Visual display unit (Monitors)

All computers are connected to some type of display unit, which is called a monitor. The monitor is a part of the computer video system and monitors are available in many different types and size. The on-screen display enables us to see how applications are processing our data, but Visual display unit is important to remember that the screen display is not permanent record i.e. the outputs are lost where the power is off.

The thinner monitors used on notebook and other small computer are known as flat-panel displays. Compared to CRT (Cathode Ray Tube) based monitors, flat panel displays consume less electricity and take up much less room. Most flat panel displays use Liquid Crystal Display (LCD) technology. LCD displays sandwich cells containing tiny crystals between two transparent surfaces. By varying the electrical current supplied to each crystal, an images forms.

Factors affecting monitor quality

Quality of monitor is often judged in terms of following four factors:

1. Monitor size
2. Dot pitch
3. Resolution
4. Refresh rate

1. Monitor size: The most important aspect of a monitor is its size. Like televisions, screen sizes are measured in diagonal inches, the distance from lower left corner to the upper right corner diagonally. Typical monitors found in the market are of size 14 inches, and 17 inches and above. The size of display are determines monitor quality. In larger monitors the objects look bigger or more objects can be fitted on the screen.
2. Dot pitch: The screen image is composed of a number of pixel elements. A team pixel is the smallest unit of display screen (the word comes from combination of picture and elements). Each pixel is composed of three phosphor dots, Red, Green and Blue. Now the Dot- pitch is the distance between the phosphor dots that make up a single pixel. The Dot-pitch of color monitors for PCs ranges from about 0.15 mm to 0.30 mm.
3. Resolution: The maximum under of points that can be displayed without overlap on a monitor screen is referred to as the resolution. The resolution of a monitor indicates how densely the pixels (a single point in a graphic image) are packed. On color monitors, each pixel is actually composed of three dots – a Red, Green and Blue. Ideally the three points should converge at the same point. Resolution indicates the quality of monitor. Greater the number of pixels, better the resolution and sharper the image. Actually the resolution is determined by the video controller or video adapter card. IBM has provided following adapter cards:

Video Graphics Array (VGA) -------------------------640 × 480 pixels

Super Visual Graphics Array (SVGA) ---------------800 × 600 pixels

----------------1024 × 768 pixels

Extended Graphics Array (XGA) ------------------------1152 × 864 pixels

iv. Refresh Rate: Refresh Rate is the number of times per second at which each pixel on a screen is refreshed. Display monitors must be redrawn many times per second. The refresh rate for a monitor is measured in hertz (HZ) or cycles/second. Generally monitors refresh rates are 60 HZ or 70 HZ. The faster the refresh rate the less the monitor flickers.

Classification of Monitors based on color

Monitors are classified into three categories according to its display color

• Monochrome monitors
• Gray-scale monitors
• Color monitors

Monochrome: Monochrome monitors actually display two colors, one for the background and another for foreground. The color can be black (background) and white (foreground), black (background) and green (foreground).

Grey-scale: Gray-scale is a special type of monochrome monitor capable of displaying different shades of gray. Background color is usually white in such monitors.

Color: Color monitors can display from 1 to 16 million different colours. These monitors are sometimes called RGB (Red, Green, and Blue) monitors because three primary colors Red, Green and Blue are used to make other colours. An RGB monitor consists of cathode ray tube with three-electron guns-one each for Red, Green and Blue at one end and screen at the other end.

Color and gray-scale monitors are often classified by the number of they use to represent each pixel. For example, a 14-bit monitor represents each pixel with 24 bits. The more bits per pixel, the more colors and shades the monitor can display.

Classification of Monitors based on Technology:

1. Cathode Ray Tube (CRT)
2. Flat Panel Display

a. Cathode Ray Tube (CRT)

A traditional picture tube is like a big glass bottle. There are electron guns in the narrow end. They fire towards the large flat surface facing the user. The inside of the glass surface is coated with tiny phosphorus dots. There are arrange in groups of three- Red, Blue and Green (for color monitor) phosphorus dot. These dots light up or grow when hit by electrons from the electron gun. The more powerful beam is, the brighter they get. The electron beams are guided by electromagnets, where bend the beams, so they hit the exact desired phosphorus dot. Data imaged is product by moving the electron beam across the phosphor coated screen. Phosphor coating can be made to glow with different intensities by varying the strength of the electron beam, which finally forms character on screen. CRT monitor contains a shadow mask, which is fine mesh made of up metal., fitted to the shape and size of the screen. The holes in the shadow mask’s mesh are used to align the electron beams, to insure that they strike precisely the correct phosphorus dot. Mostly, these holes are arranged in triangles.

CRT monitor are popular with desktop computers. These are available in monochrome and color monitors. The CRT has display screen of 25 lines of 80 characters each.

Types of CRT monitors:

There are two types of CRT monitors:

i. Composite CRT monitors

Composite CRT monitor is used only one electron gun to control the intensity of all these phosphorus dots in each pixel.

ii. RGB CRT monitors

RGB CRT monitor uses three individual guns, one for its dot, to control the intensity. Each of the sub dots are hit by its own electron gun that is why these monitors give sharper picture.

Advantages of CRT:

a) CRT monitors have wider viewing angle.

b) CRT monitors are cheaper and durable.

c) CRT monitors give sharp and crisp images.

d) CRT monitors resolution is adjustable.

Disadvantage of CRT:

a) As the phosphor dots start to fade after sometime they are hit by the electron gun, they need to be refreshed again.

b) The screen may flicker causing eyestrain.

c) They are bulky and heavy.

d) They consume high power.

b. Flat panel display

A CRT screen is reliable but it is bulky & consumes a lot of power so it is not used for portable computers. For small computers, flat panel display is used. The most common type of flat panel display are:

i. Liquid crystal display (LCD)

ii. Gas Plasma Display (GPD)

i. Liquid Crystal Display (LCD)

LCD produces images by aligning molecular crystals. When a voltage is applied, the crystals line up in a way that blocks light from passing through them and the absence of light is seen as characters on the screen. The LCD screen is flat, since it does not have picture tube. Instead the screen image is generated on a flat plastic disk. Thus LCD screens are much thinner, soft and don’t flicker as compared to CRT.

Advantages of LCD

1. LCD is light weight, flat, thin and require less space.
2. LCD does not require periodic refreshing.
3. LCD emits zero radiation thereby avoiding eye strain.
4. LCD consumes less power.

Disadvantages of LCD

1. LCD have smaller viewing angle so picture is best viewed when the person is in straight position from the centre of monitor.
2. The liquid crystals do not emit light so the images are less sharp.
3. There is a need of backlight setting to enhance sharpness of images.
4. Resolution is normally set.

ii. Gas Plasma Display (GPD)

One of the advancement in flat panel display is GPD. Gas Plasma diplay offers flicker free viewing and has higher contrast than LCDs. GPD contains ionized gas (Neon or Xenon) in between two glass plates. Among the two glass plates, one has serial of vertical wires or electrodes while other has series of horizontal wires. When the two placed together, the intersection of horizontal and vertical wire identifies a pixel. When current is applied through appropriate vertical and horizontal lines, the gas at the pixel emits light. Thus characters are formed by glowing combination of appropriate pixels.