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Cathode Ray Tube (CRT)

 

How it works

CRT is a vacuum tube device where an electron beam can be focused onto a small spot on the fluorescent screen at the opposite end of the tube. The screen is coated with a matrix of thousands of tiny phosphor dots, which glow when it is struck by a stream of fast moving electrons. Different phosphors emit different coloured light and each dot consists of three blobs of coloured phosphor: red, green and blue which will make up as a single pixel.

 

         

Figure 1: Components of CRT

The electron gun in the “bottle neck” of the CRT is composed of a cathode, heat source and focusing elements. There are three different electron guns in a colour monitor, one for each phosphor colour. Images are created when high speed electrons emitted from the heated cathode, converged to strike their respective phosphor blobs. Precise convergence is important as CRT display works on the principal of additive coloration, whereby combinations of different intensities of red, blue and green phosphors create the illusion of millions of colour. When each of the primary colours is added in equal amounts, white spot is formed, whereas the absence of any colour will result a black spot. Misconvergence will show up as shadows which appear around text and graphic images.

The electron gun emits electrons when the cathode (negatively charged) is heated by the heater to liberate electrons. In order for the electron to reach the phosphor screen, it has to pass through the focusing element. While the emitted electron beam will be circular in the middle of the screen, it has a tendency to become elliptical as it spreads its outer areas, creating a distorted image. The focusing elements are set up in a way so that the electron flow is focused into a very thin beam in a specific direction. Then the positively charged anode (located near the screen) will draw the electrons toward the phosphor dots, and cause the electron beam to light up a specific dot.

Figure 2: Electron gun

The deflection yoke around the CRT’s neck creates a magnetic field that directs the electron beams to strike the appropriate position on the screen. This starts at the top left corner (as viewed from front) and flashes on and off as it moves across the row or “raster”, from left to right. As it reaches the edge of the screen, it stops and moves down to the next line. Its motion from right to left is called horizontal retrace and is timed to match with the horizontal blanking interval so that the retrace lines will be invisible. This process is repeated until every line on the screen is traced, at which point it moves from the bottom to the top of the screen – during the vertical retrace interval – ready to display the next screen image. During this process, the electron guns are controlled by the video data stream coming into the monitor from the video card, which varies the intensity of the electron beam at each position on the screen.

The control of the intensity of the electron beam at each dot controls the colour and brightness of each pixel on the screen and it happens very quickly, and in fact the entire screen is drawn in a small fraction of a second. There are separate video streams for each colour coming from the video card, which allows the different colours to have different intensities at each point on the screen. The full rainbow colour is made possible, by varying the intensity of the red, green and blue streams. The surface of the CRT only glows for a small fraction of a second before beginning to fade. Thus the monitor must redraw the picture many times per second to avoid having the screen flicker as it begins to fade and then is renewed. This rapid redrawing is called “refreshing” the screen.

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CRT characteristics

Dot pitch

The dot pitch of a monitor is the physical distance between adjacent phosphor dots of the same colour on the inner surface of the CRT. Typically, this is between 0.22mm and 0.3mm. The smaller number gives a finer and better resolved image. However, providing too many pixels to a monitor without sufficient dot pitch to cope causes very fine details, like the writing below icons to appear blurred.

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Shadow mask

The electron beam travels through a perforated sheet located in front of the phosphor before it strikes the phosphor dots. Originally known as “shadow mask”, these sheets are now available in a number of forms, designed to suit the various CRT tube technologies that have emerged over the years. They have a number of important functions, namely:

  • “mask” the electron beam, forming a smaller, more rounded point that can strike individual phosphor dots cleanly
  • Filter out stray electrons, thereby minimizing “overspill” and ensuring that only the intended phosphors are hit
  • Guide the electrons to the correct phosphor colours, permit independent control of brightness of the monitor’s three primary colours.           

Figure 3: Shadow mask

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Aperture grill

Another alternative is by using an aperture grill where hundreds of fine metal strips run vertically from the top of the screen surface to the bottom. Like the shadow mask, it also forces the electron beam to light up only the correct parts of the screen. The advantage of using aperture grill is that it allows more than one electron beam to pass through the phosphor resulting in a brighter overall picture. Finally, as these strips are aligned vertically from top to bottom of a monitor, this type of tube is flat vertically; it curves outward as you go from left to middle to   right, but not as you go from top to middle to bottom of the monitor. This has reduced glare and results in a more pleasant and less distorted images. However, the major disadvantage is that this bunch of thin metal strips does not have the same physical stability like the shadow mask and tend to vibrate. Hence to correct this problem, one, two or three thin stabilizing wires are aligned horizontally across the screen. Although it eliminates problems with the metal strips moving around, it cause the appearance of very faint lines where the stabilizing wires are.

 Figure 4: Aperture Grill

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Monitor Power and Safety  

  • Display Power Management System(DPMS)

Initiative has been taken to reduce power consumption of monitor during idle periods because of the large amount of energy consumed by monitors during its operations. Most modern monitors are compliant with VESA’s DPMS protocol. It is used selectively to shut down parts of the monitor circuitry after a period of inactivity. 

  • Phosphor burn in and screen saver

If a particular image is displayed on a screen for a long time, the same dots will be strike by the electron beam repeatedly millions of time and cause the surface of the CRT to be damaged after some time. As this happen, ‘ghosting’ can be seen on the surface of the screen and the outline of the image that was displayed so many times could even be seen when the CRT is turned off. When this happens the phosphor is ‘burnt in’. Thus, a screen saver has been created to prevent burn in of the screen phosphor. A screen saver is just a software program that blanks the screen or display moving pattern on it, after a specified period of inactivity.  

  • Electronic emission

The electron beam that creates the image produces electrical and magnetic fields as a side-effect. However, it is unknown of to what extent does these emissions can be linked to health problems. Some believed that prolonged exposure to these electromagnetic fields can lead to increased risk of cancer. 

  • Keep the cover on for safety’s sake

Monitors operate on a very high voltage and have special hazards that can cause serious injury or even death, if you make a mistake while working on one. This is true even when the power of your monitor is turned off, due to the large capacitor that holds charges inside the CRT. Thus, it is dangerous to tamper with devices containing CRT tubes unless you have an engineering training and have taken appropriate pre-cautions.

 

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