Color CRTs

Color CRTs:--

Spectra of constituent blue, green and red phosphors in a common CRT

Color tubes use three different phosphors which emit red, green, and blue light respectively. They are packed together in stripes (as in aperture grille designs) or clusters called "triads" (as in shadow mask CRTs). Color CRTs have three electron guns, one for each primary color, arranged either in a straight line or in a triangular configuration (the guns are usually constructed as a single unit). A grille or mask absorbs the electrons that would otherwise hit the wrong phosphor. A shadow mask tube uses a metal plate with tiny holes, placed so that the electron beam only illuminates the correct phosphors on the face of the tube. Another type of color CRT uses an aperture grille to achieve the same result.

Convergence in color CRTs
The three beams in color CRTs would not strike the screen at the same point without convergence calibration. Instead, the set would need to be manually adjusted to converge the three color beams together to maintain color accuracy in the generation of symbology, for sweep retrace, and so forth. We take you through a detailed look at how the vector scan CRT uses these signals to paint the display on the CRT later in this chapter. COLOR CRT’S Thus far our discussion has been about monochrome CRTs. Color CRTs offer a variety of colors and are used extensively with personal computers, simulators, and other training devices. Most color CRTs use a raster-scan type deflection. The major differences between color and monochrome CRTs are in the phosphor coating of the CRT, the electron gun(s), and the high voltage requirements. The phosphor coating of a color CRT is made up of small dots that contain a dye so they radiate one of the three primary colors of light (red, green, or blue). These dots are arranged in groups called triads. Figure 1-6 illustrates a typical grouping of triads. The size of the phosphor dots is often used as a measure of the CRT’s resolution. Newer monitors have CRTs with dots of .20 mm and smaller. The dots are the smallest addressable element of a picture. These picture elements are called pixels or pels, depending on the manufacturer. Both terms have the same meaning. Three electron beams are required to properly strike the different colored phosphor dots. Some color CRTs use three electron guns, known as a delta gun CRT. The beams pass through a shadow mask that is designed so that only the red gun strikes the red dots, the blue gun strikes the blue dots, and the green gun strikes the green dots. Newer color CRTs have combined all three electron beams into a single gun, as shown in figure 1-6. The single-gun CRT does not need convergence alignments and greatly reduces the amount of circuitry required in a color monitor. This design is common in almost all of the newer color monitors.