Transfer process

A cartridge of ink is attached to a print head with up to hundreds of nozzles, each thinner than a human hair. The number of nozzles and the size of each determines the printer's resolution. As the print head moves across the paper, a digital signal from the computer tells each nozzle when to propel a drop of ink onto the paper. On some printers, this is done with mechanical vibrations. Piezoelectric crystals change shape when a voltage is applied to them. As they do so, they force ink through the nozzles onto the paper. Each pixel in the image can be made up of a number of tiny drops of ink. The smaller the droplets, and the more of them, the richer and deeper the colors should be.

Ink cartridges courtesy of Tektronix.

Piezoelectric crystals force ink through the nozzles onto the paper. Courtesy of Tektronix.

Inkjet printing, like conventional printing on a press, is binary. These printers can only put ink down or not put ink down. They can't control the density of each dot. To achieve the illusion of continuous tones, the percentage of area covered by ink is modulated in one or both of two ways:

1. A screening process maps the desired variations in density into variations in dot size. Thus, as the desired density increases, the dot sizes increase and a higher percentage of the white space is covered with ink.

2. If the printing process supports smaller dots of a fixed size, area modulation is achieved by varying the number (rather than the size) of dots that are printed in any given small area.

There are two pitches of concern with such printing: the dot pitch and the screen pitch. For example, an inkjet printer may have a raw dot pitch of 1200 dpi. An equivalent screen pitch may be defined as say, 75 lpi, where "lpi" refers to the equivalent dot pitch of a screen (lines per inch). Thus, each screen cell (75/inch x 75/inch) contains 1200/75 x 1200/75 = 64 raw dots. In such a case, each screen cell could be printed at any of 65 levels (0 to 64 dots). This process would then be equivalent to a 65 level, 75 pixel/inch printer.

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