Light-emitting Diode (LED) and Liquid-crystal Displays (LCDs)

Light-emitting Diode (LED) and Liquid-crystal Displays (LCDs)

In LED, a matrix of diodes is arranged to form the pixel positions in the display and picture definition is stored in a refresh buffer. Information is read from the refresh buffer and converted to voltage levels that are applied to the diodes to produce the light patterns in the display.

Liquid crystal displays (LCD) are the device that produces a picture by passing polarized light from the surroundings or from an internal light source through a liquid crystal material that transmit the light. Liquid-crystal displays (LCDs) are commonly used in small systems, such ' as calculators and portable, laptop computers.  (LCDs are commonly used in small systems, such as calculators and portable, laptop computers. These non-emissive devices produce a picture by passing polarized light from the surrounding or from an internal light source through a liquid crystal material that can be aligned to either block or transmit the light.) The term liquid crystal refers to the fact that these compounds have a crystalline arrangement of molecules, yet they flow like a liquid. Flat panel displays commonly use liquid crystal compounds that tend to keep the long axes of the rod– shaped molecules aligned. In a flat panel display, two glass plates, each containing a light polarizer at right angles to the other plate, sandwich the liquid crystal material. Rows of horizontal transparent conductors are built into one glass plate and columns of vertical conductors are put into the other plate. The intersection of two conductors defines a pixel position. Polarized light passing through the material is twisted so that it will pass through the opposite polarizer. The light is then reflected back to the viewer. To turn off the pixel, we apply a voltage to two intersecting conductors to align the molecules so that the light is not twisted.

Direct-View Storage Tubes & Flat-Panel Displays

Direct-View Storage Tubes (DVST)

This is an alternative method to monitor a screen image, as it sores the picture information inside the CRT instead of refreshing the screen. A direct-view storage tube (DVST) stores the picture information as a charge distribution just behind the phosphor coated screen. Two electron guns are used in a DVST. One, the primary gun, is used to store the picture pattern; the second, the flood gun, maintains the picture display. A DVST monitor has both disadvantages and advantages compared to the refresh CRT. Because no refreshing is needed, very complex pictures can be displayed at very high resolutions without flicker. Disadvantages of DVST systems are that they ordinarily do not display color and that selected parts of a picture cannot be erased. To eliminate a picture section, the entire screen must be erased and the modified picture redrawn. The erasing and redrawing process can take several seconds for a complex picture. For these reasons, storage displays have been largely replaced by raster systems.

Flat-Panel Displays

The term flat panel display refers to a class of video device that have reduced volume, weight and power requirement compared to a CRT. A significant feature of flat-panel displays is that they are thinner than CRTs, and we can hang them on walls or wear them on our wrists. Since we can even write on some flat-panel displays, they will soon be available as pocket notepads. Current uses for flat-panel displays include small TV monitors, calculators, pocket video games, laptop computers, armrest viewing of movies on airlines, as advertisement boards in elevators, and as graphics displays in applications requiring rugged, portable monitors.

We can separate flat-panel displays into two categories: emissive displays and non-emissive displays. The emissive displays (or emitters) are devices that convert electrical energy into light. Plasma panels, thin-film electroluminescent displays, and light- emitting diodes are examples of emissive displays. Non-emissive displays use optical effects to convert sunlight or light from some other source into graphics patterns. The most important example of a non-emissive flat-panel display is a liquid-crystal device.

Random-Scan/Calligraphic displays & Raster-Scan Displays( Difference between two)

Random-Scan/Calligraphic displays

Random scan system uses an electron beam which operates like a pencil to create a line image on the CRT. The image is constructed out of a sequence of straight line segments. Each line segment is drawn on the screen by directing the beam to move from one point on screen to the next, where each point is defined by its x and y coordinates. After drawing the picture, the system cycles back to the first line and design all the lines of the picture 30 to 60 time each second. When operated as a random-scan display unit, a CRT has the electron beam directed only to the parts of the screen where a picture is to be drawn. Random-scan monitors draw a picture one line at a time and for this reason are also referred to as vector displays (or stroke-writing or calligraphic displays) A pen plotter operates in a similar way and is an example of a random-scan, hard-copy device. Refresh rate on a random-scan system depends on the number of lines to be displayed. Picture definition is now stored as a set of line-drawing commands in an area of memory referred to as the refresh display file. Random-scan systems are designed for line-drawing applications and can-not display realistic shaded scenes. Since picture definition is stored as a set of line-drawing instructions and not as a set of intensity values for all screen points, vector displays generally have higher resolution than raster systems. Also, vector displays produce smooth line drawings because the CRT beam directly follows the line path.

Raster-Scan Displays

In a raster-scan system, the electron beam is swept across the screen, one row at a time from top to bottom. As the electron beam moves across each row, the beam intensity is turned on and off to create a pattern of illuminated spots. Picture definition is stored in a memory area called the refresh buffer or frame buffer. This memory area holds the set of intensity values for all the screen points. Stored intensity values are then retrieved from the refresh buffer and "painted" on the screen one row (scan line) at a time. Each screen point is referred to as a pixel. The capability of a raster-scan system to store intensity information for each screen point makes it well suited for the realistic display of scenes containing subtle shading and color patterns. Home television sets and printers are examples of other systems using raster scan methods. In raster scan each frame is displayed in two passes using an interlaced refresh procedure. In the first pass, the beam sweeps across every other scan line from top to bottom. Then after the vertical re- trace, the beam sweeps out the remaining scan lines. Interlacing of the scan lines in this way allows us to see the entire screen displayed in one-half the time it would have taken to sweep across all the lines at once from top to bottom. Interlacing is primarily used with slower refreshing rates. On an older, 30 frame- per-second, no interlaced display, for instance, some flicker is noticeable. But with interlacing, each of the two passes can be accomplished in l/60th of a second, which brings the refresh rate nearer to 60 frames per second. This is an effective technique for avoiding flicker, providing that adjacent scan lines contain similar display information.

 Differentiate between raster scan and random scan displays.

The most common form of graphics monitor employing a CRT is the raster scan display, based on television technology. In a raster scan system, the electron beam is swept across the screen, one row at a time from top to bottom. As the electron beam moves across each row, the beam intensity is turned on and off to create a pattern of illuminated spots. Picture definition is stored in a memory area called the refresh buffer or frame buffer. This memory area holds intensity values for all the screen points. Stored intensity values are then retrieved from the refresh buffer and pointed on the screen one row at a time. Each screen point is referred to as a pixel.

When operated as a random scan display unit, the CRT has the electron beam directed only to the parts of the screen where a picture is to be drawn. Random scan monitors draw a picture one line at a time and for this reason is also known as vector displays. The component lines of a picture can be drawn and refreshed by a random scan system. A pen plotter operates in a similar way and is an example of a random scan, hard copy device.

Resolution & Aspect Ratio

Resolution

The maximum number of points that can be displayed without overlap on a CRT is referred to as the resolution. A more precise definition of resolution is the number of points per centimeter that can be plotted horizontally and vertically, although it is often simply stated as the total number of points in each direction. This depends on the type of phosphor used and the focusing and deflection system.

Aspect Ratio

Another property of video monitors is aspect ratio. This number gives the ratio of vertical points to horizontal points necessary to produce equal-length lines in both directions on the screen. (Sometimes aspect ratio is stated in terms of the ratio of horizontal to vertical points.) An aspect ratio of 3/4 means that a vertical line plotted with three points has the same length as a horizontal line plotted with four points.

Basic Operation of CRT

Basic Operation of CRT

A beam of electrons (cathode rays), emitted by an electron gun, passes through focusing and deflection systems that direct the beam toward specified positions on the phosphor-coated screen. The phosphor then emits a small spot of light at each position contacted by the electron beam. Because the light emitted by the phosphor fades very rapidly, some method is needed for maintaining the screen picture. One Way to keep the phosphor glowing is to redraw the picture repeatedly by quickly directing the electron beam back over the same points. This type of display is called a refresh CRT.

Components of CRT are as follows:-

1. HEATER ELEMENT AND CATHODE:-Heat is supplied to the cathode by passing current through heater element. Cathode is cylindrical metallic structure which is rich in electrons. On heating the electrons are released from cathode surface.
2. CONTROL GRID:- It is the next element which follows cathode. It almost covers cathode leaving small opening for electrons to come out. Intensity of the electron beam is controlled by setting voltage levels on the control grid. A high negative cottage applied to the control grid will shut off the beam by repelling electrons and stopping them from passing through the small hole at the end of control grid structure. A smaller negative voltage on the control grid will simply decrease the number of electrons passing through. Thus we can control the brightness of a display by varying the voltage on the control grid.
3. ACCELERATING ANODE:-They are positively charged anodes who accelerate the electrons towards phosphor screen.
4. FOCUSING & DEFLECTION COILS:- They are together needed to force the electron beam to converge into a small spot as it strikes the screen otherwise the electrons would repel each other and the beam would spread out as it approaches the screen. Electrostatic focusing is commonly used in television and computer graphics monitor.
5. PHOSPHOR COATING:-When the accelerating electron beam (collides) is incident on the phosphor cooling, a part of Kinetic Energy is converted into light and heat. When the electrons in the beam collide with the phosphor coating they are stopped and their kinetic energy is absorbed by the phosphor.

 Working

Beam passes between two pairs of metal plates, one vertical and other horizontal. A voltage difference is applied to each pair of plates according to the amount that the beam is to be deflected in each direction. As the electron beam passes between each pair of plates, it is bent towards the plate with the higher positive voltage. The beam is first deflected towards one side of the screen. Then, as the beam passes through the horizontal plates, it is deflected towards, the top or bottom of the screen. To get the proper deflection, adjust the current through coils placed around the outside of the CRT loop.

Explain the operation of electron gun in the working of a CRT.

The primary components of an electron gun in a CRT are the heated metal cathode and a control grid. Heat is supplied to the cathode by directing a current through a coil of wire, called the filament, inside the cylindrical cathode structure. This causes electrons to be “boiled off” the hot cathode surface. In the vacuum inside the CRT envelope, the free, negatively charged electrons are then accelerated towards the phosphor coating by a high positive voltage. The accelerating voltage can be generated with a positively charged metal coating on the inside of the CRT envelope, near the phosphor screen, or an accelerating anode can be used. Intensity of the electron beam is controlled by setting voltage levels on the control grid, which is a metal cylinder that fits over the cathode. A high negative voltage applied to the control grid will shut off the beam by repelling electrons and stopping them from passing through the small hole at the end of the control grid structure. A smaller negative voltage on the control grid simply decreases the number of electrons p[assign through. Since the amount of light emitted by the phosphor coating depends on the number of electrons striking the screen, we control the brightness of a display by varying the voltage on the control grid.

Color CRT Monitors

A color CRT monitor displays color picture by using a combination of phosphors that emit different colored light. By combining the emitted light a range of colors can be generated. Two basic methods for producing color displays are:

a)      a) Beam Penetration Method

Random scan monitors use the beam penetration method for displaying color picture. In this, the inside of CRT screen is coated two layers of phosphor namely red and green. A beam of slow electrons excites only the outer red layer, while a beam of fast electrons penetrates red layer and excites the inner green layer. At intermediate beam speeds, combinations of red and green light are emitted to show two additional colors- orange and yellow. The speed of the electrons, and hence the screen color at any point, is controlled by the beam-acceleration voltage. Beam penetration has been an inexpensive way to produce color in random-scan monitors, but only four colors are possible, and the quality of pictures is not as good as with other methods.

b)     b) Shadow Mask Method

Raster scan system are use shadow mask methods to produce a much more range of colors than beam penetration method. In this, CRT has three phosphor color dots. One phosphor dot emits a red light, second emits a green light and third emits a blue light. When the three beams pass through a hole 'in the shadow mask, they activate a dot triangle, which appears as a small color spot on the screen. The phosphor dots in the triangles are arranged so that each electron beam can activate only its corresponding color dot when it passes through the shadow mask. In some low-cost systems, the electron beam can only be set to on or off, limiting displays to eight colors. More sophisticated systems can set intermediate intensity levels for the electron beams, allowing several million different colors to be generated. 

Compare and contrast shadow mask and beam penetration method

Both methods are used in color CRT monitors. Beam penetration method is used for random scan monitors. In beam penetration two layers of phosphor red and green are coated inside CRT screen, the display of color depend on how far electron excites outer red layer, then green layer. This method can produce four colors i.e. red, green, orange, and yellow. This is less costly method as compared to shadow mask. But it can produce less color as compared to shadow mask. The quality of picture is also poor as compared to shadow mask. Shadow mask is used for raster scan systems. It can produce wide variety of colors .There is three phosphor color dots at each pixel position. One phosphor dot emits red light, another emit green light, third emit blue light. Three guns one for each color is used. Three beams pass through holes in shadow mask, and a small color spot on screen is appeared. Shadow masks are used as display device for home computers, color TV set etc