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LCD Technology
- What is a Pixel?
- What is Dot Pitch or Pixel Pitch?
- How does Touch Screen Technology work?
- What is a NIT?
- How do I know how many NIT's I require for my application?
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What is considered a true sunlight readable or outdoor readable LCD?
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What is Luminance?
- What is Contrast Ratio (CR)?
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What is a Viewing Angle and why does it matter?
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Are there thermal management issues with high bright LCD's?
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Display Modes: An overview of screen resolutions
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A clear understanding of today's video signals
Back to the FAQ Main Page
What is a Pixel?
The pixel (a word invented from "picture element") is the basic
unit of programmable color on a computer display or in a computer
image. Think of it as a logical - rather than a physical - unit.
The physical size of a pixel depends on how you've set the
resolution for the display screen. If you've set the display to
its maximum resolution, the physical size of a pixel will equal
the physical size of the dot pitch (let's just call it the dot
size) of the display. If, however, you've set the resolution to
something less than the maximum resolution, a pixel will be larger
than the physical size of the screen's dot (that is, a pixel will
use more than one dot).
What is Dot Pitch or Pixel Pitch?
The dot pitch specification for a display monitor tells you how
sharp the displayed image can be. The dot pitch is measured in
millimeters (mm) and a smaller number means a sharper image. In
desk top monitors, common dot pitches are .31mm, .28mm, .27mm,
.26mm, and .25mm. Personal computer users will usually want a
.28mm or finer. Some large monitors for presentation use may
have a larger dot pitch (.48mm, for example). Think of the dot
specified by the dot pitch as the smallest physical visual
component on the display. A pixel is the smallest programmable
visual element and maps to the dot if the display is set to its
highest resolution. When set to lower resolutions, a pixel
encompasses multiple dots.
Selecting a Touch Screen Technology
The four most common touch screen technologies include resistive, infrared, capacitive and SAW (surface acoustic wave). Each technology offers its own unique advantages and disadvantages as described below. Resistive and capacitive touch screen technologies are the most popular for industrial applications. They are both very reliable. If the application requires that operators can wear gloves when using the touch screen, then we generally recommend the resistive technology (capacitive doesn't support). Otherwise the capacitive technology (better optical characteristics) is more often recommended.
Resistive
A resistive touch screen typically uses a display overlay consisting of layers, each with a conductive coating on the inner surface. The conductive inner layers are separated by special separator dots, evenly distributed across the active area. Finger pressure causes internal electrical contact at the point of touch, supplying the electronic interface (touch screen controller) with vertical and horizontal analog voltages for digitization. For CRT applications, resistive touch screens are generally spherical (curved) to match the CRT and minimize parallax. The nature of the material used for curved (spherical) applications limits light throughput such that two options are offered: Polished (clear) or antiglare. The polished choice offers clarity but includes some glare. The antiglare choice will minimize glare, but will also slightly diffuse the light throughput (image). Either choice will demonstrate either more glare (polished) or more light diffusion (antiglare) than associated with typical non-touch screen displays. Despite the tradeoffs, the resistive touch screen technology remains a popular choice, often because it can be operated while wearing gloves (unlike capacitive technology). Note that resistive touch screen materials used for flat panel touch screens are different and demonstrate much better optical clarity (even with antiglare). The resistive technology is far more common for flat panel applications.
Capacitive
A capacitive touch screen includes an overlay made of glass with a coating of capacitive (charge storing) material deposited electrically over its surface. Oscillator circuits located at corners of the glass overlay will each measure the capacitance of a person touching the overlay. Each oscillator will vary in frequency according to where a person touches the overlay. A touch screen controller measures the frequency changes to determine the X and Y coordinates of the touch. Because the capacitive coating is even harder than the glass it is applied to, it is very resistant to scratches from (SIC) sharp objects. It can even resist damage from sparks. A capacitive touch screen cannot be activated while wearing most types of gloves (non-conductive).
Infrared
An infrared touch screen surrounds the face of the display with a bezel of light emitting-diodes (LEDs) and diametrically opposing phototransistor detectors. The controller circuitry directs a sequence of pulses to the LED's, scanning the screen with an invisible lattice of infrared light beams just in front of the surface. The controller circuitry then detects input at the location where the light beams become obstructed by any solid object. The infrared frame housing the transmitters can impose design constraints on operator interface products.
SAW (Surface Acoustic Wave)
A SAW touch screen uses a solid glass display overlay for the touch sensor. Two surface acoustic (sound) waves, inaudible to the human ear, are transmitted across the surface of the glass sensor, one for vertical detection and one for horizontal detection. Each wave is spread across the screen by bouncing off reflector arrays along the edges of the overlay. Two receivers detect the waves, one for each axis. Since the velocity of the acoustic wave through glass is known and the size of the overlay is fixed, the arrival time of the waves at the respective receivers is known. When the user touches the glass surface, the water content of the user's finger absorbs some of the energy of the acoustic wave, weakening it. The controller circuitry measures the time at which the received amplitude dips to determine the X and Y coordinates of the touch location. In addition to the X and Y coordinates, SAW technology can also provide Z axis (depth) information. The harder the user presses against the screen, the more energy the finger will absorb, and the greater will be the dip in signal strength. The signal strength is then measured by the controller to provide the Z axis information. Today, few software applications are designed to make use of this feature.
Touch Screen Controllers
Most manufacturers offer two controller configurations--ISA Bus and Serial-RS232. ISA bus controllers are contained on a standard printed circuit plug-in board and can only be used on ISA or EISA PCs. Depending on the manufacturer they may be interrupt driven, polled or be configured as another serial port. Serial controllers are contained on a small printed circuit board and are usually mounted in the video monitor cabinet. They are then cabled to a standard RS232 serial port on the host computer.
Software
Most touch screen manufacturers offer some level of software support which include mouse emulators, software drivers, screen generators and development tools for Windows, OS/2, Macintosh and DOS. Most of the supervisory control and data acquisition (SCADA) software packages now available contain support for one or more touch technologies.
What is a NIT?
A NIT is a measurement of light in candelas per meter square (Cd/m2)
For an LCD monitor it is brightness out of the front panel of the
display. A NIT is a good basic reference when comparing brightness
from monitor to monitor. Most desktop LCD's or Notebook LCD's have
a brightness of 200 to 250 Nits. These standard LCD's are not
readable in direct or even indirect sunlight as they become washed
out.
How do I know how many NIT's I require for my application?
Applications will vary depending on the location of the LCD and
how much ambient light is available that could cause the display
to become washed out or unreadable. As a rule of thumb; notebooks
and desktop LCD's which are generally used in office light
conditions are in the 200-250 nit range. For indoor use with
uncontrolled or indirect sunlight it is recommended that a display
of 500 - 900 nits be used. If the application is outdoors or in
direct sunlight then at least 1000 nits and up should be
considered.
What is considered a true sunlight readable or outdoor readable
LCD?
First, the display screen on a sunlight readable/outdoor readable
LCD should be bright enough so that the display is visible in
direct or strong sunlight. Second, the display contrast ratio must
be maintained at 5 to 1 or higher.
Although a display with less than 500 nits screen brightness and a
mere 2 to 1 contrast ratio can be read in outdoor environments,
the quality of the display will be dreadfully poor and not get the
desired information across effectivley. A true sunlight readable
display is normally considered to be an LCD with at least 1000
nits of screen brightness and a contrast ratio greater than 5 to
1. In outdoor environments under the shade, such a display can
provide an excellent image quality.
What is Luminance?
Luminance is the scientific term for "Photopic Brightness" which
specifies the visual brightness of an object. In layman's terms,
it is commonly referred to as "brightness". Luminance is specified
in candelas per square meter (Cd/m2) or nits. In the US, the
British unit Foot-lamberts (fL) is also frequently used. To
convert from fL to nits, multiply the number in fL by 3.426 (i.e.
1 fL = 3.426 nits).
Luminance is an influential factor of perceived picture quality in
an LCD. The importance of luminance is enhanced by the fact that
humans will react more positively to a brightly illuminated
screen. In indoor environments, a standard active-matrix LCD with
a screen luminance of around 250 nits will look good. In the same
scenario an LCD with a luminance of 1,000 nits or more will look
utterly captivating.
What is Contrast Ratio (CR)?
Contrast ratio (CR) is the ratio of luminance between the
brightest "white" and the darkest "black" that can be produced on
a display. CR is another influence of perceived picture quality.
If a picture has high CR, you will consider it to be sharper and
crisper than a picture with lower CR. For example, a typical
newspaper picture has a CR of about 5 to 7, whereas a high quality
magazine picture has a CR that is greater than 15. Therefore, the
magazine picture will look better even if the resolution is the
same as that of the newspaper picture.
A typical AMLCD exhibits a CR of approximately 300 to 700 when
measured in a dark room. The CR on the same unit measured under
ambient illumination is drastically lowered due to surface
reflection (glare). For example, a standard 200 nit LCD measured
in a dark room has a 300 CR, but will have less than a 2.0 CR
under intense direct sunlight. This is due to the fact that
surface glare increases the luminance by over 200 nits both on the
"white" and the "black" that are produced on the display screen.
The result is the luminance of the white is slightly over 400
nits, and the luminance of the black is over 200 nits. The CR
ratio then becomes less than 2 and the picture quality is
drastically reduce and not acceptable.
What is a Viewing Angle and why does it matter?
The viewing angle is the angle at which the image quality of an
LCD degrades and becomes unacceptable for the intended
application. Viewing angles are usually quoted in horizontal and
vertical degrees with importance dependent on the specific
application. As the observer physically moves to the sides of the
LCD, the images will degrade in three ways. First, the luminance
drops. Second, the contrast ratio usually drops off at large
angles. Third, the colors may shift. Most modern LCD's have
acceptable viewing angles even for viewing from the sides.
For LCD's used in outdoor applications, defining the viewing angle
based on CR alone is not adequate. Under very bright ambient light
conditions the display is hardly visible when the screen luminance
drops below 200 nits. Therefore, the viewing angles are defined
based on both the CR and the Luminance.
Are there thermal management issues with high bright LCD's?
Yes -any high brightness backlight system will consume a
significant amount of power, thereby increasing the LCD
temperature. The brighter the backlight, the greater the thermal
issue. As well, if the LCD is used under direct sunlight
additional heat will be generated as a result of sunlight
exposure. Temperature issues have been handled through proper
thermal management design incorporating passive and active cooling
methods. This is extremely important in maintaining overall
reliability and long-term operation.
Display Modes: An overview of screen resolutions
 The term display mode or the more commonly used screen resolution
refers to the characteristics of a computer display. Screen real estate is
usually measured in pixels. In particular, the maximum number of colors
and the image resolution in pixels measured horizontally and vertically.
There are several display modes that are used today from a small amount of
data up to extremely large amounts that are jam-packed into the display
area.
A Brief History
The earliest displays for personal computers were monochrome monitors that
were used in text-based computer systems in the 1970s. In 1981, IBM
introduced the Color Graphics Adapter (CGA). This display system was
capable of rendering four colors, and had a maximum resolution of 320
pixels horizontally by 200 pixels vertically. While CGA was ok for simple
tasks it certainly could not display adequate graphics.
In 1984, IBM introduced the Enhanced Graphics Adapter (EGA).
It allowed up to 16 different colors and offered resolution of up to 640 x
350. This improved the appearance over earlier displays, and made it
possible to read text easily. Nevertheless, EGA did not offer sufficient
image resolution for use in graphic design either.
In 1987, IBM introduced the Video Graphics Array (VGA) display system.
This has become the accepted minimum standard for PCs. The VGA standard is
still used today in some applications.
In 1990, IBM introduced the Extended Graphics Array (XGA) display as a
successor to its 8514/A display. A later version, XGA-2 offered 800 x 600
pixel resolution in true color (16 million colors) and 1024 x 768
resolution in 65,536 colors. These two image resolution levels are perhaps
the most popular in use even today.
Recently, new display technology has given the ability to display vast
amounts of pixels into a given area. The table shows display modes and the
resolution levels (in pixels horizontally by pixels vertically) that are
commonly found today.
The 4 x 3 settings are most common with standard PC type monitors whether
they are LCD or CRT's. In recent times larger displays have become
available in the letterbox or landscape style. These are typically used
for multimedia and home theatre applications. The letterbox style displays
usually operate efficiently at a 16:9 aspect ratio. This therefore changes
the overall resolution of the display as noted in the table.
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Industry
Standard 4 x 3 Screen Resolutions |
| CGA |
Color
Graphics Adaptor |
320 x 200 |
| EGA |
Enhanced
Graphics Adaptor |
640 x 350 |
| VGA |
Video
Graphics Array |
640 x 480 |
| SVGA |
Super
Video Graphics Array |
800 x 600 |
| XGA |
Extended
Graphics Array |
1024 x 768 |
| SXGA |
Super
Extended Graphics Array |
1280 x 1024 |
| SXGA+ |
Super
Extended Graphics Array |
1400 x 1050 |
| UXGA |
Ultra
Extended Graphics Array |
1600 x 1200 |
| QXGA |
Quad
Extended Graphics Array |
2048 x 1536 |
| QSXGA |
Quad
Super Extended Graphics Array |
2560 x 2048 |
| QUXGA |
Quad
Ultra Extended Graphics Array |
3200 x 2400 |
|
Industry
Standard 16 x 9 Screen Resolutions |
| WXGA |
Wide
Extended Graphics Array |
1366 x 768 |
| WSXGA |
Wide
Super Extended Graphics Array |
1600 x 1024 |
| WSXGA+ |
Wide
Super Extended Graphics Array |
1680 x 1050 |
| WUXGA |
Wide
Ultra Extended Graphics Array |
1920 x 1200 |
| WQSXGA |
Wide
Quad Super Extended Graphics Array |
3200 x 2048 |
| WQUXGA |
Wide
Quad Ultra Extended Graphics Array |
3840 x 2400 |
A clear understanding of today's video signals
Understanding the differences between Composite Video, S-Video and
Component Video
With the growth of home theatre, video cameras and the consumer
electronics market many of today's computers and peripherals (especially
LCDs) have multiple video input options available to them. Here is a
clearer picture of what these signals represent.
 Composite video, also referred to as baseband or RCA video, is
the most common of all video signals.
A composite video signal consists of an analog waveform that conveys the
image data in a conventional National Television Standards Committee
(NTSC) television signal. Composite video contains chrominance
(hue and saturation) and luminance (brightness) information, along with
synchronization
and blanking pulses, all together in
a single signal.
Composite video is the standard that connects almost all consumer video
equipment through a phono-jack, also known as an RCA connector. In
composite video, interference between the chrominance and luminance
information is inevitable resulting in poor quality video when signals
are weak. The cable has 3 jacks: yellow, white, and red. One jack sends
the audio (left), the second the stereo (right), and the third the
video, respectively.
The picture quality is decent but pales in
comparison to S-Video
 S-Video (Super-Video, Super-VHS) and sometimes referred to as Y/C
Video was introduced in the 1980s and solved some of the problems that
were inherent with composite video. S-Video provides better color
separation and a much cleaner signal by keeping the transmitted
luminance and chrominance video signals separated.
Today, S-Video signals are generally connected using 4-pin mini-DIN
connectors using a 75 ohm termination impedance. S-Video provides for a
high quality method of delivering a clean crisp video signal.
 Component video improves the picture quality even more than
S-Video. Component refers to video transmitted as three separate signals
(subsignals if you prefer) to represent all colors. The first component
video was RGB since the three signals represented pure red, pure green,
and pure blue content respectively. Today, most video experts use the
term "component video" as short for "analog component video" consisting
of the three signals Y (luminance), Pr or R-Y, and Pb or B-Y. For NTSC
or PAL (interlaced video formats) the Y signal is the same as that used
to construct composite video or that found in S-Video.
The most common connection from DVD players is three RCA-type jacks.
(For a very technical explanation of color television and component video,
see
Tektronix's Web site.)
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