3.3 Cathodoluminescence

Cathode ray tubes (CRTs) are almost universally used in colour televisions and still dominate in the display monitors of desktop computers. They are obviously not suitable for laptop PCs, because of bulk and weight, where currently liquid crystal displays are the systems of choice. Neither are they the most suitable technology for very large area displays, where other display techniques such as plasma panels and electroluminescent devices offer advantages.

The operating principle in CRTs, shown schematically in Figure 3.3, involves an electron beam generated from the cathode located at one end of a glass vessel, which is held under vacuum. The electron beam is accelerated and then moved horizontally or vertically by electrostatic or electromagnetic means. The inner, front surface of the glass vessel is coated with inorganic phosphors and when the beam of highly accelerated electrons hit this surface the phosphors emit visible light. This process is known as cathodoluminesence.11

Figure 3.3: Schematic representation of a colour cathode ray tube.

To avoid any flicker in the image, the electron beam is scanned across and down the screen, many times per second, following a predetermined set of parallel lines, the method being known as raster scanning. The phosphor dots are the picture elements or pixels and light up as the beam scans across each one. In colour televisions and monitors additive mixing of the three colours of red, green and blue produces the final colour. The pixels contain three sub pixels of red, green and blue dots, each...