. When working behind the monitor screen, flicker is felt more strongly, since the distance from the eyes to the kinescope is much less than when watching TV. The minimum recommended monitor refresh rate is 85 hertz. Early models of monitors do not allow you to work with a refresh rate of more than 70-75 Hz. The flickering of the CRT can clearly be observed with peripheral vision.
fuzzy image
The image on a cathode ray tube is blurry compared to other types of screens. Blurry images are believed to be one of the contributing factors to eye fatigue in the user.
Currently (2008) in tasks that are not demanding on color reproduction, from the point of view of ergonomics, LCD monitors connected via a digital DVI connector are certainly preferable.
High voltage
CRT uses high voltage. Residual voltage of hundreds of volts, if no action is taken, can linger on CRT and "strapping" circuits for weeks. Therefore, discharge resistors are added to the circuits, which make the TV completely safe within a few minutes after turning it off.
Contrary to popular belief, the anode voltage of a CRT cannot kill a person due to the low power of the voltage converter - there will only be a tangible blow. However, it can also be fatal if a person has heart defects. It can also cause injury, including death, indirectly, when, by withdrawing a hand, a person touches other television and monitor circuits containing extremely life-threatening voltages - and such circuits are present in all models of televisions and monitors using a CRT.
Toxic substances
Any electronics (including CRT) contains substances that are harmful to health and the environment. Among them: lead glass, barium compounds in cathodes, phosphors.
Starting from the second half of the 60s, the dangerous part of the kinescope is covered with a special metal explosion-proof bandage, made in the form of an all-metal stamped structure or wound in several layers of tape. Such a bandage excludes the possibility of spontaneous explosion. In some models of kinescopes, a protective film was additionally used to cover the screen.
Despite the use of protective systems, it is not excluded that people will be hit by fragments when the kinescope is deliberately broken. In this regard, when destroying the latter, for safety, they first break the shtengel - a technological glass tube at the end of the neck under a plastic base, through which air is pumped out during production.
Small-sized CRTs and kinescopes with a screen diameter or diagonal of up to 15 cm do not pose a danger and are not equipped with explosion-proof devices.
DISPLAY DEVICES
Monitors
Information display devices primarily include monitors, as well as devices focused on solving multimedia or presentation tasks: devices for forming three-dimensional (stereoscopic) images and projectors.
The monitor is the most important device for displaying computer information. Types of modern monitors are very diverse. According to the principle of operation, all PC monitors can be divided into two large groups:
Based on a cathode ray tube (CRT), called a kinescope;
flat-panel, made mainly on the basis of liquid crystals.
CRT-based monitors
CRT-based monitors are the most common display devices. The technology used in this type of monitor was developed many years ago and was originally created as a special tool for measuring AC current, i.e. for an oscilloscope.
The design of a CRT monitor is a glass tube, inside of which there is a vacuum. On the front side, the inner part of the tube glass is coated with a phosphor. Quite complex compositions based on rare earth metals - yttrium, erbium, etc. are used as phosphors for color CRTs. A phosphor is a substance that emits light when bombarded with charged particles. To create an image in a CRT monitor, an electron gun is used, which emits a stream of electrons through a metal mask or grating onto the inner surface of the monitor's glass screen, which is covered with multi-colored phosphor dots. Electrons fall on the phosphor layer, after which the energy of the electrons is converted into light, i.e., the flow of electrons causes the dots of the phosphor to glow. These glowing dots of the phosphor form the image on the monitor. As a rule, three electron guns are used in a color CRT monitor, as opposed to a single gun used in monochrome monitors.
On the way of the electron beam, there are usually additional electrodes: a modulator that regulates the intensity of the electron beam and the image brightness associated with it; focusing electrode, which determines the size of the light spot; deflecting system coils placed on the base of the CRT, which change the direction of the beam. Any text or graphic image on the monitor screen consists of many discrete phosphor dots, called pixels and representing the minimum element of the raster image.
The formation of a raster in the monitor is carried out with the help of special signals received by the deflecting system. Under the action of these signals, the beam is scanned along the screen surface along a zigzag path from the upper left corner to the lower right, as shown in Fig. 4.1. The horizontal course of the beam is carried out by a line (horizontal) scan signal, and vertically by a vertical (vertical) scan. The transfer of the beam from the extreme right point of the line to the extreme left point of the next row (reverse beam travel horizontally) and from the extreme right position of the last line of the screen to the extreme left position of the first line (reverse beam travel vertically) is carried out by means of special reverse motion signals. These types of monitors are called raster. In this case, the electron beam periodically scans the screen, forming closely spaced scan lines on it. As the beam moves along the lines, the video signal applied to the modulator changes the brightness of the light spot and forms an image visible on the screen. The resolution of a monitor is determined by the number of picture elements it can display horizontally and vertically, such as 640x480 or 1024x768 pixels.
Unlike TV, where the video signal that controls the brightness of the electron beam is analog, PC monitors use both analog and digital video signals. In this regard, PC monitors are usually divided into analog and digital. The first PC display devices were digital monitors.
AT digital monitors control is carried out by binary signals that have only two values: logical 1 and logical 0 ("yes" and "no"). The logical one level corresponds to a voltage of about 5 V, the logic zero level - no more than 0.5 V. Since the same levels of "1" and "0" are used in the widespread standard series of microcircuits based on transistor-transistor logic (TTL- Transistor Transistor Logic- transistor-transistor logic), digital monitors are called TTL monitors.
The first TTL monitors were monochrome, later color ones appeared. In monochrome digital monitors, dots on the screen can only be light or dark, differing in brightness. The cathode ray tube of a monochrome monitor has only one electron gun; it is smaller than color CRTs, making monochrome monitors smaller and lighter than others. In addition, a monochrome monitor operates at a lower anode voltage than a color monitor (15 kV versus 21 - 25 kV), so its power consumption is much lower (30 W instead of 80 - 90 W for color ones).
In the kinescope color digital monitor contains three electron guns: for red (Red) green (green) and blue (Blue) colors with separate control, so it is called an RGB monitor.
Digital RGB monitors also support monochrome mode with up to 16 shades of gray.
analog monitors, as well as digital ones, they are color and monochrome, while a color monitor can work in monochrome mode.
The main reason for switching to analog video is the limited color palette of a digital monitor. The analog video signal that regulates the intensity of the electron beam can take on any value in the range from 0 to 0.7 V. Since there are infinitely many of these values, the palette of an analog monitor is unlimited. However, the video adapter can only provide a finite number of video signal level gradations, which ultimately limits the palette of the entire video system as a whole.
For understanding the principle of forming a raster of color monitors should represent the mechanism of color vision. Light is electromagnetic vibrations in a certain range of wavelengths. The human eye is able to distinguish colors corresponding to different regions of the visible radiation spectrum, which occupies only a small part of the total spectrum of electromagnetic oscillations in the wavelength range from 0.4 to 0.75 microns.
The total radiation of wavelengths of the entire visible range is perceived by the eye as white light. The human eye has three types of receptors responsible for color perception and differ in their sensitivity to electromagnetic waves of different wavelengths. Some of them react to violet-blue, others to green, and others to orange-red. If light does not reach the receptors, the human eye perceives black. If all receptors are illuminated equally, a person sees gray or white. When an object is illuminated, some of the light is reflected from it, and some is absorbed. Color density is determined by the amount of light absorbed by an object in a given spectral range. The denser the color layer, the less light is reflected and, as a result, the color tone (tone) is darker.
The physiological features of color vision were studied by M. V. Lomonosov. The basis of the theory of color vision developed by him is the experimentally established fact that all colors can be obtained by adding three light fluxes with high saturation, for example, red, green and blue, called primary or primary.
Usually, light radiation excites all receptors of the human eye simultaneously. The human visual apparatus analyzes light, determining the relative content of various radiations in it, and then in the brain they are synthesized into a single color.
Thanks to the remarkable property of the eye - the three-component color perception - a person can distinguish any of the color shades: there is enough information only about the quantitative ratio of the intensities of the three primary colors, so there is no need for a direct transfer of all colors. Thus, due to the physiological features of color vision, the amount of information about color is significantly reduced and many technological solutions related to the registration and processing of color images are simplified.
Another important property of color vision is spatial color averaging, which lies in the fact that if a color image has closely spaced color details, then from a large distance the colors of individual details are indistinguishable. All closely spaced colored parts will appear painted in the same color. Due to this property of vision, the color of one image element is formed in the cathode-ray tube of the monitor from three colors of phosphor grains located next to each other.
These properties of color vision were used in the development of the operating principle of a CRT color monitor. Three electron guns with independent control circuits are located in the cathode-ray tube of a color monitor, and a phosphor of three primary colors is applied to the inner surface of the screen: red, blue and green.
Rice. 4.2. Scheme of color formation on the monitor screen
On fig. 4.2 shows the scheme of color formation on the monitor screen. The electron beam of each gun excites the dots of the phosphor, and they begin to glow. The dots glow differently and represent a mosaic image with extremely small sizes of each element. The glow intensity of each dot depends on the control signal of the electron gun. In the human eye, dots with three primary colors intersect and overlap each other. By changing the ratio of the intensities of the points of the three primary colors, the desired shade is obtained on the monitor screen. In order for each gun to direct the electron flow only to the phosphor spots of the corresponding color, each color kinescope has a special color-separating mask.
Depending on the location of the electron guns and the design of the color separation mask (Fig. 4.3), there are four types of CRTs used in modern monitors:
· CRT with shadow mask (Shadow Mask)(see fig. 4.3, a) most common in most monitors manufactured by LG, Samsung, Viewsonic, Hitachi, Belinea, Panasonic, Daewoo, Nokia;
· Enhanced Shadow Mask (EDP) CRT- Enhanced Dot Pitch)(see fig. 4.3, 6);
· CRT with slit mask (Slot Mask)(see fig. 4.3, in), in which the phosphor elements are located in vertical cells, and the mask is made of vertical lines. The vertical stripes are divided into cells containing groups of three phosphor elements of three primary colors. This type of mask is used by NEC and Panasonic;
· CRT with an aperture grid of vertical lines (Aperture Grill) (see Fig. 4.3, d). Instead of dots with phosphor elements of the three primary colors, the aperture grille contains a series of filaments consisting of phosphor elements arranged in vertical stripes of the three primary colors. Sony and Mitsubishi tubes are produced using this technology.
Structurally, the shadow mask is a metal plate made of a special material, invar, with a system of holes corresponding to the dots of the phosphor deposited on the inner surface of the kinescope. The temperature stabilization of the shape of the shadow mask during its bombardment by an electron beam is ensured by a small value of the linear expansion coefficient of the invar. The aperture grille is formed by a system of slots that perform the same function as the holes in the shadow mask.
Both types of tubes (shadow mask and aperture grille) have their own advantages and applications. Shadow-mask tubes produce a more accurate and detailed image because light passes through the sharp-edged holes in the mask. Therefore, monitors with such CRTs are recommended for intensive and long-term work with texts and small graphics elements. Aperture-grille tubes have a more openwork mask, they obscure the screen less and allow you to get a brighter, more contrasting image in saturated colors. Monitors with these tubes are well suited for desktop publishing and other color oriented applications.
The minimum distance between phosphor elements of the same color in shadow masks is called Dot Pitch(dot pitch) and is an index of image quality. Dot pitch is usually measured in millimeters. The smaller the dot pitch value, the higher the quality of the image displayed on the monitor. The average distance between the dots of the phosphor is called the grain. For various monitor models, this parameter has a value from 0.2 to 0.28 mm. In a CRT with an aperture grille, the average distance between the strips is called Strip Pitch(band pitch) and is measured in millimeters. The smaller the stripe pitch, the higher the image quality on the monitor. You cannot compare the pitch size for tubes of different types: the pitch of dots (or triads) of a shadow mask tube is measured diagonally, while the pitch of the aperture grille, otherwise called the horizontal dot pitch, is measured horizontally. Therefore, for the same dot pitch, a tube with a shadow mask has a higher dot density than a tube with an aperture grating. For example: 0.25 mm dot pitch is approximately equivalent to 0.27 mm strip pitch.
In addition to the cathode ray tube, the monitor contains control electronics that process the signal coming directly from the PC video card. This electronics must optimize the signal amplification and control the operation of the electron guns.
The image displayed on the monitor screen looks stable, although in fact it is not. The image on the screen is reproduced as a result of a process in which the glow of the phosphor elements is initiated by an electron beam passing sequentially through the lines. This process happens at a high speed, so it seems that the screen is constantly lit. The image is stored in the retina for about 1/20 s. This means that if the electron beam moves slowly across the screen, the eye will perceive it as a single moving bright dot, but when the beam starts moving at high speed, drawing a line on the screen 20 times per second, the eye will see a uniform line on the screen. If the beam scans the screen sequentially along horizontal lines from top to bottom in less than 1/25 s, the eye will perceive a uniformly illuminated screen with little flicker. The movement of the beam itself is so fast that the eye is not able to notice it. It is believed that flicker becomes almost imperceptible at a frame repetition rate (beam passes through all image elements) of about 75 times per second.
The illuminated pixels of the screen must continue to glow for the time it takes for the electron beam to scan the entire screen and return again to activate this pixel when drawing the next frame. Therefore, the minimum persistence time should not be less than the frame change period of the image, i.e. 20 ms.
CRT monitors have the following Main characteristics.
Monitor screen size- the distance between the lower left and upper right corner of the screen, measured in inches. The size of the screen area visible to the user is usually somewhat smaller, on average 1 ", than the size of the handset. Manufacturers may indicate two diagonal sizes in the accompanying documentation, while the visible size is usually indicated in brackets or marked "Viewable size", but sometimes only one is indicated size - the size of the diagonal of the tube. Monitors with a diagonal of 15" stand out as a standard for PCs, which approximately corresponds to 36 - 39 cm diagonal of the visible area. For Windows, it is desirable to have a monitor of at least 17".
Screen grain size defines the distance between the nearest holes in the color separation mask type used. The distance between the mask holes is measured in millimeters. The smaller the distance between the holes in the shadow mask, and the more holes there are, the better the image quality. All monitors with grain greater than 0.28mm are classified as coarse and cost less. The best monitors have a grain of 0.24mm, reaching 0.2mm on the most expensive models.
Resolution A monitor is defined by the number of picture elements it can display both horizontally and vertically. 19" monitors support resolutions up to 1920 x 14400 and above.
Type of cathode ray tube should be considered when choosing a monitor. The most preferred types of kinescopes are Black Trinitron, Black Matrix or Black Planar. These types of monitors have a special phosphor coating.
Monitor Power Consumption indicated in its technical specifications. For 14" monitors, the power consumption should not exceed 60 watts.
Screen covers necessary to give it anti-reflective and antistatic properties. The anti-reflective coating allows you to watch only the image generated by the computer on the monitor screen, and not tire your eyes by observing reflected objects. There are several ways to obtain an anti-reflective (non-reflective) surface. The cheapest of them is etching. It makes the surface rough. However, the graphics on such a screen look blurry, the image quality is poor. The most popular method of applying a quartz coating that scatters incident light; this method has been implemented by Hitachi and Samsung. An anti-static coating is necessary to prevent dust from adhering to the screen due to the accumulation of static electricity.
Protective screen (filter) should be an indispensable attribute of a CRT monitor, since medical studies have shown that radiation containing rays in a wide range (X-ray, infrared and radio radiation), as well as electrostatic fields that accompany the operation of the monitor, can have a very negative effect on human health.
According to the manufacturing technology, protective filters are: mesh, film and glass. Filters can be attached to the front wall of the monitor, hung on the top edge, inserted into a special groove around the screen, or put on the monitor.
Screen filters practically do not protect against electromagnetic radiation and static electricity and somewhat worsen the contrast of the image. However, these filters are good at reducing glare from ambient light, which is important when working with a computer for a long time.
Film filters also do not protect against static electricity, but significantly increase the contrast of the image, almost completely absorb ultraviolet radiation and reduce the level of X-ray radiation. Polarizing film filters, such as Polaroid, are able to rotate the plane of polarization of reflected light and suppress glare.
Glass filters produced in several versions. Simple glass filters remove static charge, attenuate low-frequency electromagnetic fields, reduce ultraviolet radiation and increase image contrast. Glass filters of the “full protection” category have the greatest combination of protective properties: they practically do not produce glare, increase the image contrast by one and a half to two times, eliminate the electrostatic field and ultraviolet radiation, and significantly reduce low-frequency magnetic (less than 1000 Hz) and x-ray radiation. These filters are made of special glass.
Monitor security for of a person is regulated by the TCO standards: TCO 92, TCO 95, TCO 99, proposed by the Swedish Confederation of Trade Unions. TCO 92, issued in 1992, defines the parameters of electromagnetic radiation, gives a certain guarantee of fire safety, ensures electrical safety and defines energy saving parameters. In 1995, the standard was significantly expanded (TSO 95) to include requirements for the ergonomics of monitors. In TCO 99, the requirements for monitors have been further tightened. In particular, the requirements for radiation, ergonomics, energy saving, and fire safety have become stricter. There are also environmental requirements that limit the presence of various hazardous substances and elements in the monitor parts, such as heavy metals.
Monitor Lifespan largely depends on the temperature of its heating during operation. If the monitor gets very hot, you can expect it to have a short lifespan. The monitor, the case of which has a large number of ventilation holes, is accordingly well cooled. Good cooling prevents its rapid failure.
