Frequently Asked Questions
About LCD Displays

1. What is a Pixel?
2. What is Dot Pitch?
3. How does Touch Screen Technology work?
4. What is a NIT?
5. How do I know how many NIT's I require for my application?
6. What is considered a true sunlight readable or outdoor readable LCD?
7. What is Luminance?
8. What is Contrast Ratio (CR)?
9. What is a Viewing Angle and why does it matter?
10. Are there thermal management issues with high bright LCD's?
11. Display Modes: An overview of screen resolutions
12. A clear understanding of today's video signals

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1. What is a Pixel?

Often referred to as dot, as in "dots per inch", "Pixel" is short for picture elements, which make up an image, similar to grains in a photograph or dots in a half-tone. Each pixel can represent a number of different shades or colors, depending on how much storage space is allocated for it. Pixels per inch (ppi) are sometimes the preferred term, as it more accurately describes the digital image. The actual physical size of a pixel depends on the resolution set for the display screen. If your display is set for the maximum resolution, the physical size of the pixel is equal to the dot pitch of the display. If your display is set to something less than the maximum resolution, then a pixel will be larger than the actual size of the screen dot, i.e. a pixel will use more than one dot.

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2. What is Dot Pitch?

The dot pitch, also known as the pixel pitch is the distance between a red (or green or blue) dot and the closest red (or green or blue) dot on a color monitor or LCD screen. On desktop monitors, the dot pitch is typically from 0.25 to 0.31mm, while large presentation monitors may go up to 1.0mm. On LCD monitors, the dot pitch is typically from 0.16 to 0.29mm. In general, the smaller the dot pitch, the crisper the image. A 0.28 dot pitch means dots are 28/100ths of a millimeter apart. A dot pitch of 0.31 or less provides a sharp image, especially on text.

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3. How does touchscreen technology work?

Selecting a Touch Screen Technology

The three most common touch screen technologies include resistive, 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 resistive is the recommended technology as capacitive doesn't support gloves. If operation with gloves is not required, then capacitive technology is the preferred choice due to better optical characteristics.

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. 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).

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.

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4. 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.

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5. 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.

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6. 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 effectively. 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.

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7. What is Luminance?

Luminance is the term which specifies the visual brightness of an object. It is another term for NIT. Like NIT, it is measured in candelas per square meter (Cd/m2) or 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.

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8. 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.

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9. 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.

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10. 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.

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11. 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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12. 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.

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