Understanding Display Types
There are three major types of displays you should be knowledgeable of:
- CRT monitors
- LCD monitors
- Data projectors
The following sections help you understand the common and unique features of each.
Cathode ray tube (CRT) displays are now fading in popularity but are still in widespread use on older systems. CRTs use a picture tube that is similar to the picture tube in a tube-based TV set. The narrow end of the tube contains an electron gun that projects three electron beams (red, blue, green) toward the wide end, which is coated with phosphors that glow when they are hit by the electron beams. Just before the phosphor coating, a metal plate called a shadow mask is used to divide the image created by the electron guns into red, green, and blue pixels or stripes that form the image. Shadow masks use one of three technologies:
- A phosphor triad (a group of three phosphors—red, green, and blue). The distance between each triad is called the dot pitch.
- An aperture grill, which uses vertical red, green, and blue phosphor strips. The distance between each group is called the stripe pitch.
- A slotted mask, which uses small blocks of red, green, and blue phosphor strips. The distance between each horizontal group is also called stripe pitch.
If you look closely at a CRT display, you can see the individual triads or strips. However, from normal viewing distances, they blend into a clear picture.
Figure 3-17 shows the design of a typical CRT monitor.
Figure 3-17 A cutaway of a typical CRT display.
Generally, the smaller the dot or stripe pitch, the clearer and sharper the onscreen image will be. Typical standards for CRT monitors call for a dot pitch of .28 millimeters (mm) or smaller. Generally, low-cost monitors have poorer picture quality than higher-cost monitors of the same size because of wider dot pitch, low refresh rates at their highest resolutions, and poor focus at their highest resolutions.
Typical CRT displays range in size from 15 inches (diagonal measure) to 19 inches, and feature support for a wide range of resolutions. CRTs are analog display devices that can display an unlimited range of colors, and use the 15-pin VGA connector. To learn more about VGA connectors, see the section "VGA," later in this chapter.
Liquid crystal displays (LCD) use liquid crystal cells to polarize light passing through the display to create the image shown on the monitor. In color LCD displays, liquid crystal cells are grouped into three cells for each pixel: one each for red, green, and blue light.
All LCD displays use active matrix technology, which uses a transistor to control each cell, as the basic technology. Variations in how quickly a display can refresh, how wide the viewing angle, and how bright the display help distinguish different brands and models from each other.
An LCD monitor is a digital design, but many models, particularly low-end models and older designs, use the same VGA analog interface as CRTs. In such cases, the monitor must include an analog-digital converter to change the analog signal received by the VGA cable into a digital signal. High-end LCD displays and most recent midrange models also support digital signals and use DVI-D ports. To learn more about DVI-D connectors, see the section "DVI," later in this chapter.
Compared to CRT monitors, LCDs are much lighter, require much less power, emit less heat, and use much less desk space.
An LCD display has only one native resolution; it must scale lower resolutions to fit the panel, or, depending upon the options configured in the video card driver, might use only a portion of the display when a lower resolution is selected. When a lower resolution is scaled, the display is less sharp than when the native resolution is used.
LCD displays are found in both standard (4:3 or 1.33:1) and widescreen (16:9 or 16:10) aspect ratios, and range in size from 14 inches (diagonal measure) to 24 inches or larger.
Data projectors can be used in place of a primary display or can be used as a clone of the primary display to permit computer information and graphics to be displayed on a projection screen or a wall.
Data projectors use one of the following technologies:
- Liquid crystal display (LCD)
- Digital light processing (DLP)
LCD projectors use separate LCD panels for red, green, and blue light, and combine the separate images into a single RGB image for projection, using dichroic mirrors. A dichroic mirror reflects light in some wavelengths, while permitting light in other wavelengths to pass through. In Figure 3-18, red and blue dichroic mirrors are used to split the image into red, blue, and green wavelengths. After passing through the appropriate LCD, a dichroic combiner cube recombines the separate red, green, and blue images into a single RGB image for projection.
Figure 3-18 How a typical three-LCD data projector works.
LCD projectors use a relatively hot projection lamp, so LCD projectors include cooling fans that run both during projector operation and after the projector is turned off to cool down the lamp.
DLP projectors use a spinning wheel with red, green, and blue sections to add color data to light being reflected from an array of tiny mirrors known as a digital micromirror device (DMD). Each mirror corresponds to a pixel, and the mirrors reflect light toward or away from the projector optics. The spinning wheel might use only three segments (RGB), four segments (RGB+clear), or six segments (RGB+RGB). More segments help improve picture quality. Figure 3-19 illustrates how a DLP projector works.
Figure 3-19 How a typical DLP projector works.
Touch screen (or touchscreen) monitors enable the user to transfer data into the computer by pressing onscreen icons. Touch screen monitors are popular in public-access and point-of-sale installations.
Touch screen monitors use liquid crystal display (LCD) or cathode ray tube (CRT) technology and also incorporate one of the following surface treatments to make the monitor touch sensitive:
- Four-wire resistive technology—Uses a glass panel coated with multiple layers that conduct and resist electricity. A flexible polyester cover sheet fits over the glass panel and is separated from the panel with insulating separator dots. The outer side of the cover has a durable coating; the inner side has a conductive coating. When the cover is pressed, an electrical signal is generated and is sent through the interface to the computer. The lowest-cost touch screen technology, this type of screen is designed for public use.
- Five-wire resistive technology—A more sensitive and more accurate version of four-wire resistive technology suitable for use by trained personnel (offices, point-of-sale, and so on).
- Surface wave—Uses horizontal and vertical piezoelectric transducers to create ultrasonic waves. Touching the screen overlay disrupts the waves and the coordinates of the touch determine what signal is sent to the computer. It's a durable surface able to compensate for surface damage and dirt and is suitable for self-service applications such as banking or information kiosks.
- Touch-on-tube—Combines surface wave technology with direct touch contact to the CRT; no overlay is necessary. LCDs use an overlay with a simple air gap between the overlay and the panel surface. Suitable for self-service applications.
- Scanning infrared—A light grid created by infrared (IR) signals is used to sense touches. Works with plasma as well as other types of displays.
Touch screens are available in freestanding versions similar to normal desktop CRT and LCD displays as well as in kiosk and built-in designs.
Touch screens, like ordinary LCD and CRT monitors, use standard VGA analog or DVI digital interfaces to the video card. However, the touch signals are transmitted to the computer through a separate interface known as the touch screen controller. Touch screen controllers can use either of the following interfaces:
- Serial (RS-232)—Some touch screen monitors have an internal serial controller; others use an external serial controller. The internal serial controller might use a standard 9-pin serial cable or a special PS/2–to–9-pin serial cable to connect the controller to a serial (COM) port on the computer, depending upon the monitor model. The external serial controller uses a controller with a built-in serial cable.
- USB—A touch screen monitor with an internal USB interface uses a standard USB cable to connect to a USB port on the computer.