PAPER : PRINTING AND SCANNING

Presented by: Nazia Mohammed Anil 1

Printing

Computer systems have made it easier to produce paper documents; as it is easy to print out many copies in order to get the final copy looking ‘just right’.

Older printers had a fixed set of characters available on a printhead these varied from the line printer to golf-ball and daisy-wheel printers.

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All of the popular printing technologies, like screens, build the image on paper as a series of dots.

It enables any character set or graphic to be printed, limited only by the resolution of the dots.

This resolution is measured in

dots per inch (dpi).

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Types of dot-based printers

Dot-matrix printers Uses an ink ribbon, like a typewriter, a line of pins are used, each which strike the ribbon and result in dotting the paper. Ink-jet and bubble-jet printers Operate by sending tiny blobs of ink from the printhead to the paper. The ink is squirted at pressure from an ink-jet, whereas bubble-jets use heat to create a bubble. 4

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Laser printer Uses similar technology to a photocopier: - dots of electrostatic charge are placed on a drum - which then picks up toner (black powder) - it is then rolled onto the paper and cured by heat - Curing is why laser printed documents come out warm Dot-matrix printers have a resolution of 80-120 dpi Ink-jet printers have a resolution of 300-600 dpi Laser printers have a resolution of 600-2400 dpi 5

Comparing printers

As well as resolution printers vary in speed and cost.

Ink-jet or Laser printers print between four and eight pages per minute.

Dot-matrix are rated in

characters per second (cps),

printing no more than a page or so per minute.

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Fonts and page description languages

Printed documents can be complex – they incorporate text in many different fonts and many sizes, often italicised, emboldened and underlined. Sophisticated printers can accept a

page description language,

the most common of which is PostScript, this is a form of programming language for printing. Text is printed in a font with a particular size and shape. The size of a font is measured in

points (pt).

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The shape of a font is determined by the its font name, e.g. Times New Roman, Courier, Comic Sans MS. Times Roman font is similar to the type of many newspapers, such as

The Times,

whereas Courier has a typewritten shape. Some fonts, such as Courier are

fixed pitch,

where each character has the same width. The alternative is the Times New Roman a others, such as the ‘i’.

variable pitched-font,

where some characters such as the ‘m’ are wider than 8

Screen and page

A common requirement of word processors and desktop publishing software is that

what you see is what you get,

the acronym

WYSIWYG.

This means the document on the screen should be the same as its eventual appearance on the printed page.

Most screens use an additive colour model using red, green and blue light, whereas printers use a subtractive colour model with cyan, magenta, yellow and black inks, so conversions have to be made. 9

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Differences between screen and printer mean that different forms of graphic design are needed for each. E.g. headings and changes in emphasis are made using font style and size on paper, but using colour, brightness and line boxes on screen. This is not usually a problem for the display of the users own documents as the aim is to give the user as good an impression of the printed page as possible, given the limitations.

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Scanners and OCR

Printers take electronic documents and put them on paper –

scanners

reverse this process. It is started by turning the image into a bitmap, but with the aid of

optical character recognition

can convert the page right back into text. Two types of main scanners: -

Flat-bed:

the page is placed on a flat glass plate and the whole page is converted into a bitmap.

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Hand-held:

pulled over the image by hand, as the head is passed over an area it is read in, yielding a bitmap strip. A roller at the end ensures that the scanner knows how fast it is being pulled and how big the image is. 11

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Scanners work by shining a beam of light at the page and then recording the intensity and colour of the reflection. Scanners also differ in resolution, commonly between 600 and 2400 dpi.

Scanners are used extensively in

desktop publishing (DTP)

for reading in hand drawn pictures and photographs, images can be rotated, scaled and transformed using a variety of image manipulation software tools.

Optical Character Recognition (OCR) is the process whereby the computer can ‘read’ the characters on the page, scanned images are electronically read to convert them into editable text. This conversion is performed after scanning, and may output formatted text or text-files only.

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