Brother IMPACT DOT MATRIX PIRNTER User Manual download pdf (Page 10)

Printers

cient as the techniques offered by the laser technology. Moreover, the lifetime of these printers is

shorter than that of printers using the laser technology.

The liquid crystal display (LCD) printers operate similarly, using an LCD panel placed

between a constant light source (which is not a laser beam) and the photoconductive drum.

Usually, electro-photographic printers have resolutions of 600 or 1200 dots per inch. In

most printers, the resolution is fixed primarily by the raster image processor (RIP), which trans-

lates the printing commands into the bitmap image to be printed. Another element that may limit

the resolution is the size of printer’s memory. By changing the RIP and extending the memory, it

is possible to increase the printer’s resolution. Nevertheless, higher resolutions also require a ton-

er with an adequate quality, since at high resolutions the size of toner particles may limit the im-

age clarity.

The Resolution Enhancement Technology (REt) increases the apparent printing quality

within the limits of a certain available resolution. This technology, introduced by Hewlett-

Packard in 1990 with the LaserJet III printer series, consists in altering the size of toner dots at

the edges of characters and diagonal lines to reduce the aliasing effect. Thus, by using this tech-

nology the on-paper resolution remains at the printer’s rated value, but the images will appear

sharper.

Compared to inkjet printers, the main advantages of electro-photographic printers are

their higher speed and precision. Common printing speeds are between 20 and 50 pages per mi-

nute, but complex printers may have much higher speeds. For instance, some sophisticated mod-

els may have speeds of 200 pages per minute or higher. The diameter of the laser beam is con-

stant, so that it is possible to achieve a high precision of the dots from which graphical images are

built. Furthermore, the solid toner does not diffuse into the paper pores as the liquid ink does, and

therefore the printing quality is dependent to a much smaller extent on the paper quality. Alt-

hough the cost of electro-photographic printers is higher, their cost per page is lower than that of

inkjet printers. However, color electro-photographic printers are not as widely used as color

inkjet printers.

6.5. Color Printers

6.5.1. Generating Color

Unlike color monitors, which use the additive color synthesis, color printers use the

subtractive color synthesis. With monitors, a color is generated by combining the three prima-

ry additive colors, red, green, and blue; the standard used is called RGB (Red, Green, Blue).

Printers use pigments with the three primary subtractive colors, cyan, magenta, and yellow;

the color system used is called CMY (Cyan, Magenta, Yellow). Cyan is the complementary

color for red, magenta is the complementary color for green, and yellow is the complementary

color for blue. For instance, to print in the red color, a pigment should be used that is colored

magenta (which absorbs green) and yellow (which absorbs blue), reflecting back only the red

color.

More often than not, printers use a fourth pigment as well, colored black; this color

system is called CMYK (Cyan, Magenta, Yellow, Black). Although, theoretically, black can

be created by the superposition of the three primary subtractive colors (CMY), in practice

creating the black color is difficult by such superposition, because it is difficult to create abso-

lutely monochromatic pigments (for instance, there might exist traces of cyan in the magenta

pigment, etc.). In such cases, the black obtained will have shades of green, blue, or red. On

the other hand, creating the black color by superposition of three pigments is not economic.

To get a large number of colors, the three primary colors used by printers are mixed

in various proportions. This mixing can be physical or optical. The physical mixing of colors

is only possible with liquid inks and it implies that two or more colors of ink actually mix to-

gether before drying. Since printers use inks that dry rapidly, the colors to be mixed must be

applied to the paper simultaneously or in rapid succession. Only a few printers are based on

the physical mixing of inks to increase the number of colors they generate.

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Brother IMPACT DOT MATRIX PIRNTER User Manual Page 10

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