Wherever possible you should determine which target output device is to be used before you scan an image - best results are obtained when the image is scanned using settings specifically tailored to the intended output device. This section discusses some of the commonly used output devices and their main characteristics.
Screen output
When you want to display a scanned image on your screen, remember that the performance of your monitor will be limited by the component which has the lowest capability. For example, even if your monitor and video card are capable of showing 32,000 colours, if the software driver can only handle 16 colours then 16 colours is all you will get.
Screen resolution is determined by the number of pixels in the horizontal and in the vertical directions: these are not always the same. These pixels are also called 'addressable' dots. Most monitors have a resolution of 72 - 120 dpi. If you are scanning an image for screen output, remember that the scanning resolution should not exceed the resolution of your display monitor. If it does, you will increase scanning times and storage space unnecessarily.
{2}On a colour monitor, each of the addressable pixels on the screen is made up of three separate phosphor dots: one red, one green and one blue. (Refer to the previous section for more details). Colour is displayed when the monitor switches on various combinations of the phosphors. Some of the higher end colour monitors can display an almost infinite number of colours. When you display coloured scanned images on the screen, because the light emitted is quite pure, they can be far more vivid than those which can be printed. This explains why what looked green on the screen may well look more like khaki on your printed page!
Printer output
For simple monochrome line art scans, the main determinant of final print quality is the physical resolution of your printer. If you are trying to print halftones or colour images, however, the picture is rather more complicated.
Printing colour images
If you are using a printer to output scanned images, you must convert the RGB colours in your scanned images into the CMYK format first. You may find that your software application does this automatically when you choose your intended output device. If not, there are several colour calibration packages available which enable you not only to convert raw RGB to CMYK data, but also let you calibrate the colours most effectively for the individual characteristics of your selected output device.
{3}Most printers produce images using cyan, magenta, yellow and black dots. The exceptions to this are pen plotters and dye-sublimation printers (see later in this section).
To create additional colours, printers either lay down patterns of primary dots - for example cyan and magenta for purple, or they add white by leaving some areas unprinted - for example cyan and white space for pale blue.
{4}Printing halftones
Although a scanned halftone contains dots of varying sizes, a printer can only print dots of a fixed size. This means that special dithering techniques are needed to simulate the different dot sizes in the halftone image. The dithering process is usually carried out by your applications software. It groups printer dots into what are sometimes called 'halftone cells', each containing between 16 and 64 printer dots according to the printer resolution and the number of grey levels displayed. Sometimes scanners which cannot handle greyscale information carry out dithering at the input end which is usually not very satisfactory (see How scanners work for more details).
Consider, for example, a halftone cell consisting of 16 printer dots arranged in a 4 x 4 square. This arrangement allows for the representation of 16 different grey levels (including black and white). If all printer dots are black, the halftone cell is black. If 8 of the 16 dots are black, the halftone cell appears medium grey (50% grey). If 2 of the 16 printer dots are black, the cell is light grey (12.5% grey).
Dithering effectively reduces the printer resolution, because it uses an entire halftone cell to represent just one halftone dot. The higher the number of grey levels reproduced (i.e. the greater the number of printer dots per halftone cell), the lower the effective printer resolution.
{5}A number of methods have been suggested for working out the best scanning resolution for halftones, but as a rule of thumb, you need to double the resolution of your output device each time that you upgrade from 16 to 64 to 256 grey levels. The trade-off between grey levels and image resolution is shown in the next table.
For example, if you are scanning a black and white photograph for output on a 600 dpi laser printer, you could represent 16 different grey levels by scanning at 600/4 =150 lpi. If you wanted to print the same scanned halftone at 150 lpi with 64 different grey levels, you would need a printer with at least 1,200 dpi resolution (1,200/8 = 150 lpi). In practice, you will find that you need to allow for a lot of trial and error before you can produce the best results.
{6}Printer types
Dot matrix printers
Dot matrix printers imprint the image of a character on paper by selecting from a vertical matrix of pins on the printing head, the appropriate patterns to form the shape of the character required.
It is unlikely that you would use a dot matrix printer to output anything other than very simple documents containing scanned line-art. Colour reproduction on most dot matrix printers is limited due to the small choice of colours available on the ribbon, and printer resolution is usually low. However, to their advantage, dot matrix printers are cheap to run and are reliable.
{7}Inkjet printers
There are two basic types of inkjet printer: liquid inkjet and solid inkjet (phase change) printers.
Liquid Inkjet Printers
Liquid inkjet printers can be further divided into 'drop-on-demand' and 'continuous flow' printers.
Drop-on-demand
Drop-on-demand inkjet printers use one of two different ink-emitting systems based on different technologies: Piezo technology and thermal jet ('bubble-jet') technology.
Thermal jet technology is based on the rapid alternate heating and cooling of a small thermal element. The heat element is positioned directly next to the exit hole of the print head and creates a gas bubble which forces trapped ink out of the opening as a droplet.
{8}Piezo technology relies on a pump action to expel ink. This pump is set up by the application of an electrical charge to a diaphragm made of crystalline material. Drop-on-demand printers are well-suited to printing simple monochrome or coloured scanned images, especially on overhead transparencies.
Continuous flow
Continuous flow liquid inkjet printers can provide high quality colour but are very wasteful - the print head directs ink droplets towards the paper constantly and if the ink is not required it is passed into a waste reservoir. As a consequence, this type of printer is expensive to run although they are good for proofing documents containing scanned images before you send them to a commercial printer or for producing low volume runs of colour print jobs.
{9}Solid inkjet printers
Solid inkjet or 'phase-change printers' use solid sticks of cyan, magenta, yellow and black inks which can change phase to liquid when they are melted and propelled towards the paper. When the ink reaches the paper it freezes quickly, thus reducing absorbency and maintaining high colour quality on various types of paper.
Like liquid inkjet printers they are useful for colour-proofing an image or page which you eventually want to output on a traditional printing press.
Thermal wax transfer printers
Thermal wax printers work by heating coloured wax panels and fusing them to a special type of paper or overhead transparency film. The paper makes three or four passes under the head - one for each of CMY and K. Dithering techniques are used to create any other colours required from the enormous range available. Thermal wax transfer printers are an excellent way of printing overhead transparencies containing coloured scanned images.
{10}Dye-sublimation printers
Dye-sublimation printing is similar to thermal wax printing except that instead of laying coloured wax in layers on the paper, it heats the wax to such high temperatures that the colour impregnates the paper itself.
A major advantage of the dye-sublimation process is that it does not have to resort to dithering techniques to simulate colour tones. The amount of colour transferred to the paper can be controlled carefully using varying amounts of heat. This means that continuous colour tone output can be produced. Dye-sublimation is thus an excellent output device for scanned colour images. Its relatively high running costs means that it is best reserved for high end short-run printing jobs.
{11}Laser printers
Monochrome laser printing is a well-established technology, especially in the DTP arena. It involves applying a static charge to a photostatic drum or belt and passing a laser beam through the areas not to be printed.
This leaves the areas to be printed electrically charged so that toner can be applied to the charged areas and the image can then be transferred electrostatically onto the paper. Colour laser printers work in the same way except that they repeat the process four times - once for each of the CMYK colours.
{12}Higher-end monochrome laser printers can output at resolutions as high as 1,200 dpi which is adequate for proofing black and white scanned halftones or for final output of monochrome line art. Although colour laser printing is excellent for pre-press colour proofing, at the time of writing it is still an expensive option.
Traditional printing presses
Many documents containing scanned images will eventually be output by a type-setting bureau for subsequent printing on a traditional printing press. Normally you supply your document files to your typesetter who will use an image-setter to output your document files to film. If you are outputting low resolution monochrome documents, you may be able to use the cheaper method of outputting to a special type of paper - usually called 'bromide'. For adequate quality on high-resolution monochrome work and for all colour work, you must run the files out to film. Other systems (direct-to-plate) are able to run files straight out to plates ready for printing.
Colour separation is the process by which your image file is separated into its four component colours - cyan, magenta, yellow and black. Thus four separate pages of film are produced for each page of your 'composite' original. These separate films are used to make up printing plates in the four different CMYK colours. Most printing presses use four colours, but sometimes the use of special colours may mean that the printer has to make up special additional inks.
{13}As the novice will quickly discover, an enormous number of variables determine the final output quality of colour documents printed on traditional presses.There is not room to discuss them all here, but you will usually find that your commercial typesetters and printers have a wealth of experience you can draw on. In addition, several books and journals are available. Best results will be obtained if you make sure that you calibrate your scanned image properly for the image-setter which will be used. Most software drivers for PostScript office laser printers will let you separate your image into the four component 'plates' and run these pages out separately for proofing before you send your files to the typesetting bureau.
If possible, try to check the film separations carefully yourself before the film is given to the printer. In this way you can check for problems such as inaccurate registration, and faint or missing components. In this way you should minimise the chance of anything going wrong and benefit from the extremely high quality which traditional printing can provide.
