Display Devices | Research & Encyclopedia Articles

The first computer display devices were modified typewriters and Teletype machines. These devices were slow, noisy, and expensive, and could only handle text. For graphic output, an X-Y plotter, a device that pulled a penover a piece of paper, was used. It shared all the problems of the Teletype machine.

It did not take long before these mechanical machines were replaced with electronic counterparts. The replacement was called a terminal. It consisted of a typewriter-like keyboard, which activated switches, and a display screen, which was a modified television receiver. Thus, the first computer display device was a cathode ray tube or CRT.

A cathode ray tube paints an image on a phosphor screen using a beam of electrons. The concept of the CRT was formulated before the nature of the electron was known. Cathode rays are not rays at all but high-speed streams of particles called electrons. The CRT is a vacuum tube where the processing of electrons takes place in an evacuated glass envelope. If the electron beam passed through air or another gas, the electrons would collide with the molecules of that gas, making it difficult to manipulate the electron beam.

The CRT generates a source of electrons from an electron gun. The electrons are accelerated in a straight line to a very high velocity using a high voltage and then deflected from the straight line using a magnetic field. The beam can be turned on or off with an electrical signal. The screen of the CRT is coated with a phosphorus compound, which gives off light energy when it is struck with high-speed electrons. When the beam hits the face of the CRT, a spot of light results. The beam is scanned from left to right and from top to bottom. The beam is turned on when a light area is to be generated and it is turned off when an area is to be dark. This scanning is clearly visible on both computer monitors and television screens.

There are two basic methods of scanning a picture. The first is called "progressive." Every line is scanned beginning at the top left corner and finishing at the lower right. Another method is called "interlace." The picture is scanned twice with half of the lines being scanned each time. This is done to refresh the picture at twice the actual scan rate and reduce the flicker of the display.

The first CRTs were one color, also called monochrome; the display color was usually green. The first display devices were designed to replace a mechanical printer, so a single color was sufficient. However, the CRT is not limited to only printing text but is capable of producing images and complex graphics where full color is highly desirable. Again, technology was adapted from the television industry, and color monitors were quick to follow the early monochrome versions.

The color CRT is effectively three monochrome CRTs, all in the same glass envelope. There are three electron guns but each gun is individually controlled. The three electron beams are deflected together and strike the face of the CRT. The electron beams must pass through a screen with hundreds of thousands of small holes, called a shadow mask, before striking the phosphor on the front of the CRT. The holes are arranged in groups of three so that when one electron beam passes through the hole, it strikesa small dot of phosphor that gives off red light. Another beam strikes only a dot of phosphor that gives off green light. The third beam falls on a phosphor dot that gives off blue light.

The mechanics of the color CRT are such that each of the three electron beams produces scanning beams of only one color. These three colors—red, green, and blue, or RGB—are the three additive primary colors. Any color may be generated by using a combination of these three. This shadow mask technology was the first to be used for color television and is still used in most CRTs.

Over the years the shadow mask CRT has been refined. Modern tubes have a nearly flat face, have much improved color, and have very high resolution, which is the ability of a display to show very small detail. One improvement is a shadow mask using stripes rather than round holes. This arrangement is easier to align. These improvements are not only found in computer displays but television receivers as well.

Since its inception, the CRT has shown a number of disadvantages. First, the tube is large and heavy. CRT sizes are relative to the diagonal measurement and most CRT displays are deeper than their diagonal measurement. Secondly, the electrons in the tube are accelerated using high voltage. The larger the tube, the higher the accelerating voltage, which reaches to the tens of thousands of volts and requires a large power supply. The tube is made of glass, which is not suited for portable equipment or for applications with significant vibration such as aircraft. Finally, the tube requires significant power.

As hard as it is to believe today, the first "portable" computers actually used CRTs for display devices! In a word, these early portable computers were huge and would have remained so if a suitable replacement for the CRT had not been found. What was needed was a low power display device that had the capability of the CRT yet was small, not as fragile, and required low power and low voltage.

The liquid crystal display, or LCD, is a low voltage device. It requires very low power but it was not originally a graphics device or capable of color. The LCD is essentially a light gate, which can be opened to allow light to pass, or closed to shut light off. To use the LCD as a full color graphics display, the display is divided up into picture elements called pixels. Each pixel represents the color and intensity of a small part of the complete picture. Three LCD light gates are required for each pixel: one for red, green, and blue. Behind each gate is a color filter, which is illuminated by a white light source. Behind one LCD light gate is a red filter, behind another is a green filter, and the third a blue. By adjusting the amounts of the three primaries, as in the CRT, the correct intensity and color can be generated for each pixel.

LCD construction is simple. The liquid crystal material is sandwiched between two flat glass plates. Crystalline materials, which usually are not liquid, have very profound effects on light waves. The liquid crystal can affectthe manner in which light energy passes through the material, and this can be changed by the application of an electric field. A thin metal electrode is placed over the area where the LCD is to change from dark to light. The electrode is so thin that it is completely transparent and cannot be seen. An electric field is created when a voltage is applied to the electrodes on the LCD glass.

Most LCDs use the rotation of polarized light to change the intensity of the light. The light entering the pixel is plane polarized, meaning that the light waves are in one plane. This is done with a polarizer, which is the same technique used in sunglasses to reduce glare. A simple way to visualizea polarizer is to think about a venetian blind where the separation of the slats is so close that light waves can only pass through in one plane.

On the front of the LCD there is a second polarizer, which is oriented at a right angle to the first. If these two polarizers were placed together with nothing but vacuum or air between them, no light could pass through. This is because the light is polarized by the first polarizer and is incompatible with the second.

Liquid crystal material has the ability to overcome this by rotating the polarization of light waves, but only when an electric field is placed across the liquid crystal. Therefore, if a voltage is placed across the liquid crystal, the light is rotated by 90 degrees and will pass through the front polarizer. The application of a voltage can permit or shut off the light intensity.

In the color LCD display three "sub pixels" are required because the intensity of light from the three primaries must be independently controlled. If one pixel could provide both brightness and color, the LCD could be simplified. An improved LCD display uses a single light valve where the liquid crystal material generates both the color and brightness. This new LCD material is called cholesteric because it was originally derived from animal cholesterol.

The number of pixels into which an image is divided will directly affect the quality of the picture. As an example, a conventional television picture is generated with 525 scanning lines (the U.S. standard). Of these, only about 484 lines are visible. The aspect ratio of the television picture is 4:3, which means that the width of the picture is four-thirds the height. If the pixels were square, there would be 484 rows and 660 columns of pixels. Because of the interlace scan, the actual number of rows and columns is half of that, or 242 by 330.

When an image is generated with an insufficient number of pixels, the picture lacks resolution and the pixels are very evident. The individual lines of a television picture are clearly visible, particularly in a large screen television. Common computer displays have resolutions of 340 X 680, 680 X 1760, and so on. Computer monitors can have a better picture than some television receivers.

An improved television standard is set to replace the older 525 line system; this is called high definition television, or HDTV. In addition to the improved resolution or definition, the aspect ratio is 16:9, which is the same as motion pictures. Because HDTV is a digital system and optical disks are used to store video, the relationship between computer monitors and television receivers will grow closer over the years.

In the LCD display, each light gate has to be connected to electronic drivers, which activate or deactivate the gate. An LCD graphics display has a very large number of pixels, which poses a serious challenge in running conductors to each LCD light gate. Thin, transparent conductors can hardly be seen but the sheer number of them would make manufacturing LCD displays difficult, at best. One solution is a method of connecting the LCD segmentsby mounting electronic circuits right on the glass plate. This arrangement is called an "active matrix" and it significantly reduces the number of interconnects required. The transistors used for the active matrix are made from thin films that are so small they are virtually invisible. This is called a thin film transistor active matrix LCD, or TFTAM LCD or AMLCD.

Even though the AMLCD has simplified the LCD graphics display, a large number of light gates, transistors, and interconnections remain. In the manufacturing process, if one pixel fails, the display must be scrapped. In an LCD graphics display, the number of LCD light gates numbers more than one million. The chances are good that one of those LCD gates or the thin film transistors would be defective in the manufacturing process.

The percentage of good products from a factory production run is called the yield. A poor yield is reflected in a high price of a product. Increasing the yield of the LCD production was the major challenge to the LCD industry in producing a cost-effective display product. The cholesteric LCD can be made with one-third the number of pixels and therefore, one-third the number of LCD light gates. This means the cholesteric LCD will have three times the manufacturing yield, which makes the technology potentially much more cost effective than other options.

The AMLCD requires a white light source to operate. Some of the more common light sources are not suited for backlighting an LCD display. The incandescent lamp and LEDs are point sources of light whereas a distributed source is desired. These two sources are also not energy efficient, which is an important characteristic required for battery power.

For notebook computers, an electroluminescent panel is used. This device generates a low light level with good energy efficiency. The panel is thin and can be sandwiched easily behind the LCD and the display case.

Some portable devices such as small "palm" computers, cellular telephones, and watches must perform in bright sunlight. Displays that reflect, rather than emit, light are used in these devices. LCD displays are well suited to applications where the display operates in the "reflective" mode. When the ambient light is low, a backlight provides the necessary illumination. When backlighting is provided, the LCD is now operating in the "transmissive" mode. LCD displays that operate in both modes are called "transflective." As of the year 2001, transflective LCDs were not yet capable of providing full color.

If the light intensity falling on the front of a transmissive display is greater than the emitted light, the display contrast will be lost and the display will "wash out." Usually, displays are shielded from very bright light such as sunlight but in some applications this is not possible, such as an aircraft instrument panel. Displays used for these applications are called "sunlight readable." This means the display is visible in full sunlight. In these high brightness applications, a thin, serpentine, fluorescent lamp is used for backlighting. This technique provides a high light output but also generates considerable heat. Providing a very high level of backlighting for a color LCD display has become very common as the LCD is used for computer projectors.

The new cholesteric LCD material will also allow for an LCD display that operates with reflected light and will be completely sunlight readable. Improved resolution will result because the cholesteric LCD requires only one light gate per pixel.

The modern AMLCD display is one of the best display technologies but it still suffers from some weaknesses. The resolution of a good quality AMLCD is not as good as the better CRTs. The cost of AMLCDs, although dropping, is still higher than the equivalent CRT. The AMLCD, or LCD in general, is not well suited for use in harsh environments because it is negatively affected by low temperatures. The response time of an LCD display under these conditions is increased significantly. This would cause moving images to drag and blur. In very cold temperatures, such as those in which military equipment is often operated, the LCD will quit operating completely and could be damaged by the extreme conditions.

A new display technology in the later stages of development is called the field emission display, or FED. The FED uses an array of small, pointed electrodes mounted close to a dot of phosphor. Like the color CRT, the pointed electrode causes an emission of an electron beam, which excites the phosphor to emit light. Essentially, the FED is a flat CRT where the electron beam is not deflected. The FED has all the advantages of the CRT, including good resolution, bright display, full color capability, and sunlight readability, without the major disadvantages, such as low temperature problems. It is not yet clear what direction this new technology will take, but it is likely that FEDs will be used for aircraft instruments and other sunlight readable applications.

Albert D. Helfrick

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——. Video Display Engineering. New York: McGraw-Hill, 2000.

This complete Display Devices contains 2,639 words.