For many decades, cathode-ray tube (CRT)-based computer monitors were the only game in town. But things don't stay the same for ever; new display technologies are emerging all the time, and there are some mega-exciting possibilities for the future...

Liquid Crystal Displays (LCDs)
In the early 1990s, a number of different companies started experimenting with substances known as liquid crystals (LCs); eventually, liquid crystal displays (LCDs) became available to the market.

In their early incarnations, these displays were very expensive compared to CRT-based techniques; also, the picture quality wasn't as good and the response time of the individual pixels was slow enough that "ghosting" and blurring effects were seen on fast-moving objects in the images. These problems have now been solved, and sales of LCDs have risen dramatically over the last few years, to the extent that almost 90% of all new displays sold are LCD-based.

Interestingly enough, LCD technology has its roots in 1888, when an Austrian botanist called Friedrich Reinitzer (1857-1927) was studying cholesterol in plants. He ended up creating a material we now know as cholesteryl benzoate. This was a phase of matter of which we had never previously been aware, but which we now know as possessing a "liquid crystalline" structure.

One problem with LCDs is that there many different variations on the theme. For example, we now know of more than 50,000 compounds and mixtures that possess liquid crystalline properties. Thus, the following will be a generic (high-level) description intended only to give the "flavor" as to how this all hangs together. As usual, the easiest way of summarizing how these things work is by means of a high-level diagram as illustrated in Figure 1.

Figure 1. High-level representation of a single pixel in a liquid crystal display.

This illustration reflects a cutaway portion of the screen showing the tiny red, green, and blue filters forming a single pixel. Each of these filters has a bunch of liquid crystals associated with it, and each of these bunches has an associated transistor (these transistors are not shown here for reasons of simplicity).

At the back of the display is a back-light formed from some source of white light [often light-emitting diodes (LEDs) in the case of the smaller displays]. Next is a polarizing film (not shown in this illustration for simplicity), which we can think of as only passing light whose electromagnetic field is oscillating in an "up-down" manner. Then we have bunches of liquid crystals associated with each sub-pixel followed by a second polarizing film (again, this film is not shown here for simplicity – actually, in the case of the system shown in Figure 1, the transparent front screen would also typically act as the second polarizing film). This second film is rotated at 90° to the first; in this case we can think of it as only passing light whose electromagnetic field is oscillating in a "side-to-side" manner.

Now, have you ever played with the lenses from polarized sunglasses? If you place one lens in front of the other with both presented in an identical up-down/side-to-side orientation, then you can see through to the other side because both lenses are passing light that is polarized in the same direction. But if you rotate one of the lenses such that it is oriented 90° to the first, then everything goes black. This is because the first lens passes only the light that is polarized in the "up-down" plane (for example), while the second lens blocks this light; if given a chance, the second lens would pass light that is polarized in the "side-to-side" plane, but that light has already been blocked by the first lens.

Exactly the same thing would happen in our display if we were to remove all of the liquid crystals associated with our sub-pixels. In this case, the first filter would pass the backlight polarized in the up-down plane, but the second filter would block this light and our screen would appear to be black.

And so we return to the bunches of liquid crystals forming each of our sub-pixels. By default, the liquid crystals will arrange themselves into a tight ("twisted") helix pattern, but if we apply current to the crystals they will "untwist". Varying the amount of current will affect the amount of "twist".

The reason this is important is that the crystals effectively act as a variable polarizer. In their "untwisted" state, the polarized light from the first filter is passed through the crystals unmodified, only to be blocked by the second filter. As we increase the amount of "twist", the polarization of the light passed through the crystals is also "twisted", which allows it to pass through the second filter. Thus, by varying the amount of "twist" of the crystals associated with a particular sub-pixel, we can control the intensity of that sub-pixel.

The great advantage of LCDs over CRT-based displays is that they are very thin, very light, and very flat. Having said this, CRT-based displays still have an advantage in terms of the brightness, contrast, and "vibrancy" of the images that can be achieved. If only there were some other technologies...

Plasma Display Panels (PDPs)
You may have seen flat-panel plasma displays at television stores. These displays offer bright, crisp, high-contrast images. In this case, we can think of each pixel as being formed from three tiny fluorescent lights (like microscopic neon tubes). By one mechanism or another, these three tiny neon tubes can be coerced into generating red, green, and blue light, each of which can be controlled to form the final color coming out of that pixel.

Plasma displays are fantastic when it comes to presenting ever-moving images such as films. However, if they are instructed to present the same image over and over again, they suffer from "burn-in" effects that leave "ghost" images on the screen. This means that plasma-based technologies do not make an ideal display for computer applications (although there are always some folks who will try to do so).

Organic Light-Emitting Diodes (OLEDs)
These are devices that are formed from thin films of organic molecules that generate light when stimulated by electricity. OLED-based displays hold the promise of providing bright and crisp images while using significantly less power than liquid crystal displays. (See also my blog: Video Explains OLEDs Using a Pickle).

At some stage in the future, it may be possible to use OLEDs to create displays that are only a few millimeters thick and are two meters wide (or more); these displays would consume very little power compared to other technologies, and in some cases the display could be rolled up and stored away when it wasn't in use (OLEDs can be "printed" onto flexible plastic substrates).

But (despite some very exciting "proof-of-concept" demonstrations), this technology isn't ready for "prime time" usage just yet. OLED-based displays are sometimes used for small-screen applications such as cell phones and digital cameras, but their widespread use for applications like large screen computer displays may not come for another five or ten years at the time of this writing (in fact, they may not make it at all if the SED technology discussed below fulfills its promise).

Surface Emission Displays (SEDs)
This is where things start to get very exciting. Prior to the mid-1980s, graphite and diamond were the only forms of pure carbon that were known to us. In 1985, however, a third form consisting of spheres formed from 60 carbon atoms was discovered. Commonly referred to as "Buckyballs," the official moniker of this material is Buckministerfullerine, which was named after the American architect R. Buckminister Fuller who designed geodesic domes with a similar underlying symmetry.

Sometime later, scientists discovered a related structure that we now refer to as a carbon nanotube. Such nanotubes can be incredibly small, with a diameter only one thousandth of one millionth of a meter. Furthermore, they are stronger than steel, have excellent thermal stability, and are tremendous conductors of heat and electricity.

In addition to functioning as wires, nanotubes can be persuaded to act as transistors. Of particular interest to us here is that they can also be coerced into emitting streams of electrons out of one end. Hmmm, tiny little electron guns; what wonders could we perform with these little rapscallions?

Figure 2. High-level representation of a single pixel in a surface emission display.

Well, imagine a screen that is thin and flat like a LCD, but is as bright and vibrant as a CRT-based display. Well, that's what you end up with if the screen is formed from a carbon nanotube-based SED. In this case, the inside of the screen is covered with red, green, and blue phosphor dots (one of each to form each pixel), and each if these dots has its own carbon nanotube electron gun.

This technology has been skulking around in the background for some time. Toshiba hosted the first public demonstration of a large-scale carbon nanotube-based SED at the consumer electronics show (CES) in January 2006. Industry expert Dennis P. Barker attended the show; afterwards he said to me:

"High-definition television is incredibly realistic, but SED goes one step beyond. When I saw the Toshiba demonstration, it gave me chills and the hairs on the back of my neck stood to attention. I have seen the future and – to me – the future is SED!"

Originally it was predicted that we would be seeing SEDs on the streets toward the end of 2006 and the beginning of 2007. It was later announced, however, that the introduction of these devices was being held back until around the middle of 2008 (to coincide with the Summer Olympics in Beijing). At the time of this writing, this technology has not yet being widely deployed, but it appears as though most of the issues that have been holding it back have been resolved, and SEDs could well be poised to leap onto the center stage.

Even better, imagine a high-resolution SED display using six primary colors instead of the three (ref, green, and blue) we usually associate with displays. (See also my blog Displays with Six Primary Colors). I don’t know if such a beast will ever come to pass, but if it does I WANT ONE!!!