In the previous chapter, input devices were discussed. The methods and problems of converting an analog image to a digital form that can be stored and processed in a computer were presented. In this chapter, we explore the devices and methods for displaying this digital image data. The ideal method for displaying a digital image depends upon the user's intent. That intent may be information transfer, analysis or aesthetics. The requirements for each of these purposes determine the necessary modality and quality of the reproduction.

The important characteristics of an output image include:

• Permanence,

• Cost,

• Accuracy,

• Display conditions,

• Size,

• Number of copies.

These characteristics should be considered as the following output technologies are discussed: CRT monitors, LCD displays, photography, electrophotography, commercial printing, e.g., gravure, offset, ink-jet printing and thermal transfer devices.

Cathode ray tube monitors

The cathode ray tube (CRT) was invented in 1897 by Karl Ferdinand Braun. Today, color CRT monitors are a common soft-copy output device. While black and white monitors are used for some text and document applications, color has become so economical that almost all imaging applications use color monitors.

Most color CRTs use three independent electron guns for each of the three primary phosphors. The guns can be arranged in line or in a triangle (delta) geometry, the geometries of which are shown in Figs. 11.1a and b. The Trinitron™ gun actually uses three cathodes in a single gun.