We use Stochastic screening to produce all the plates we use for Litho Printing. The process is only possible because of the exceptional accuracy of the image produced by the 10 Micron Lasers we use in our Platesetters.
Just in case anyone is interested to know the word Stochastic is derived from greek - (stō kas΄tik) and means random.
The huge advantage of Stochastic Screening over Conventional as used by the majority of printers is the clarity it brings to images, which are close to photographic quality and small text especially white text out of colours made up of tints of Full Colour. The text remains perfectly sharp around the edges rather than ‘fuzzy and ill defined’ in conventional printing.
Image enlarged showing the difference between Stochastic and Conventional screening.
The difference between Stochastic Screening and Conventional lies in the way that half tones (tints) are formed. With Conventional screening the half tones are formed by laying down dots of varying size depending on the percentage of tint. This may be described as AM (Amplitude Modulated). The dots are evenly spaced on a grid. The gauge of this grid is described by lines per inch. Newspapers typically use 75 lines per inch screens while high end printing will often work on 175 lines per inch.
With Stochastic Screening there is no regular grid. Instead dots which are all exactly the same size and much smaller than the smallest dot used in Conventional Screening are randomly placed in the half tone areas. The way that different tints are produced is by varying the number of dots in a particular area. This may be described as FM (Frequency Modulated).
Using 10 micron lasers the quality of image we produce may be compared with what would be produced with a 600 line per inch conventional screen (although it is not actually possible to achieve such a high level of Conventional Screening).
A happy side effect of laying down many more smaller dots is that it gives a cleaner (higher chroma) colour. This is because the many, many small dots create much more "edge" interface where the edge of the dot and paper each contribute to the visual effect. Each dot has a fixed area of "optical dot gain" due to the refraction of light through the ink film. The distance from the true edge of the dot and the "apparent" edge of the dot remains the same regardless of dot size. Where there are many small dots instead of a few large dots, the area of "optical dot gain" is much larger, thus reducing the direct reflection of paper. This produces higher image cleanliness and more colour overall.
In addition, because Stochastic screens random it is impossible to get Moiré patterns if scanned images of previously printed work are incorporated in to artwork. Previously printed scans can be left in sharp focus rather than being ‘blurred’ to remove the original screening. This produces much better end results for this sort of image.
The reason that very few printers use Stochastic Screening is that it throws up a number of technical challenges on Press and requires an exceptional level of Press cleanliness and Maintenance. We believe it is superior technology and we put in the resources to us it.