A display is a computer output
surface and projecting mechanism
that shows text and often graphic
images to the computer user, using a
cathode ray tube (CRT), liquid
crystal display (LCD),
light-emitting diode, gas plasma, or
other image projection technology.
The display is usually considered to
include the screen or projection
surface and the device that produces
the information on the screen.
In some computers, the display is
packaged in a separate unit called a
monitor. In other computers, the
display is integrated into a unit
with the processor and other parts
of the computer. (Some sources make
the distinction that the monitor
includes other signal-handling
devices that feed and control the
display or projection device.
However, this distinction disappears
when all these parts become
integrated into a total unit, as in
the case of notebook computers.)
Displays (and monitors) are also
sometimes called video display
terminals (VDTs). The terms display
and monitor are often used
interchangably.
Most computer displays use analog
signals as input to the display
image creation mechanism. This
requirement and the need to
continually refresh the display
image mean that the computer also
needs a display or video adapter.
The video adapter takes the digital
data sent by application programs,
stores it in video random access
memory (video RAM), and converts it
to analog data for the display
scanning mechanism using an
digital-to-analog converter (DAC).
Displays can be characterized
according to:
• Color capability
• Sharpness and viewability
• The size of the screen
• The projection technology
•
Color Capability
Today, most desktop displays provide
color. Notebook and smaller
computers sometimes have a less
expensive monochrome display.
Displays can usually operate in one
of several display that determine
how many bits are used to describe
color and how many colors can be
displayed. A display that can
operate in SuperVGA mode can display
up to 16,777,216 colors because it
can process a 24-bit long
description of a pixel. The number
of bits used to describe a pixel is
known as its bit-depth. The 24-bit
bit-depth is also known as true
color.
It allows eight bits for each of the
three additive primary colors - red,
green, and blue. Although human
beings can't really distinguish that
many colors, the 24-bit system is
convenient for graphic designers
since it allocates one byte for each
color.
The Visual Graphics Array (VGA) mode
is the lowest common denominator of
display modes. Depending on the
resolution setting, it can provide
up to 256 colors.
Sharpness and Viewability
The absolute physical limitation on
the potential image sharpness of a
screen image is the dot pitch, which
is the size of an individual beam
that gets through to light up a
point of phosphor on the screen.
(The shape of this beam can be round
or a vertical, slot-shaped rectangle
depending on the display
technology.) Displays typically come
with a dot pitch of .28 mm
(millimeters) or smaller.
The smaller the dot pitch in
millimeters, the greater the
potential image sharpness.
The actual sharpness of any
particular overall display image is
measured in dots-per-inch (dots per
inch). The dots-per-inch is
determined by a combination of the
screen resolution (how many pixels
are projected on the screen
horizontally and vertically) and the
physical screen size.
The same resolution spread out over
a larger screen offers reduced
sharpness. On the other hand, a
high-resolution setting on a smaller
surface will product a sharper
image, but text readability will
become more difficult.
Viewability includes the ability to
see the screen image well from
different angles. Displays with
cathode ray tubes (CRT) generally
provide good viewability from angles
other than straight on. Flat-panel
displays, including those using
light-emitting diode and liquid
crystal display technology, are
often harder to see at angles other
than straight on.
The Size of the Screen
On desktop computers, the display
screen width relative to height,
known as the aspect ratio, is
generally standardized at 4 to 3
(usually indicated as "4:3"). Screen
sizes are measured in either
millimeters or inches diagonally
from one corner to the opposite
corner. Popular desktop screen sizes
are 12-, 13-, 15-, and 17-inch.
Notebook screen sizes are somewhat
smaller.
The Projection Technology
Most displays in current use employ
cathode ray tube (CRT) technology
similar to that used in most
television sets. The CRT technology
requires a certain distance from the
beam projection device to the screen
in order to function. Using other
technologies, displays can be much
thinner and are known as flat-panel
displays.
Flat panel display technologies
include light-emitting diode (LED),
liquid crystal display (LCD), and
gas plasma. LED and gas plasma work
by lighting up display screen
positions based on the voltages at
different grid intersections. LCDs
work by blocking light rather than
creating it. LCDs require far less
energy than LED and gas plasma
technologies and are currently the
primary technology for notebook and
other mobile computers.
Displays generally handle data input
as character maps or bitmaps. In
character-mapping mode, a display
has a preallocated amount of pixel
space for each character. In bitmap
mode, it receives an exact
representation of the screen image
that is to be projected in the form
of a sequence of bits that describe
the color values for specific x and
y coordinates starting from a given
location on the screen. Displays
that handle bitmaps are also known
as all-points addressable displays.
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