Stochastic (FM screening) VS. Conventional (AM screening) 
Thursday, July 6, 2006, 03:29 PM - Other
At Wizard Graphics, we use almost exclusively stochastic (actually staccato, Kodac/creo's 2nd generation version of stochastic) screening. Along with a handful of other new technologies, stochastic offers marked improvement over conventional screening processes.

First of all, you might be asking- What is screening? Screening is the process by which printers convert color images into halftones, or rows of colored dots. By printing dots that are the additive primary colors of Cyan, Magenta, Yellow, and Black, we can trick the viewer into perceiving a full range of colors. This is what we call 4 color process printing.

Conventional (AM) screening has been used basically from the beginning of the modern printing industry. Originally color images were photographed using a color filter (essentially, a tinted piece of translucent plastic that only allowed C,M,Y,orK information to pass through to the camera aperature), in combination with a piece of clear film with actual screen lines imaged upon it. These films were later stripped together, to produce color seperations for the press. Conventional Screening is also referred to as Amplitude Modulation. This is because with this method, the dot size is varied, to achieve different values. Small dots make lighter tones, while large dots make darker tones.

This illustration shows a conventional dot gradient.


Stochastic (FM) screening is a relatively new technology designed to reproduce the same colors as conventional screening, but with higher fidelity. Stochastic is also referred to as Frequency Modulation screening. With stochastic, different screen values are acheived by varying the number of dots. Unlike conventional, every dot printed is the smallest dot that can be reproduced. To make light tone values, fewer dots are used, while darker tone values are achieved by printing more dots.

This illustration shows a stochastic dot gradient.


The difference in detail between stochastic and conventional can be seen in the following images.

Let's say that our photograph looks like this.


When we look at a magnified section of this image printed conventional, here is what we will see.


The same section will look like this when printed stochastic.


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