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Cathode is heated to produce electrons as a cloud. Control Grid reduces the number of electrons leaving the Cathode =

Brightness Control. A high negative voltage applied to the controlgrid will stop the electrons. A smaller negative voltage on thecontrol grid decreases the number of electrons passing through.

Since the amount of light emitted by the phosphor coating dependson the number of electrons striking the screen, the brightness of adisplay is controlled by varying the voltage on the control grid. Acontrol knob is available on video monitors to set the brightness forthe entire screen.

Accelerators are used to impart enough energy to produce lightwhen they strike phosphor.

Focusing Systems in a CRT is needed to force the electron beam toconverge into a small spot as it strikes the phosphor. Otherwise theelectrons will repel each other, and the beam would spread out as itapproaches the screen.

Deflection Systems An electromagnetic field or an electrostatic fieldcan be used to deflect a stream of electrons in some linear manner.In electrostatic method, the beam passes between two pair of metalplates. A voltage difference is applied to each pair of platesaccording to the amount that the beam is to be deflected in eachdirection.

Fluorescence Light emitted while the phosphor is being struck byelectrons.

Phosphorescence Light emitted once the electron beam is removed.


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Resolution The maximum number of dots (pixels) that can

be displayed without overlap on a CRT isreferred to as resolution .

Resolution of a CRT depends on the type ofphosphor used and the focusing and deflectionsystems.

E.g. A 640-by-480 pixel screen is capable ofdisplaying 640 distinct dots on each of 480 lines,or about 300,000 pixels. Good quality systemshave a resolution of about 1600X1200 and evenmore.


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Random-scan & Raster-scan Monitors Refresh CRTs can be operated either as random-scan or

as raster-scan monitors. Random-scan Monitors

In Random-scan Monitors, the electron beam is directedonly to parts of the screen where a picture is to bedrawn. These monitors draw a picture one line at a time.They are also referred as vector displays or calligraphicdisplays. The component lines of a picture can be drawnand refreshed by a random-scan system in any orderspecified. This type of system is suited for line-drawingapplications.

A pen plotter is an example of a random-scan hard copydevice.


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Raster-scan Monitors Raster-scan Monitors shoot the electron beam over all

parts of the screen, turning the beam intensity on and offto coincide with the picture definition. The picture is

created as a set of points starting from the top of thescreen. Definition for a picture is now stored as a set ofintensity values for all the screen points, and thesestored values are painted on the screen one row (scanline) at a time. These are well suited for displayingcolors & shading. TV sets and Printers are examples ofsystems using raster-scan methods.


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higher refresh rates are needed.

This is done by interlacing scan lines. First, all points on the even-numbered

lines are displayed. Then, all points along the odd-numbered

lines are displayed.


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Color CRT Monitors Beam Penetration Method Used in Random-scan monitors Two layers of phosphor (usually red & green) are coated onto the

screen. The displayed color depends on how far the electron beam

penetrates into the phosphor layers. A beam of slow electrons emits only the outer red layer. A beam of very fast electrons penetrates through the red layer and

excites the inner green layer. At intermediate beam speeds, combinations of red and green light

are emitted to show additional colors, orange & yellow. Disadvantages Only 4 colors are possible. The quality of pictures is not so good.


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Shadow Mask Method Used in Raster-scan systems (including color TV). The screen is coated with tiny triangular patterns, each containing

three different closely spaced phosphor dots. One phosphor dot of each triangle emits a red light, another emits a

green light, and the third emits a blue light. There is a shadow-mask grid just behind the screen, which contains a

series of holes aligned with the phosphor-dot patterns.

There are three electron guns, one for each color dot. The threebeams from the guns are deflected and focused as a group onto theshadow mask.

When the three beams pass through a hole in the mask, they activatea dot triangle, which appears as a small color spot on the screen.

The phosphor dots in the triangles are arranged so that each electronbeam can activate only its corresponding color dot.

Advantages Much wider range of colors. Clearer character formation. Better cost/performance.


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Direct - View Storage Tube(DVST)


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Primary Gun is used to store the picture pattern. Flood Gun maintains the picture display. High-speed electrons from the primary gun strike the

storage grid, knocking out electrons, which are attractedto the collector grid. Since the storage grid is non-conducting, the areas

where electrons have been removed will keep a netpositive charge. This stored positive charge pattern onthe storage grid is the picture definition.

The flood gun produces continuous flow of free electronsthat pass through the control grid and are attracted to thepositive areas of the storage grid.

These electrons penetrate through the storage grid to thephosphor coating. The finer the grid the higher the resolution of the DVST

(Direct View Storage Tube).


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Plasma-Panel Displays A plasma panel display is made up of millions of phosphor-coated gas-filled

pixel cells. i.e., millions of tiny cells (pixels) containing minute amounts ofgas are sandwiched between two sheets of glass. A series of vertical andhorizontal electrodes, placed on the front and rear glass panels, are used tolight up individual points in the neon. When electrically charged, theyproduce an ultraviolet beam, which activates the phosphorous coating of thecell transmitting light through the glass surface. Color images are producedwith different colors of phosphorous. The result is a totally flat screen withincredibly brilliant images.

An individual neon point in a plasma panel is turned on by applying a "firingvoltage" of about 120 volts to the pair of electrodes. Once the point is turnedon, the voltage on these electrodes is then lowered to a "sustaining voltage"level (about 90 volts) that keeps the neon cell glowing. Erasing the screenis accomplished by lowering the voltage on each electrode below thesustaining voltage level.

Disadvantages Number of points that can be displayed by a plasma panel is limited. High cost than that of a refresh CRT. Advantages No refreshing is required. They have flat screens and are transparent, so displayed images can be

superimposed with pictures from slides.


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LED and LCD Monitors LED and LCD Monitors Two other technologies used in the design of graphics monitors are

light emitting diodes (LED) and liquid-crystal displays (LCDs).These devices use light emitted from diodes or crystals instead ofphosphors or neon gas to display a picture. LEDs and LCDs areparticularly useful in the design of miniscreens used with somegraphic games.

LCD Vs CRT There is no flicker on an LCD display because, while a CRT must

be refreshed, the LCD has a constant source of light over the wholescreen. Once a pixel is on, it stays on until turned off.

The image is always perfectly "focused" over the entire screen. The amount of heat generated by an LCD monitor is considerably

less than a CRT monitor, resulting in a lower load on airconditioning.

LCD Monitors offers brightness twice (approximately) than that of aCRT Monitor

LCD Monitors have less size and weight when compared to CRTMonitors

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