Display devices
The display devices used in
graphics system is video monitor. The most common video monitor is based on CRT
technology.
Cathode
Ray Tube (CRT)
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CRT are the most common display devices on
computer today. A CRT is an evacuated glass tube, with a heating element on one
end and a phosphor-coated screen on the other end.
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When a current flows through this heating
element (filament) the conductivity of metal is reduced due to high
temperature. These cause electrons to pile up on the filament.
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These electrons are attracted to a strong
positive charge from the outer surface of the focusing anode cylinder.
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Due to the weaker negative charge inside the
cylinder, the electrons head towards the anode forced into a beam and accelerated
by the inner cylinder walls in just the way that water is speeds up when its
flow though a small diameter pipe.
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The forwarding fast electron beam is called
Cathode Ray. A cathode ray tube is shown in figure below.
Figure :Cathode Ray Tube
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There are two sets of weakly charged deflection
plates with oppositely charged, one positive and another negative. The first
set displaces the beam up and down and the second displaces the beam left and
right.
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The electrons are sent flying out of the neck of
bottle (tube) until the smash into the phosphor coating on the other end.
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When electrons strike on phosphor coating, the
phosphor then emits a small spot of light at each position contacted by
electron beam. The glowing positions are used to represent the picture in the
screen.
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The amount of light emitted by the phosphor
coating depends on the no of electrons striking the screen. The brightness of
the display is controlled by varying the voltage on the control grid.
Persistence:
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How long a phosphor continues to emit light after
the electron beam is removed
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Persistence of phosphor is defined as the
time it takes for emitted light to decay to 1/10 (10%) of its
original intensity. Range of persistence of different phosphors can react many
seconds.
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Phosphors for graphical display have persistence
of 10 to 60 microseconds. Phosphors with low persistence are useful for
animation whereas high persistence phosphor is useful for highly complex,
static pictures.
Refresh Rate:
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Light emitted by phosphor fades very rapidly, so
to keep the drawn picture glowing constantly, it is required to redraw the
picture repeatedly and quickly directing the electron beam back over the some
point. The no of times/sec the image is redrawn to give a feeling of
non-flickering pictures is called refresh-rate.
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If Refresh rate decreases, flicker develops.
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For refresh displays, it depends on picture
complexity
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Refresh rate above which flickering stops and
steady it may be called as critical fusion frequency(CFF).
Resolution:
Maximum number of points
displayed horizontally and vertically without overlap on a display screen is
called resolution. In other ways , resolution is referred as the no of points
per inch(dpi/pixel per inch).
