The Evolution of Display Technologies
CRT LCD Plasma DLP OLED FED

A Carbon NanoTube Monitor

The Cathode Ray Tube: A vacuum glass tube (e.g. the air is removed). An electron beam is emitted and then received on some metal surface. The received energy is converted to a"glow" as a result of interacting with phosphor.

A phosphor is any material that, when exposed to radiation, emits visible light. The incoming radiation can be in the form of an electron beam or in the form of ultraviolet (UV) light.

In a CRT, phosphor coats the inside of the screen. When the electron beam strikes the phosphor, it makes the screen glow. In a black-and-white screen, there is one phosphor that glows white when struck. In a color screen, there are three phosphors arranged as dots or stripes that emit red, green and blue light. There are also three electron beams to illuminate the three different colors together. However, a television requires an addition element because a television, as is most any image, is a rasterized imaged composed of an array of individual pixels. What is therefore required to transform a CRT into a rasterized image, is a mechanism to point and steer the electron beam. Thus a TV CRT is wrapped with steering coils.

The steering coils are simply copper winding that create magnetic fields inside the tube, and the electron beam responds to the fields. One set of coils creates a magnetic field that moves the electron beam vertically, while another set moves the beam horizontally. By controlling the voltages in the coils, you can position the electron beam at any point on the screen.

So the image is created by rapidly moving an electron beam across a raster thus illuminating the phosphor dots. One advantage of the CRT is that each phosphor dot has relatively high brightness.

There are some significant limitations to CRT technology (watch the video above) which is why it never really changed throughout its operational lifetime (roughly 1950- 2000).

• CRTs require a significant amount of power and the more pixels there are to illuminate, the greater the power requirements. More power generates more heat.

• The "painting of the screen" via the steering coils re-direction of the electron beam has limitations. A "high-end" CRT may have 525 lines of resolution (rows of phosphor dots) that are painted 30 times a second, so thats 15,750 lines per second. There are limitations to this frequency with this technology so its difficult, to say, paint 2000 lines 30 times a second.

• Generally speaking, phosphor pixels can not be made particularly small because you need a minimum amount of material for its response to "glow" at standard video brightness levels. So the equivalent of an HDTV CRT would be very big and heavy and completely impractical because you can't build an arbitrarily large glass screen without it breaking. The industry size limit was approximately 40 inches diaganol.

To therefore get to more pixels, HDTV, larger display images, etc one requires new technologies. We describe the historical evolution of these two technologies in various stages