As central computers became faster and more powerful, it was possible 1 to establish many remote display stations from which operators could all use the same computer to display information and enter data. Later, even the small machines were equipped with a fairly large display screenand keyboardoriented towards use by a person with limited 5
training, rather than by a highly skilled computer operator.
 For many interactions with computers a permanent record is
unnecessary; therefore, output that is scanned once and then thrown away produces a lot of wasted paper. To solve this problem cathode ray tube (CRT)terminals can be used. In addition to eliminating paper 10
waste, these terminals are completely silent and frequently much faster than hard-copyterminals. Because of their speed and quietness, CRT terminals are very useful interactivedevices for use in offices and in other areas. The electronic circuitry used in them is very much the same as that in the familiar TV (video) set. 15
pi These display terminals are diverse and colourful. The original video output was single-colour (black and white) upper-case letters, but in more highly developed devices, lower-case letters can be displayed, and some give options of blinking and dual-densitycharacters. Certain screens can produce 'negative' (dark) characters on a bright background 20 or even make each character a different colour, if so desired. The latter is an important feature in order to catch someone's attention when a value is abnormal.
 More sophisticated screens can generate continuous lines for graphic
displays. The simplest of these are monochromaticand may have 25
strictly limited graphic applications. For example, they may use special characters strung together in order to form lines that look continuous, or they may be restricted regarding the number and the shape of curves that can be drawn at once. Multicolour pictorial graphics are extremely useful in emphasizing contrast, and have been used with good results in 30 nuclear medicine, where differences in intensity would be too subtle if shades of only one colour were used.
 The full power of visual display terminals may soon be realized. Already dynamic(motion) graphicsoutput devices that display simple changing scenes have been developed for flight trainers and computer-generated 35
movies. These devices could have an important impact on the use of computer-assisted instruction(CAI) techniques in schools and colleges.
 Besides screens, a wide variety of devices called plottersare used to produce permanent copies of graphic output. The first plotters developed used a pen that moved back and forth across the rotating 40
drum to which paper was attached. These devices produced results which were both quite accurate and reasonably fast. Other plotters featuring a moving pen in two dimensions are relatively slow, because not only are they mechanical devices, but most can produce only one continuous line at a time. However, the modern electrostatic plotter,45 an extension of the previously mentioned electrostatic printer, can obtain slightly less artistic results in seconds because it doesn't depend on a moving pen. This is because it electronically generates patterns of dots in a line across the page to make up a picture.
 Graphic output may be an effective alternative to high-speed hard-copy 50 output where graphs are more useful than columns of numbers for showing results. Most of the time, a graph is not only better than columns and pages of printed numbers, but also has the advantages of being quicker to produce and easier to understand and file. Pen plotters have been used a great deal in scientific research, where results are often 55 expressed in terms of graphs.
 In recent years, new output devices have been developed to bridge the gaps between the various devices just described. For instance, terminals with both video screen and hard-copy output are now available. These devices don't waste paper, since hard copy is produced only when a 60
permanent record is needed. Terminals of this nature are used in applications where relevant personal data is needed which can be corrected or edited on a screen before a hard copy is made. University registration or patient registration are two areas where these devices save time and money, because a new hard copy need be produced only if 65
something has changed from the previous registration or visit.
 Finally, there are electrostatic printer-plotters available which produce both print and graphic displays with equal facility. It is possible with these devices to change the style and size of the letters at the users' wish and to have graphs and displays interspersed between the printed lines. 70
[101 It is therefore very clear that in modern times, the converging
technologies of printers, plotters, and graphic displays have resulted in the creation of a few hybrid devices capable of doing many things.