The Evolution of Display Technologies
CRT LCD Plasma DLP OLED FED

A Carbon NanoTube Monitor

Stage 1: The creation of the rear projection TV.

Here a projector is mounted inside the TV console and projects to a mirror which then reflects the beam to the larger screen. This is a way to make the image size on the CRT screen bigger and the resolution is determined by the number of pixels in the (LCD) projector. Sports bars of the 1980s featured these devices but generally the image quality just sucked.

Stage 2: The creation of LCD display screens

This was first done and made commercially available in 1988 and this really fundamentally changed display technology and allowed for relatively flat screens (hence it becomes possible to make a laptop) of a wide variety of sizes and resolutions.

A liquid crystal represents an odd state of matter but one that can be manufactured easily into some materials. In a solid the molecules always maintain their orientation with respect to each other but in a liquid, the molecules can easily change their orientation and position with respect to each other. A liquid crystal represents a material where the molecules due maintain their orientation with respect to each other but they can move to different positions. This movement can be controlled by applying a voltage to the material. In this way, an LCD device is like acts like a "light valve" as its light emitting/transmitting properties can be controlled. In general, the amount of light intensity per pixel depends upon the applied voltage to that pixel.

In essence, electrical currents can cause liquid crystals to change their shape. This allows them to act as light valves -- different amounts of current allow different amounts of light to pass through the crystal. This lets the LCD device create a greyscale image of different intensities at different locations on the screen. To add color, most projectors use a series of mirrors that split the light into red, green and blue beams. Each beam passes through a separate LCD, and a lens collects the three beams and projects the image on the screen.

Significant R&D has gone in to the characterization of different materials that can act as an LCD. As shown above, the basic physical size of the LCD is determined by the size and number of molecules that make it up. But we are not yet at single molecule devices (yet). Current LCD pixel size is then limited by the electronic interface needed to control the pixel. Improvements (using thin film transistors) have allows for this electronic control interface to become pretty small:

This is why its possible to get 1,000,000 pixels (e.g. 1024x 768) on to a standard laptop screen. LCD also represents a fairly scalable technology and now 6 million pixel screens as large as 65 inches have been developed.

Stage 3: The Development of Plasma Screens

The basic idea of a plasma display is to illuminate tiny colored fluorescent lights to form an image. Each pixel is made up of three fluorescent lights -- a red light, a green light and a blue light. Just like a CRT television, the plasma display varies the intensities of the different lights to produce a full range of colors. The central element in a fluorescent light is a plasma - which is an ioniozed gas made up of ions and electrons. If you introduce many free electrons into the gas by establishing an electrical voltage across it, the situation changes very quickly. The free electrons collide with the atoms, knocking loose other electrons and therefore creating postively charged nuclei. In a plasma with an electrical current running through it, negatively charged particles are rushing toward the positively charged area of the plasma, and positively charged particles are rushing toward the negatively charged area. This causes energetic collisions which excited the gas atoms in the plasma.

As we learned before, an electron in an excited state will de-excite a lower energy level thus releasing a photon (remember, in a laser this process is tuned so that only a narrow bandwidth of photons are emitted). In order to produce flouresence, one needs an initial source of UV photons and hence one needs to find the right material to do this. Typically xenon and neon atoms are used in plasma screens as these elements, when excited, produce mostly UV photons.

The xenon and neon gas in a plasma television is contained in hundreds of thousands of tiny cells positioned between two plates of glass. Long electrodes are also sandwiched between the glass plates, on both sides of the cells. The address electrodes sit behind the cells, along the rear glass plate. The transparent display electrodes, which are surrounded by an insulating dielectric material and covered by a magnesium oxide protective layer, are mounted above the cell, along the front glass plate. Both sets of electrodes extend across the entire screen. The display electrodes are arranged in horizontal rows along the screen and the address electrodes are arranged in vertical columns. As you can see in the diagram below, the vertical and horizontal electrodes form a basic grid.

To ionize the gas in a particular cell, the plasma display's computer charges the electrodes that intersect at that cell. It does this thousands of times in a small fraction of a second, charging each cell in turn. When the intersecting electrodes are charged (with a voltage difference between them), an electric current flows through the gas in the cell. The current creates a rapid flow of charged particles, which stimulates the gas atoms to release ultraviolet photons. The released ultraviolet photons interact with phosphor material coated on the inside wall of the cell. The phosphors in a plasma display give off colored light when they are excited. Every pixel is made up of three separate subpixel cells, each with different colored phosphors. One subpixel has a red light phosphor, one subpixel has a green light phosphor and one subpixel has a blue light phosphor. These colors blend together to create the overall color of the pixel. Note that the pixels are layed out horizontally .

By varying the pulses of current flowing through the different cells, the control system can increase or decrease the intensity of each subpixel color to create hundreds of different combinations of red, green and blue. In this way, the control system can produce colors across the entire spectrum. The main advantage of plasma display technology is that you can produce a very wide screen using extremely thin materials. And because each pixel is lit individually, the image is very bright and looks good from almost every angle.

However, advances in LCD technology have outpaced those in plasma for the most part and its likely that this kind of technology will be phased out over the next few years.