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1. CRT (Cathode Ray Tube)
Control grid – determines the rate at which the electron will
pass thro.
Electron beam- electrons travel without any hindrance from
the air/dust as the tube is a vacuum.
Phosphor coated screen – It glows when struck by
electrons.
conductive coating - to soak up the electrons that pile up at the
screen-end of the tube.
Focusing anode – It attracts scattered electron to a focal
point.
Accelerating anode – It gives the anode a high velocity so
that we can use the velocity/momentum to give the light we
want.
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2. CRT Monitors
A CRT monitor contains millions of tiny red, green, and blue phosphor
dots that glow when struck by an electron beam that travels across
the screen to create a visible image.
In a cathode ray tube, the "cathode" is a heated filament. The heated
filament is in a vacuum created inside a glass "tube." The "ray" is a
stream of electrons generated by an electron gun that naturally pour
off a heated cathode into the vacuum. Electrons are negative. The
anode is positive, so it attracts the electrons pouring off the cathode.
This screen is coated with phosphor, an organic material that glows
when struck by the electron beam.
There is a conductive coating inside the tube to soak up the electrons
that pile up at the screen-end of the tube.
There are three ways to filter the electron beam in order to obtain the
correct image on the monitor screen: shadow mask, aperture grill and
slot mask. These technologies also impact the sharpness of the
monitor's display. Let's take a closer look at these now.
1. Shadow-mask
A shadow mask is a thin metal screen filled with very small holes.
Three electron beams pass through the holes to focus on a single
point on a CRT displays' phosphor surface. The shadow mask helps
to control the electron beams so that the beams strike the correct
phosphor at just the right intensity to create the desired colors and
image on the display. The unwanted beams are blocked or
"shadowed."
2. Aperture-grill
Monitors based on the Trinitron technology, which was pioneered by
Sony, use an aperture-grill instead of a shadow-mask type of tube.
The aperture grill consists of tiny vertical wires. Electron beams pass
through the aperture grill to illuminate the phosphor on the faceplate.
Most aperture-grill monitors have a flat faceplate and tend to
represent a less distorted image over the entire surface of the display
than the curved faceplate of a shadow-mask CRT. However,
aperture-grill displays are normally more expensive.
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3. 3. Slot-mask
A less-common type of CRT display, a slot-mask tube uses a
combination of the shadow-mask and aperture-grill technologies.
Rather than the round perforations found in shadow-mask CRT
displays, a slot-mask display uses vertically aligned slots. The design
creates more brightness through increased electron transmissions
combined with the arrangement of the phosphor dots.
Advantages of phosphor
- electron r easily knocked off to give light
- once electrons starts losing energy, phosphor stay glowing for
some time – persistence
PERSISTENCE
Different kind of phosphor are used in a CRT. Each has a diff time
durations during which it continues to emit light after the beam is
removed.
Persistence is defined as the time it takes for the emitted light from
the screen to decayto 1/10th
of its origin in intensity. Lower
persistence phosphor require higher refresh rates to maintain a
picture on the screen w/o flicker. The phosphor with low persistence
is useful for animation. A high persistence phosphor is useful for
displaying high complex static pictures.
Resolution.
The max No. of points that can b displayed on the screen on a CRT
w/o overlap is called resolution.
Typical resolution of high definition system is 1280 by 1024.
Screen size.
The physical size of a graphics monitor is given by the length on the
screen diagonally n normally quoted in inches.
Aspect Ratio
It gives the ratio of vertical points to horizontal points necessary to
produce equal length lines in both direction of the screen.
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4. raster graphics
a raster graphics image or bitmap, is a data structure representing
a generally rectangular grid of pixels, or points of color, viewable via a
display medium..
A bitmap corresponds bit-for-bit with an image displayed on a screen,
generally in the same format used for storage in the display's video
memory, or maybe as a device-independent bitmap. Bitmap is
technically characterized by the width and height of the image in
pixels and by the number of bits per pixel (a color depth, which
determines the number of colors it can represent).
In a raster scan display system the electron beam is swept across the
screen one row at a time from top to bottom and from left to right. As
the elec beam moves across each row, the beam intensity is turned
on or off to create a pattern of illuminated spots.
The spots to be turned on are dependent on the picture to be drawn.
The definition of this picture is stored in a memory area called the
refresh buffer or frame buffer.
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5. This memory area holds the intensity valuesfor all screen points.
The intensity values are read from memory area and used to ‘paint’
each point on the screen one row at a time.
This row is called a scan line and each point is called a pixel (picture
element).
The ordering of pixels by rows is known as raster order, or raster
scan order...
Rasterization
The term rasterization can in general be applied to any process by
which vector information can be converted into a raster format.
In normal usage, the term refers to the popular rendering algorithm
for displaying three-dimensional shapes on a computer. Rasterization
is currently the most popular technique for producing real-time 3D
computer graphics. Real-time applications need to respond
immediately to user input, and generally need to produce frame rates
of at least 20 frames per second (and preferably 60 fps or more).
Compared to other rendering techniques such as ray tracing,
rasterization is extremely fast. However, rasterization is simply the
process of computing the mapping from scene geometry to pixels and
does not prescribe a particular way to compute the color of those
pixels.
Interlacing
Its a method of encoding a bitmap image such that a person who has
partially received it sees a degraded copy of the entire image. When
communicating over a slow communications link, this is often
preferable to seeing a perfectly clear copy of one part of the image,
as it helps the viewer decide more quickly whether to abort or
continue the transmission.
Interlacing is supported by the following formats:
• GIF
• PNG
• JPEG
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6. • PGF
Interlacing is also known as "progressive" encoding, because the
image becomes progressively clearer as it is received.
FLAT PANEL DISPLAY
PLASMA PANEL
The basic idea of a plasma display is to illuminate tiny, colored
fluorescent lights to form an image. Each pixel is made up of three
fluorescent lights -- a red light, a green light and a blue light.
What is Plasma?
The central element in a fluorescent light is a plasma, a gas made up
of free-flowing ions (electrically charged atoms) and electrons
(negatively charged particles).
Under normal conditions, a gas is mainly made up of uncharged
particles. That is, the individual gas atoms include equal numbers of
protons (positively charged particles in the atom's nucleus) and
electrons. The negatively charged electrons perfectly balance the
positively charged protons, so the atom has a net charge of zero.
If you introduce many free electrons into the gas by establishing an
electrical voltage across it, negatively charged particles rush toward
the positively charged area of the plasma, and positively charged
particles rush toward the negatively charged area.
In this mad rush, particles are constantly bumping into each other.
These collisions excite the gas atoms in the plasma, causing them to
release photons of energy
How the system works
Its composed of 2 glass plates.
The 1st
plate is brought to the 2nd
plate until the space btn them is
small. The edges are sealed off and space is left with air. The air
inside is then removed and replaced with the plasma gas (e.g neon).
Properties of the gas
- Must produce light when ionized
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7. - Must be easily ionized
- Produce the correct color of gas when ionized.
LIQUID CRYATAL DISPLAY.
In a LCD display, there are 2 polarizers i.e vertical and horizontal
polarizer.
A polarizer is a component that filters light. A vertical polarizer filters
vertical component of light and allow horizontal component of light
thro.
As light strikes the first filter, it is polarized. The molecules in each
layer of the liquid crystal then guide the light they receive to the next
layer. As the light passes through the liquid crystal layers, the
molecules also change the light's plane of vibration to match their
own angle. When the light reaches the far side of the liquid crystal
substance, it vibrates at the same angle as the final layer of
molecules. If the final layer is matched up with the second polarized
glass filter, then the light will pass through.
For a particular voltage the liquid material at that intersection of
electrons changes the orientation of that liquid crystal of that
intersection. The horizontal component is converted to a vertical
component hence transmit light.
If we apply an electric charge to liquid crystal molecules, they
untwist. When they straighten out, they change the angle of the light
passing through them so that it no longer matches the angle of the
top polarizing filter. Consequently, no light can pass through that area
of the LCD, which makes that area darker than the surrounding
areas.
INTERACTIVE DEVICES
They are devices that help in input of data in the system and also
help in giving out the processed information.
They include:
Mouse
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8. Space balls – right handed co-ordinate system. The space ball
doesn’t move. It has a strain gauge that measure the amount of
pressure applied to the space ball to provide input.
Trackball – it’s an upside down mouse
Touch pad
Touch panel
LOGICAL INPUT DEVICES
The need for interactive comp graphics sys has resulted in the dev of
diff kinds of physical interactive devices which function differently
from each other.
The actions of pointing, selecting, sketching, placing or erasing in a
direct manner and the instantaneous sys response to this action
make the interaction possible and natural – however it becomes diff
for a comp graphics application to deal with all the diff kinds of
operating sys.
Most sys therefore introduce an interface btn the application program
and the sys itself with the peculiarities of diff interactive devices.
To make graphical packages independent of the devices used to
input, functions can be structured according to the data description to
be handled by each function.
This approach provides a logical input devices classification in terms
of the kind of data to be input by the device.
1. Locator – it’s a device for specifying a co-rdinate position x,y,
in world co-ord input to the graphics package by the user
selecting a point.
2. stroker – a device for specifying a series of cor-ord positions
3. string – a device for specifying input text
4. valuater – a device for specifying scalar values.
5. choice – a device for selecting menu options
6. pic – a device for selecting picture components.
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