Stochastic Screening or Traditional Screening?

Stochastic Screening or Traditional Screening?

Characteristics and differences of the two most common screens technology


If we look at a printed product with a magnifying glass we will notice a curious fact: what at first glance appeared to be a continuous image turns out to be a constellation of tiny colored dots. It is a phenomenon that is very difficult to observe with the naked eye (you can try it with photographs printed in magazines) but it is completely normal in offset printing and is closely connected to a technology called screening. Let's see it in more detail.

What is screening

Dither consists in the correct sizing and distribution on the substrate to be printed of those colored dots that the magnifying glass has revealed to our eyes. Only in this way, in fact, will our brain organize them into an image with meaning. Not unlike what happens with the frames of a film, screening tricks our perceptual apparatus by transforming a set of colored dots into a colorful autumn landscape. The order of magnitude of the dots is one micron (one thousandth of a millimeter), so they are almost never visible to the naked eye.

To understand how dithering works, let's imagine we need to print a color gradient: a gradient ranging from full red to pure white. The printing press can only print the red, leaving the white to be given by the unprinted sheet. But how does this work for the infinite intermediate steps of our color gradient? Simple: where red is more vivid the dots will be larger or more frequent; as we move toward white the dots will be smaller or less frequent.

Traditional screen and stochastic screen

There are two screening techniques to choose from depending on what we need to print, the type of paper and many other parameters to be evaluated case by case. Let's continue for convenience to imagine our single color shade, knowing that the principle of operation remains valid even in the case of an image of a forest of Canadian maples in full foliage. In this case several screens will have to be superimposed, so that dots of several colors, usually 4 (yellow, cyan, magenta and black), are juxtaposed.

The traditional screen (or AM screen)

In the traditional screen (also called Amplitude Modulation) the colored dots vary in size but not in position; that is, they always have the same distance between them. Larger dots will result in sharper colors; smaller and smaller dots will result in increasingly blurred areas.

The stochastic screen (or FM screen)

The stochastic (Frequency Modulation) screen does not work on the size of the dots but on their frequency in space: where the dots are denser, the color will be fuller, where, on the contrary, they are sparser, the color will become lighter. It should be noted that the position of the dots does not follow any pattern but is random ("stochastic" means "random"): to the eye our color gradient will appear more natural and better overall.

Differences between traditional and stochastic screens

Let's go back to our autumn landscape. In 4-color printing with traditional screen, the screen of each ink (yellow, cyan, magenta and black) must be properly angled with respect to the others to avoid the so-called "moiré effect", an annoying optical interference that can occur when printing small textures, such as those of fabrics, or certain graphic patterns. This risk is avoided by stochastic screening precisely because of the randomness with which it distributes the printing dots.

Even small out-of-register defects in the print are mitigated by stochastic screening. For some types of work, this type of screening allows a better definition of photographic images; at the same time it softens the contrast and gives a slight feeling of graininess. At the production level, stochastic screens have a positive effect on ink consumption, in some cases significantly reducing it.

If you still have doubts or questions please contact us. We will evaluate the type of screen that best suits the printing project you have in mind.