Understanding Flat Panel Technology

• What are LCD monitors and how do they work?
• How to choose a flat panel monitor: what features determine quality?
• What's the difference between resolution and size?
• What is the difference between CRT and LCD size measurements?
• How does the total cost of ownership (TCO) compare to an equivalently sized CRT monitor (19 or 21-inch)?
• What is the difference between active matrix LCDs (AMLCD) and passive matrix LCDs?
• What is 24-bit color?
• What are pixel defects? Is my display defective?

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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