Stochastic screening

Stochastic screening is a halftone process based on random distribution dots, using a sort of frequency modulation to make a density of dots according to the gray level desired.

The traditional amplitude modulation halftone screening is based on a geometric and fixed order of dots, which vary in size depending on the tone color represented (for example, from 10 to 200 microns).

The stochastic screening or frequency modulated (FM) screening instead uses a fixed size of dots (for example, about 25 microns) and a distribution density that varies depending on the color’s tone.

The technique of stochastic screening, which has existed since the nineties, has had a revival in recent times thanks to increased use of computer-to-plate (CTP) techniques. In previous techniques, computer to film, during the exposure there was a drastic decline in the quality of the plate. It was a very delicate and difficult procedure that was not much used. Today, with the CTP during the creation of the plate you just need to check a few parameters on the density and tonal correction curve. When you make a plate with stochastic screening you must achieve a tone correction curve, this curve allows a better management of the "optical dot gain", increasing the reproducible tonal range. The optical dot gain is the perimeter of the points. With the same percentage of screen, an FM screening has a greater area of optical dot gain compared to a traditional screening. This feature allows the FM screen to reflect at our eyes a light composed of more color of the ink, that is not polluted by the color of the support. The creation of a plate with stochastic screening is done through an initial inspection of the machines and parameters for the creation of a proof plate. It is made a start-up machine and printed some hundreds copies. The values of the proofs are measured, and it is built on them an appropriate tone correction curve. The definitive plates will be created by applying the tone curve created.

Advantages

• The screening of 4 colors should no longer be made with 4 different angles as for the traditional screen.
• The FM screening is not based on geometric compositions such as rosettes. The Moirè effect is the interference between the inclinations of rosettes of various colors. The Moirè effect is impossible with the stochastic technique.
• You can have size of points up to 10 microns, which gives at the product a quality comparable to that of photographic prints.
• The effects of misregistration is not completely eliminated, but the effect is certainly less apparent than in the traditional screening, this feature is very favorable for printing on rotary machines where the misregistration is very common due to effects such as web growth.

Disadvantages

The small dots used in FM screening require special care and cleanliness, especially when plates are made from films.