Lecture 1 --- 6.837 Fall '01

Raster Display Disadvantages Requires screen-sized memory array Discrete spatial sampling (pixels) Moire patterns result when shadow-mask and dot-pitch frequencies are mismatched Convergence (varying angles of approach distance of e-beam across CRT face) Limit on practical size (< 40 inches) Spurious X-ray radiation Occupies a large volume Advantages Allows solids to be displayed Leverages low-cost CRT H/W (TVs) Whole Screen is constantly updated Bright light-emitting display technology
	Lecture 1		Slide 15		6.837 Fall '01

Color CRT's are more complicated than the simple monochrome models summarized before. The phosphors on the face of a color CRT are laid out in a precise geometric pattern. There are two primary variations, the stripe pattern of in-line tubes shown on the left, and the delta pattern of delta tubes as shown on the right.

Within the neck of the CRT there are three electron guns, one each for red, green, and blue (the actual beams are all the same color-- invisible). There is also a special metal plate just behind the phosphor cover front face, called a shadow mask. This mask is aligned so that it simultaneously allows each electron beam to see only the phosphors of its assigned color and blocks the phosphor of the remaining two colors.

The figure shown above shows the configuration of an example in-line tube. On page 44 of Hearn & Baker you'll see a similar diagram for a delta electron gun configuration

A significant portion of the electron beam's energy strikes the mask rather than the phosphors. This has two side effects. The shadow mask has to be extremely rigid to stay aligned with the phosphor patterns on the CRT face. The collision of electrons with metal mask causes the mask to emit some of it absorbed energy as electromagnetic radiation. Most of this energy is in the form of heat, but some fraction is emitted as x-rays. X-rays can present a health hazard. This wasn't a large problem for television because the intensity of the x-ray radiation falls off quickly as you move away from the screen. However, computer monitors are supposed to be viewed from a short distance. This health concern along with the high voltages and power dissipations of CRTs has motivated the development of new display technologies.