Page 38 - DCAP504_Computer Graphics

Unit 3: Overview of Graphics I/O Devices

3.3.2 Projectors
Projectors are used to produce soft copy output. They are commonly used in classroom training or in
conference rooms for presentation. Projectors are mainly of two types:
1. LCD Projectors: LCD is the acronym for Liquid Crystal Display. It is the established technology
used by most of the top manufacturers. Most of today’s LCD projectors contain three separate
LCD glass panels, one each for red, green, and blue components of the image signal being fed into
the projector. As light passes through the LCD panels, individual pixels can be opened to allow
light to pass through, or closed to block the light. This activity modulates the light and produces
the image that is projected onto the screen.
2. DLP Projectors: DLP is the acronym for Digital Light Processing. A single Digital Mirror Device
(DMD) chip is used in DLP that has thousands of tiny mirrors, each representing a signal pixel.
Video images are handled extremely well by DLP projectors.

Texas Instruments Inc. (an American company based in Dallas, Texas, United States,
Did you know?
well known for developing and commercializing semiconductor and computer
technology) developed DLP projectors.

3.4 Image Persistence
Persistence is the amount of time taken by the emitted light from the screen to break down to its
original intensity by one-tenth times. When the LCD monitors are run continuously for long time with a
fixed image, a trace of electric charge is created near the electrode in the LCD module. This results in a
ghost image of the previous image when the image is changed. This is known as image persistence.
Image persistence is not permanent. However, when a particular image is displayed for a long period of
time, ionic impurities accumulate inside the LCD and are seen in the image that is displayed on the
LCD screen. However, this may some times result in the retention of the image on the screen.
In an LCD monitor the objects that can generate a persistent image are the
desktop icons, task bar and background image. This is because these elements
are static and are displayed on the screen for a long period of time. When other
images are loaded on these locations a faint outline or image of the previous
image can be seen.

Image persistence can be avoided by turning off the screen after a few minutes of idle time, or by using
a screen saver with movements. This helps to avoid the display of a particular image for a long time.
3.5 Resolution

Resolution refers to the sharpness and clarity of an image. Resolution is mostly used to describe
monitors, printers, and bit-mapped graphic images. Resolution indicates the maximum number of
points that may be displayed without overlap on the CRT. In case of dot-matrix and laser printers, the
term resolution indicates the number of dots per inch. Resolution is defined as the number of points per
centimeter that may be plotted horizontally and vertically. Resolution depends on the phosphor type,
the displaying intensity and the focusing and deflection systems used in the CRT.
The display resolution of a digital television or display device is the number of different pixels in each
dimension that may be displayed. It may be an indefinite term as the displayed resolution is controlled
by all different factors in CRT and flat panel or projection displays using fixed picture-element (pixel)
arrays.
A 300-dpi printer is one that can print 300 distinct dots in a line that is 1 inch
long. This means that it can print 90,000 dots per square inch.

For graphics monitors, the screen resolution indicates the number of dots on the entire screen.

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