Electronic Information Display Technologies

The following sections are included:

• PREFACE

• CONTENTS

• ACKNOWLEDGEMENTS

This book discusses the present condition and future prospects of electronic display technologies for two broad application areas: entertainment and professional uses. Entertainment requires displays capable of good-quality, full-color, full-motion, video images in a typical home environment with at least the resolution of today's television (about 480 scan lines). Price is important in entertainment-display purchasing decisions. Professional displays are primarily used in the business environment for applications usually related to computers. The sale of these displays can be less price sensitive than entertainment displays, and users are often willing to pay a premium and even to sacrifice in some areas of display quality in order to optimize in other areas.

We also consider the potential of various display technologies to reach the size, resolution, and brightness levels required for high-definition television (HDTV), where the number of scan lines is approximately double that of standard NTSC television. Such high-definition displays may provide a future where entertainment and professional displays overlap, as the ability to display both video and high-quality graphics will be present on one screen. Finally, these categories obviously do not encompass all the displays that are in use today. Those that are not included in these categories, which we will not discuss in detail, include small-scale alphanumeric displays such as those used on calculators, watches and copiers, and extremely large displays such as those used in stadiums and arenas.

In this chapter, we give an overview and summary of the contents of the book.

Before beginning to describe individual display technologies, we will first try to quantify the importance of displays. We will then describe what we feel are the most important trends in display manufacturing, research and development. While we believe that flat-panel displays will eventually replace small-to medium-sized CRTs in applications where space is at a premium, such as in bedrooms and kitchens, on portable computers, and maybe even on the desktop, the display that hangs on the wall like a master's painting is still a somewhat futuristic dream.^c CRTs will probably continue to be used most often for entertainment, and larger displays are likely to be optical projection systems for the next 5 or 10 years. Active-matrix liquid-crystal devices look like strong contenders for both direct view up to about 25 inches in diagonal and for projection applications. Sadly, US involvement in manufacturing these products is likely to be extremely limited because most of the domestic display development programs were terminated in the 1980s.

This chapter discusses the technical requirements that displays must generally meet. Later assessments of display technologies are based on how well they meet these requirements. Put simply, an excellent electronic display must produce a visually effective image, be large enough to meet the requirements of the application, and remain within the context of a practical electronic system. By a practical system, we mean one that is electronically feasible without dissipating too much power.

While the following chapters will provide more detailed discussions of most of the main display technologies, this chapter will briefly mention a few types of displays that are not likely to make great inroads in personal computing or entertainment applications. We have supplemented the limited information in this chapter with appropriate references, in case readers have other applications in mind and want to follow up on these display technologies elsewhere.

Cathode-ray tube technology dominates most display applications. Available CRTs range from 1-inch diagonal units in video-camera viewfinders to recently-developed 35-inch models for large-screen television, and even larger tubes have been made. CRT technology also continues to improve. In a very thoughtful article, Peter Brody asked "how is it that solid-state components have replaced vacuum devices across the whole spectrum of information processing, but have failed to replace the CRT?"³⁸ Part of the answer is that while very significant reductions in size and power were realized by switching from vacuum tubes to transistors for processing and memory, direct-view displays are unshrinkable in two dimensions. And we cannot expect much reduction in power because CRT phosphors are highly evolved and quite efficient.

Another key to the CRT's dominance, according to Brody, is the central problem of the interconnection or signal distribution that possible replacement technologies must face. The CRT has a single serial data input plus a handful of control inputs that actually are merged into one signal in the case of television.

The CRT's principal disadvantage is its bulky glass envelope, which must have enough strength to prevent the weight of the atmosphere outside from crushing the tube and filling the vacuum inside. Without using optical projection schemes, which are discussed in Chap. 9, the CRT's depth and weight become unsupportable at dimensions less than the full size (, 73-inch diagonal) of interest for HDTV. Moreover, large CRTs tend to be dim because the electron beam is shared over too large an area. A CRT's screen brightness also depends on the efficiency with which the phosphors on the screen convert electron energy to light. In the following subsections, we will review these and other technical issues and refinements of CRT technology and discuss its ultimate limitations.

In the following sections, we will discuss the leading emissive flat-panel technologies. Elegant designs and the use of unique combinations of material properties have led to improved plasma and electroluminescent displays (EL). This has gone on for many years, and AC-plasma and AC-EL displays continue to be preferred in specific applications. Furthermore, color plasma displays currently offer the best hope for the large wall-screen display of the future. In addition, the growing production of liquid-crystal displays for portable computers and video products emphasizes the related issue of backlights. Backlights have interesting optical issues of their own, but they generate light in ways similar to emissive displays. For these reasons, we have included a section on backlights to conclude this chapter.

Liquid crystals are fluids that exhibit a part of the order that is characteristic of crystalline solids. Liquid-crystal displays (LCDs) exist because liquid crystals frequently possess both dielectric and optical anisotropies. The dielectric anisotropy allows an electric field to control the orientation of the liquid-crystal molecules and the optical anisotropy allows the orientation of the molecules to control the polarization of light passing through the material. LCDs can operate on very little power, because they only modulate light rather than emitting it.

Multiplexed LCDs are simple devices containing linear arrays of transparent electrodes on opposing sheets of glass. The configuration of the molecules in an LCD is partly controlled by alignment layers, which are thin polymer coatings that have been applied to the two substrates and then rubbed in specific directions. Various liquid-crystal configurations are created in this way. A particular type known as supertwisted LCDs can provide good contrast and resolution. However, they have little gray-scale capability and are too slow for video. They even have trouble keeping up with pointing devices in computer applications. However, a new addressing technique for supertwist displays promises sufficiently quick response and some gray scale. Displays based on ferroelectric liquid crystals are currently in the research stage. FLCDs have quick response and are capable of high resolution, but can be difficult to make and also have problems with gray scale.

In the following section, we will introduce the basics of nematic liquid-crystal displays. We then go on to discuss the most prominent types of liquid-crystal displays in subsequent sections.

Many more rows of pixels can be multiplexed when active components are integrated into a liquid-crystal display, although this may be the least elegant way to make a display. An active matrix comprises a transistor, or a network of diodes, in series with each pixel. These elements are incorporated primarily to create a sharp threshold to improve the multiplexibility of the display. The active matrix also prevents charge from leaking away from pixels until their rows are selected again, making it possible to display a gray-scale range by varying the voltage at each pixel. Active elements require complex processing steps that raise the cost and lower the yield of acceptable panels. By early 1994, however, no other way to make a fully-functional flat display had made it into production.

Scaling these displays to larger sizes is problematic. High yields must be achieved before AMLCDs can be produced at larger screen sizes. Also, producing large screens will require the use of production equipment that did not yet exist in the early 1990s and would not be useful outside this industry. At very large sizes, it may be difficult to drive the panel fast enough, because the electrodes must be kept very narrow to allow the light to pass through, and narrow electrodes have high resistances.

Active-matrix elements and opaque conductors also block off a substantial fraction of the pixel area. The active area, expressed as a percentage of the total area per pixel, is called the aperture ratio. There is a trade-off between speed and aperture ratio, and it is a serious problem because the transmission of twisted-nematic liquid-crystal displays is also reduced by the use of color filters and polarizers.

Despite the problems mentioned above, several groups have demonstrated impressive examples of this technology. In the following section, we will discuss the details of the various active-matrix technologies and their relative merits. For discussion, we will divide active-matrix displays into three separate types. By custom, diode arrays are included in the "active-matrix" category, and form our first category. Our second category includes two kinds of thin-film transistors used in AMLCDs: amorphous silicon, which can be processed on low-cost substrates, and polysilicon, which allows high-speed drive circuits to be fabricated right on the glass. We put all other nonlinear conduction devices in a third category. The newest of these is a plasma device that is closely coupled to the LCD.

High-definition images require larger screens to be fully appreciated. Until large, flat-panel, video displays become available, optical projection remains the most practical alternative to direct-view CRTs for large screen sizes. Actually, CRTs are projection systems too, but the glass envelope surrounding the vacuum can become too heavy and expensive. Optical projection systems have the further advantage that the optical path can be folded with the aid of mirrors. As a result, largest direct-view CRTs are deeper than rear-projection systems of the same screen size.

Projection displays generally use a lens to focus a magnified image of an internal display device onto a projection screen. The projection unit can be either behind the screen or in front of it. Front projection units, like slide projectors, use empty room space to expand the image and project on to a flat screen on the wall. Rear-projection units typically use special screens that minimize the effect of room light, and they also have "gain", or the ability to redirect most of the projected light into places where it is most likely to be used.

Another distinction in projection systems depends on how the image and light are generated. "Light-valve" projection systems use a fixed light source and a separate modulating element to form the image. Light-valve projectors can have high brightness and contrast because the light source and the modulator can be separately optimized. CRT projectors, on the other hand, combine the light-generating and image-forming elements. CRT-based optical projectors use established technology and can have good brightness and efficiency by optically merging the images from three monochrome CRTs that separately generate the red, green and blue images.

How long can CRTs continue to dominate the display market? This question has been asked for as long as flat-panel displays have been under development, and the answer still depends on their prospects for improvement. It would be easier if there were just one flat-panel display technology to assess, but in fact there are several of them that differ from one another in fundamental ways. To help dispel the confusion caused by this variety, we offer here a subjective rating system designed to estimate the relative suitability of the technologies in a few broad areas of applications.

This rating system provides a structure for assessing new display technologies, and we consider two examples of current high interest: field-emission displays and active-addressed LCDs. We will also emphasize an additional trend that may eventually bring about significant change in displays. Solid-state light sources have been improving steadily for some time and are reaching respectable efficiencies. If this trend continues, particularly if an efficient blue LED or a high-power visible diode laser should be developed, new categories of display technologies would become feasible.

The following sections are included:

• APPENDIX A ABBREVIATIONS

• INDEX