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Unformatted text preview: oltage applied to the control grid, larger will be
the number of electrons that will be allowed to strike the screen, and larger will be the
beam's intensity. If the grid voltage is negative, then no electrons will be allowed to pass
the grid and the beam is said to be blanked.
The electrons, which are allowed to pass through the control grid are accelerated and
focused by an assembly unit consisting of accelerating electrodes and a focusing electron
lens. The accelerating electrodes are applied with a positive voltage to accelerate the
electrons and the focusing electron lens consists of a cylindrical metal enclosure with a
hole through which the electrons pass in the form of an electron beam. The electron beam
generator, the control grid, and the accelerating electrodes with focusing electron lens
together form the electron gun of the CRT monitor.
To focus the electron beam generated by the electron gun at a desired point on the screen,
the beam is passed through a pair of vertical deflection plates and a pair of horizontal
deflection plates. Appropriate positive voltages are applied to one of the vertical and one
of the horizontal plates to achieve this. For example, the beam can be moved up/down by
applying a suitable positive voltage to the top/bottom plate of the vertical plates pair.
Similarly, the beam can be moved left/right by applying a suitable positive voltage to the
left/right plate of the horizontal plates pair.
The focused and accelerated electron beam can thus be made to hit at any point on the
screen. The backside of the monitor's screen is coated with phosphor, a chemical that
glows for a short time when it is hit by the electron beam. The screen's phosphor coating
is organized into a grid of dots called pixels (a contraction of picture elements). Since the
phosphor glows only for a short time (few microseconds), an image drawn on the screen
would be visible only for a few microseconds and then fade away, unless the image is
repeatedly drawn. This may appear to be an inconvenience, but it is also an advantage
since there is no need to erase anything. Instead, we just need to draw new images a...
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- Spring '14