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What are LCD monitors and how do they work?
The liquid crystal display (LCD) technology now used in flat panel monitors has long been
used in digital watches, calculators, and many other devices. An LCD device (also called a
cell) is made out of two layers of very fine glass material called substrates that form a
"sandwich" around a thin layer of rod-shaped molecules (liquid crystals) that
flow like liquid. When a charge of electric current passes through the layer of liquid
crystals, they align or twist, preventing or allowing the light to pass through. A white paper on Silicon Graphics 1600SW is
available for a more in-depth explanation of the technology behind SGI flat panel
displays.
How to choose a flat panel display: what features determine quality?
We recommend looking at the following major features when evaluating the quality and
price/performance ratio of an LCD monitor.
Resolution:
Make sure you measure the maximum true resolution--the resolution achieved by the number
of pixels the monitor contains--without scaling the screen contents up or down.
Size:
Size is measured diagonally from one corner of the glass to the opposite corner.
Aspect ratio:
This is related to resolution; it is the ratio of the monitor's width to height. Most
displays have a 1.25:1 ratio. Some, like Silicon Graphics 1600SW, have a wider format,
1.6:1 ratio. The amount of horizontal space is critical in determining how much
information can be displayed at once, particularly when viewing side-by-side pages, or
spreads.
Dot
pitch:
This is a measure of the amount of space between each pixel. The smaller the dot
pitch, the sharper the image. The dpi, or dots per inch (also called pixels per inch), is
related to dot pitch: the smaller the dot pitch, the higher the dpi is.
Brightness:
Brightness is determined by the intensity and quality of the backlighting; in displays, it
is usually measured in luminance (candelas per square meter). A brighter monitor will be
easier to view and have greater color intensity.
Color resolution:
The number of colors that can be represented on a display without dithering. Higher
quality LCDs have 24-bit color,
allowing a color gamut, or range, of 16.7 million.
Contrast:
Related to brightness, this is a measure of the range between the lightest tones and the
darkest tones that the LCD is capable of producing. A higher contrast ratio makes the
information on the display more readable.
Viewing angle:
This determines how far above, below, or to either side of the display a person can be and
still accurately view the image on it.
Pixel response rate:
Measured in milliseconds or microseconds, this is the time it takes for a pixel to respond
to voltage (to be turned "on") and then return back to its normal state. The
shorter the pixel response rate, the more quickly the panel will be able to display and
refresh images.
Additional features:
Additional features that are important to some users are color calibration capability, a
removable stand (for mounting the monitor on a wall or adjustable arm), a pivoting head
(for viewing in portrait mode), and additional connectors (for USB, video inputs, etc.)
A critical consideration in determining a display's overall value is TCO or total cost of
ownership for the product. For more information on this, see "How does the total cost of ownership (TCO) compare to an
equivalently sized CRT monitor (19 or 21-inch)?".
What's the difference between resolution and size?
Resolution usually refers to pixel resolution, or the number of addressable pixels on a
display, whereas size is a measure of the diagonal distance from one corner of the display
glass to the opposite corner. Some monitors are very large, but not very high resolution
(most 20-inch and larger displays, for example, have only a 1280x1024 resolution). This
means that the space between the pixels is larger and results in a grainer screen
image-much like a photograph loses sharpness when it is enlarged. SGI monitors are built
to have a very high resolution relative to their size. This results in much finer dot
pitch (the space between pixels is much smaller) and a much higher image quality. Even
very small details in data or images appear very crisp, and a greater number of pixels
allows users to comfortably display more information at once.
What is the difference between CRT and LCD size measurements?
CRTs have two specifications for screen size: the CRT size (the actual size of the picture
tube) and the viewable screen size (the usable screen area). Because the CRT picture tube
is enclosed in the plastic casing, the viewable screen size is smaller than the overall
CRT size. Though CRTs are commonly referred to by the picture tube size, it is the
viewable area that is important in comparing a CRT to an LCD. Unlike CRTs, the viewing
area of an LCD is the only valid measurement of its size. This is why many LCD
specifications list a CRT equivalent size to identify the picture tube size that is
required to achieve the same viewable size display. For example, the Silicon Graphics
1600SW has a diagonal screen size of 17.3 inches, which is equivalent to a 19-inch CRT
display's viewing area.
How does the total cost of ownership (TCO) compare
to an equivalently sized CRT monitor (19 or 21-inch)?
Total cost of ownership includes the actual purchase price for the monitor as well as all
other costs of owning and using the monitor. Studies show that the purchase cost for
computer-related equipment often is less than a quarter of the total cost of ownership.
LCD panels offer excellent value compared to CRT monitors when viewed from the total cost
of ownership perspective. The 1600SW operates on less than 20% of the power of typical
21-inch CRT monitors, in addition to offering three times the brightness and up to five
times the contrast. This efficiency not only saves on power consumption costs, but also
translates into significant cooling power savings. Flat panel displays take up far less
space than CRT monitors and can be easily mounted on flexible arms, saving on furniture
expense and allowing a higher density of both people and information displayed. For
applications requiring close communication of workers or large amounts of information,
flat panels can enable dramatic productivity gains.
What is the difference between active matrix LCDs (AMLCD) and passive
matrix LCDs?
For an LCD to work, each pixel must be energized to either let light through or block
light out. The difference between active matrix and passive matrix displays is the way in
which the pixels are electrically addressed, or "energized." Passive matrix flat
panel displays consist of a grid of horizontal and vertical wires. At the intersection of
each grid is an LCD element that constitutes a single pixel. Active matrix flat panels are
a higher quality and more expensive type of display in which transistors are built into
each pixel within the screen. For example, the 1600x1024 screen size of the 1600SW
requires over 14 million transistors, one for each red, green, and blue subpixel. Active
matrix, sometimes also called TFT (thin film transistor) displays typically have higher
resolution, higher contrast, and much faster pixel response rates than passive matrix
LCDs.
What is 24-bit color?
The number of colors an LCD monitor can display is dependent on the number of grayscale
levels that it can display, which is essentially a measure of how well it is electrically
addressed. Each subpixel in the 1600SW is addressed by 8 bits of data, allowing it to have
256 (or 28) gray levels. In a color monitor, each pixel is made up of three subpixels, one
for each primary color. These red, green, and blue dots are energized to different
intensities (or grayscales) to create a range of colors that we perceive as the mixture of
these dots. Because the shade of each of the three subpixels is determined by 8 bits of
data, the monitor displays in 24 (3x8) bit color. That means the monitor is capable of
displaying 256 (R) x 256 (G) x 256 (B), or over 16,700,000 colors! Imagine painting a
picture with only four colors. With such a limited selection, your level of expression is
greatly suppressed. With display devices, higher color depth effectively gives you more
colors with which to "paint" your image. Some LCD monitors can only display 18
bits of color and cannot show 24 bits of true color. While this may be fine for some
noncritical applications, serious professionals require the full color range for their
work. SGI flat panels all are 24-bit color capable, giving you a fantastic color range,
with no dithering.
What are pixel defects? Is my display defective?
A certain number of pixel defects are expected on LCD panels and do not constitute a
defective display. Each graphic pixel on an LCD screen is made up of red, green, and blue
subpixels. Due to the variables inherent to the manufacturing process, some of these
subpixels become stuck in either the on or the off state. This pixel defect, also called a
stuck pixel, appears as a small red, green, or blue dot for stuck-on pixels, or a small
black dot for stuck-off pixels. During the manufacturing testing process, LCD panels that
exceed a specified number of stuck pixels are rejected. In order to provide the latest
flat panel technology at a reasonable price, LCD manufacturers allow a small number of
pixel defects in their quality-control specifications. Although these specifications are
improving all the time, with fewer pixel defects allowed with every new generation of
displays, a zero-defect policy today would be prohibitively expensive. Even though SGI
flat panel displays have some of the most stringent specifications for pixel defects in
the industry, many panels can have several stuck pixels. |
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