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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.
| How do I choose a flat panel display? |
Look 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 16:9 ratio.
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.
Analog/digital:
All LCDs are natively digital
(unlike CRTs) but whether it is analog or digital is determined by
what kind of input it requires. Digital monitors usually have a
better picture quality, because the output is not the result of an
analog to digital conversion.
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.
| 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. Some 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.
| How does the total cost of ownership
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. LCD panels offer excellent value compared to CRT monitors
when viewed from the total cost of ownership perspective. The
LCD 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 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
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.
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 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.
| What is color
management? |
Color management consists of hardware and
software components that enable the monitor to achieve a very high
level of color accuracy and consistency. It allows the precise control
of gamma and color temperature over a wide dynamic range and
supports the adjustment of display characteristics to meet a variety
of display values.
Color Calibration
The process of adjusting a
display's output characteristics so as to modify its appearance to
conform to predetermined standards or settings. Usually accomplished
through the use of a color or luminance measuring device and a
lookup table (LUT) of values accessible to the computer's graphics
controller. See Gamma.
Color Filter
A red-, green-, or blue-dyed
gelatin or pigment placed above each LCD subpixel. Combinations of
various light levels passing through these color elements can
produce all the visible spectral colors.
Color Gamut
The entire range of colors
available on a particular device such as a monitor. On an LCD with
true 24-bit color, the color gamut is 16,700,000 colors.
Color Resolution
The colors that can be
represented on a display depending on the number of Grayscales
resolved by the LCD element.
Color Temperature
The definition of a
monitor's white point, whose chromaticity coordinates can be
somewhat arbitrary, existing in color space from red-white to
blue-white. Expressed in Kelvins (K), color temperature refers to
the amount of light radiated by a perfect thermal radiator. Values
at or below 5000K appear reddish; higher numbers, for example 7000K,
appear bluish.
Column Drivers
Microelectronic circuits that
provide the correct voltages to the individual subpixels through the
source lines. For example, 8-bit drivers provide 256 gray shades, or
256 distinct colors per subpixel.
Contrast
The range between the lightest tones
and the darkest tones in an image. The lower the number value, the
more closely the shades will resemble each other. The higher the
number, the more the shades will stand out from each other.
Contrast Ratio
The ratio between the amount
of light transmitted by a pixel in its unselected ("off") state and
its selected ("on") state.
Convergence
The clarity and sharpness of each
pixel.
LCD
(Liquid Crystal Displays). These displays
are fabricated using semiconductor processes, with each pixel
comprising transistors set up in a grid. LCDs are inherently digital
displays. In their most common computer application, notebook
computers, video drivers take the digital information in the
graphics frame buffer and digitally interface to the row and column
drivers that set the colors at each pixel in the display.
Pixel
The smallest addressable unit on a
display screen. The resolution of a monitor is determined by the
number of pixels covering the width and height of the complete
on-screen image.
In storage, pixels are made up of one or more bits. The greater
this bit depth, the more shades or colors can be represented.
Grayscale and color displays typically use from 4 to 24 bits per
pixel, providing from 16 to 16 million colors.
On screen, pixels are made up of one or more dots of color
(subpixels). For grayscale, the pixel is energized with different
intensities, creating a range from dark to light. Color systems use
a red, green, and blue dot per pixel, each of which is energized to
different intensities, creating a range of colors perceived as the
mixture of these dots. Black is all three dots dark, white is all
dots light.
Pixel Clock
The pixel clock that resides in
the LCD is critical to ensure that as each line is scanned, the red,
green, and blue components of each pixel precisely align with each
other. If there is a slight error in the clock, this error
accumulates as the line is scanned from the left to the right edge
of the display.
Pixel Pitch
The distance from the edge of one
pixel to the similar edge on an adjacent pixel.
Pixel Resolution
The number of addressable
pixels in a display. Several standard display sizes are:
| VGA: |
640 x 480 |
| SVGA: |
800 x 600 |
| XGA: |
1024 x 768 |
| SXGA: |
1280 x 1024 |
| SXGA-Wide: |
1600 x 1024 |
UXGA:
|
1600 x 1200 |
| HDTV: |
1920 x 1080 |
| UXGA-Wide: |
1920 x 1200 |
| QXGA: |
2056 x 1536 |
Refresh Rate
How many times per second the
screen is refreshed (redrawn).
Response Time
The speed at which the
orientation of a liquid crystal material can change in response to a
charging/discharging cycle. Typically noted as "rise plus fall," for
twisted-nematic structures, this time ranges from 20 to 50 ms.
SVGA
Super video graphics array is a set of
graphics standards designed to offer greater resolution than VGA.
There are several varieties of SVGA, each providing a different
resolution. All SVGA standards support a palette of 16 million
colors, but the number of colors that can be displayed
simultaneously is limited by the amount of video memory available.
The SVGA standards are developed by a consortium of monitor and
graphics manufacturers called VESA.
SXGA
(Super Extended Graphics Adapter) A
graphics standard offering a display resolution of 1280x1024 pixels.
SXGA-Wide has a display resolution of 1600x1024.
TFT
(Thin Film Transistor) An a-Si, p-Si, or
CdSe used as a switch to a charge storage device beneath each
subpixel of an AMLCD. A type of LCD flat-panel display screen, in
which each pixel is controlled by from one to four transistors. The
TFT technology provides the best resolution of all the flat-panel
techniques, but it is also the most expensive. TFT screens are
sometimes called active-matrix LCDs.
TTL
(Transistor-Transistor Logic). A common
type of digital circuit in which the output is derived from two
transistors. The term is commonly used to describe any system based
on digital circuitry, as in TTL monitor.
UXGA
(Ultra Extended Graphics Adapter). A
graphics standard offering a display resolution of 1600x1200
pixels.
VGA
(Video Graphics Array). VGA has become
one of the de facto standards for PCs. In text mode, VGA systems
provide a resolution of 720 by 400 pixels. In graphics mode, the
resolution is either 640 by 480 (with 16 colors) or 320 by 200 (with
256 colors). VGA remains the lowest common denominator among PC
displays.
Viewing Angle
The bounding angles generated
from a point normal to the display surface within which can be found
acceptable contrast ratios and linear grayscales.
XGA
(Extended Graphics Array). A
high-resolution graphics standard introduced by IBM in 1990. XGA was
designed to replace the older 8514/A video standard. It provides the
same resolutions (640 by 480 or 1024 by 768 pixels), but supports
more simultaneous colors (65,000 compared to 256 colors).
In addition, XGA allows monitors to be noninterlaced.
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