Stochastic Printing

Stochastic printing, also called frequency modulation (FM) screening, uses small (10, 20 or 25 Micron), same size dots in a random pattern and varies the density of the dot to create an image that is closer to continuous tone. In the reproduction of an image, we scan a continuous tone original photograph. The scan results in light striking a photosensitive device which issues a number that digitally represents the tonality of the original image. Printing a halftone image on paper requires that this number be passed to a computer, which stores a grid of numbers in a rectangular matrix representing the original image in digital form. To convert the digital matrix of numbers into a printed halftone, the computer sends the grid of numbers to the imagesetter where it is overlaid with a "virtual" halftone screen, a mathematical matrix. The computer captures the value of the numbers and passed it to the halftone generator in the imagesetter. The numbers can range from 0 to 256, which is translated by the halftone generator into a halftone "dot". That dot may exist in any of 256 potential values. The dot is actually drawn by the halftone generator that determines its shape as it grows in size from nonexistence to solid. Varieties of dot shapes exist, from round or elliptical to diamond shaped. In addition, straight-line and star-shaped dots are also possible. Stochastic printing uses a random dot, which takes the mathematical value of the dot and distributes its components inside the halftone cell. Stochastic patterns make possible halftone printing without the use of conventional dots. As a result, problems associated with screening striped or finely detailed fashion sportswear that would result in a moir patterns are eliminated. This also makes it possible to print in more than the conventional four colors of ink. By removing the barriers of screen angle interference from the printing process, stochastic printing has made it possible for the use of more than the basic 4 ink colors. Pantone introduced hexachromatic inks (six process colors). This allows more accurate reproduction of pastel colors.

NOTE: Screening is the process after rasterizing PDF files in the RIP (raster image processor) during prepress. Halftone screening is done through software and creates very small dots, or cells, that are imaged onto a printing plate. The tiny dots create the illusion of continuous tone photographs when printed on press.

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