Computer Peripherals 437
15.4.3 Laser Printer
Out of the three types of printers being discussed, laser printer uses most complex technology. The proce-
dure is very much similar to that of a photocopying machine. In laser printer, a rotating drum is coated with
extra- ne coating of positively charged photoconductive powder and subjected to a ray of a laser beam,
controlled by a rotating mirror. The presence or absence of laser is controlled by the white and black portion,
respectively, of the original document being printed. In other words, the black portion of the document forces
the laser off and white portion forces it on. The duty of this laser beam is to neutralize the positive charges
on the drum, wherever it is applied. Therefore, after the laser scanning is complete, only the black portion
of the original document, to be printed, would be represented by positively charged particles over the drum.
The next step is to sprinkle some negatively charged powder (popularly referred as the toner ) over
the surface of the rotating drum. These particles are attracted and become attached only to those areas
that are still having the positive charge, and generate a de facto page-image. Next comes the paper, to be
pressed with the drum to acquire the impression of it in the form of toner. Finally, the paper is heated so
that the toner is permanently xed on the paper and we see the printout. Due to the achievable accuracy,
the quality of printing by a laser printer is far better than the inkjet printers.
15.4.4 Colour Printing
Whatever we have discussed so far, is all related with monochrome printing, i.e., always using black
ink. However, in certain cases, colour printing is essential, which evidently is more complex than mono-
chrome printing processes. In the case of colour printing, generally, four colours are used, namely cyan,
yellow, magenta and black. As a matter of fact, cyan absorbs the red colour, yellow absorbs the blue
colour and magenta absorbs the green colour. As in printing on a white paper we see only the re ected
colours, which are not absorbed, these colours are used for printing on white background. Although a
proper combination of cyan, yellow and magenta should absorb all colours from white light, re ecting
nothing and producing the image of black, however, in practice, this exact combination is dif cult to
obtain and, therefore, black is used as the fourth colour.
15.5 DISPLAY
Just like a keyboard is an integral part of any computer functioning as the primary input device, the
display device is also another integral part, serving as the essential output device. Popularly known as
monitors, these display devices vary widely, depending upon their type and technology used.
In earlier days, the cathode ray tube (CRT) type display was the only alternative available to serve as
the computer’s output screen. We are accustomed of seeing the text and images on this type of screens.
At a later date, with the introduction of laptops, the at-screen displays were made available in the com-
puter market. We shall discuss about both of these in the following sections, brie y.
15.5.1 CRT Display
The basic mechanism of displaying any character through CRT type display is similar to the technique
adopted for dot-matrix printing. Here also, any character is divided into 5 7 grids, designated as pixels,
and only the relevant pixels are made visible on the screen of the CRT. An electronic gun (three, namely
red, green and blue termed as RGB for a colour monitor) is used to create the impression of the relevant dot.
To cover the complete screen of the monitor, this gun has to sweep from left to right and top to bot-
tom in a synchronized fashion. It starts its sweeping action from top-left corner of the screen and after
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