A black and white photograph may have over a hundred shades of gray. A color photograph may have upward to three
million colors in it, but most printing presses use only four process inks: Cyan, Magenta, Yellow and Black.
Black-and-white printing requires only black ink. To create various shades of gray during the reprodution of an
image, we use a process called screening. Screening breaks an image into a series of dots. Varying the dot sizes
approximates shades of color.
Using halftoning to reproduce shades of gray.
Printers create halftone printouts using screen frequencies, which are measured in lines per inch (lpi).
A screen frequency can be represented by a grid, much like the one shown below. Each small square in this grid is
a halftone cell, capable of holding one halftone dot. Higher screen frequencies produce finer halftone screens.
Lower screen frequencies produce coarser, or more loose halftone screens.
a
b
c
The same picture printed at
33 lpi (a), 53 lpi (b) and 75 lpi (c). Low screen frequencies give coarse raster.
Printers are able to produce a certain amount of dots per linear unit. This value is called a printer resolution
and is abbreviated as dpi (dots per square inch). Though copy machines are able to create 600 dots per square inch,
these are constituted only as pure black dots of uniform size. To produce a halftone dot, which has a variable size,
the printer combines a few of dots. On one hand, the more dots that are combined, the more shades of a color are
printed. On the other hand, the larger halftone dots are more visible. In other words, the low screen frequencies
(bigger halftone dots) allow the printer to reproduce more halftone shades and tints, while the higher screen
frequency dots are less visible. This rule limits the quality of images that can be printed on laser copy machines.
| In laser color copiers a special
dithering technique is often applied to provide higher quality (less visible dots) without
sacrificing the range of available colors. This is why the halftone screen is less
visible on color prints. Some printers (like most of ink-jets) use screening
technologies other than halftoning (mostly so-called stochastic screening).
Such prints are composed from small dots with irregular sizes and locations.
Line screens are always comprised of regular shaped dots organized in lines and
columns. |
