Page 26 - DCAP313_LAB_ON_COMPUTER_GRAPHICS

Lab on Computer Graphics

Notes and other for vertical deflection. In case of electric field, two pairs of parallel plates are used,
one for horizontal deflection and next for vertical.

The CRT Screen: The inside of the large end of a CRT is covered with a fluorescent material that
gives off light when struck by electrons. When the electrons in the beam is collides with phosphor
coating screen, they stopped and their kinetic energy is absorbed by the phosphor. Then a part
of beam energy is changed into heat energy and the remainder part causes the electrons in the
phosphor atom to move up to higher energy levels. After a short time the excited electrons come
back to their ground state. During this period, we see a glowing spot that quickly fades after
all excited electrons are returned to their ground state.

Persistence: It is defined as the time they continue to produce light after the CRT beam is removed.
Persistence is defined as the time it takes the emitted light from the screen to decay to one-tenth
of its original intensity. Lower-persistence phosphors require higher refresh rates to maintain a
picture on the screen without flicker. A phosphor with low persistence is useful for animation;
a high-persistence phosphor is useful for displaying highly complex, static pictures. Although
some phosphor have persistence greater than 1 second, graphics monitor are usually constructed
with persistence in the range from 10 to 60 microseconds.
Resolution: The number of points per centimetre that can be used be plotted horizontally and
vertically or total number of points in each direction.
The resolutions of a CRT is depend on

• type of phosphor
• intensity to be displayed
• focusing and deflection system
Aspect Ratio: It is ratio of horizontal to vertical points. Example: An aspect ratio of 3/4 means
that a vertical line plotted with three points has equal length as horizontal line plotted with
four points.
The first cathode ray tube to use a hot cathode was developed by John B.
Johnson (who gave his name to the term Johnson noise) and Harry Weiner
Weinhart of Western Electric, and became a commercial product in 1922.

2.3 Vector Scan/Random Scan Display

A vector scan CRT display directly mark outs out only the desired lines on CRT i.e. if we want
a line connecting point A with point B on the vector graphics display; we simply drive the
beam deflection circuitry, which will cause beam to go directly from point A to B. If we want to
move the beam from point A to point B without showing a line between points, we can vacant
the beam as we move it. To move the beam across CRT, the information about, magnitude and
direction is required. This information is generated with the help of vector graphics generator.

Figure 2.3: Vectors Scan Display

A

B

The typical vector display architecture. It consists of display controller, central processing unit
(CPU), display buffer memory and a CRT. A display controller is connected as an I/O peripheral

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