THE MONITOR
A monitor or display (sometimes called a visual display unit) is an electronic visual display for
computers. An electronic visual display
is a piece of electrical equipment which performs as output device for
presentation of images transmitted electronically, for visual reception, without producing a permanent
record. Common applications for electronic visual displays used to be
television sets or computer monitors.
Pixel: Computers can use pixels to display an image as shown
in Figure (1).
·
It is the smallest unit of picture that can
be represented or controlled.
This example shows an image with a portion
greatly enlarged, in which the individual pixels are rendered as little squares
and can easily be seen
Figure (1)
Resolution of computer monitors: The display resolution is determined by the video card of the computer. The number of pixels or picture
elements on the screen determines the resolution or clarity of the picture that
monitors achieve. High resolution monitors have resolution of 2000×2000 pixels.
The old fashioned International Business Machines (IBM) PC, for example, with a
Color Graphic Adaptor (CGA) can output image with a resolution of 640×200
pixels. This was adequate for character-based displays but for graphics it was
inferior to the displays of more advanced machines such as the Apple Macintosh.
In the mid-1980s IBM introduced the
Enhanced Graphic Adaptor (EGA), which offered a high resolution. This gave a
much better clarity, though it was still not as good as the Macintosh.
Then in
1987, IBM brought out the Video Graphic Array (VGA) on its Personal System/2 or
PS/2 (IBM’s third generation of PC) range of microcomputers with a resolution
of 640 by 480.
In 1990
IBM brought out its Extended Graphics Array(XGA) with a resolution of 1024 by
768 pixels and support for 65000 colors. Super VGA with the same resolution
became the standard.
Cathode Ray Tubes: Most computer monitors are based on Cathode
Ray Tubes (CRT) similar to those used in TV sets. The unique function of a CRT
is to convert electronic signals to a visual display. Figure (2) illustrate the
basic operation of a CRT. All CRT's have four main elements:
1. an electron gun,
2. Focusing System
3. a deflection system
4. A screen.
Figure
(2)
The
electron gun provides an electron beam, which passes through focusing and
deflection systems that direct the beam toward specified points on the phosphor
coated screen.
Electrons emitted by the cathode (electron
gun) are attracted to the focusing anode. This is the reason that they travel
through the small hole in the grid.
Deflection: Something is
necessary to produce the picture. That something is called deflection. The
deflection system positions the electron beam on the screen, and the screen
displays a small spot of light at the point where the electron beam strikes it.
Deflection
of the electric beam is done with electric field. The beam passes between two
pairs of metal plates: one pair vertical, the other pair 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 toward the plates with the higher positive
voltage.
The CRT screen: The inside of the large end of a CRT is coated with
a fluorescent material that gives off light when struck by electrons. This
coating is necessary because the electron beam itself is invisible. The
material used to convert the electrons' energy into visible light is a PHOSPHOR.
Since 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 refresh CRT. It redraws 30 to 60 times in a second in order
to avoid flicker. If the redraw is below 25 times per second then the picture
flickers.
Persistence of phosphor: How long the phosphor continues to emit
light after the electron beam is removed. Lower persistence phosphor requires
higher refresh rate to maintain a picture on the screen without flicker. A
phosphor with low persistence is useful for animation, while high persistence
phosphor is better suited for displaying static pictures.
In color screen, there are three guns, one
for each color Red, Green and Blue (RGB). The combination of these three gives
the full color spectrum.
Old monochrome
models display text and graphics as either green on a black background or white
on a black background. The monochrome monitors are gradually declining as most
of the models developed these days uses many colors.
FLAT SCREEN: Flat screen are slowly replacing the CRT
monitors as they are bulky. These were introduced for use on battery operated
portable computers, as these consume very little power. Liquid Crystal Display
(LCD) use flat screen. In an LCD, the image is formed by liquid crystals. These
are long rod-like molecules through which solid can flow like a liquid. Each
pixel on the screen consists of a microscopic electrode positioned below these
molecules.
Color
CDs require very fast liquid crystals. In passive matrix display; there are
three screens, colored red, green and blue which are placed on top of each
other. Below the three screens are the thousands of pixels electrodes which
generate light. The pixel in each screen acts as filters for this light. If a
pixel is to be red, the green and blue filters above it are turned on, blocking
out those colors but the red filter is off, allowing the red light to pass on.
By turning the filters on and off in other combinations, other colors can be
obtained.
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PLOTTER
A
plotter is an output device that uses colour ink pens to draw maps, charts,
diagrams and other drawings on the paper. Plotters have been used in automotive
and aircraft design, topological surveys, architectural layouts, civil
engineering drawings and other similar complex drafting jobs. A plotter is
composed of a pen, a moveable carriage, a drum, and a holder for chart paper.
Both the pen and the paper can move up and down and back and forth. This permit
very detailed drawings.
Plotters
work on the principle of a human hand holding a pen and moving it on the paper.
Plotters are normally very slow output devices because of excessive mechanical
movement required during plotting but the output is of better quality.
Plotters are of two
types:
1.
Flatbed
Plotter
2.
Drum
Plotter
1.
Flatbed
Plotter:
It
is less expensive. It plots on paper that is spread and fixed over a
rectangular flatbed as shown in figure (A). Pens of different colours are
mounted in the pen holding mechanism that moves on the surface in two
orthogonal directions. The pen can be raised and lowered on to the paper during
the drawing operations. The microprocessor in the plotter selects the desired
pen and controls its movement under the control of the computer.
Figure (A)
2.
DRUM
Plotter
Drum
plotter are normally used with mainframe and minicomputer systems. The paper on
which the design has to be made is placed over a drum that rotates back and
forth to produce vertical motion. The pen is mounted on the carriage, which
moves across the width of the paper. The vertical and horizontal movement of
the paper create the required design under the control of the computer. Pen
having ink of different colors can be used to produce output in multicolored.
Figure
(B)
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