CONVENTIONAL PRINTING vs. STOCHASTIC PRINTING

Advantages of stochastic printing versus conventional printing:

• Continuous tone photographic reproduction
• Produces a larger CMYK color gamut on press
• Renders greater detail
• Eliminates moire patterns
• Reduces ink consumption by as much as 10% – notice the ‘pooling’ of ink in conventional dot
• Produces smoother gradients
• More consistency in color throughout pressrun
• Faster ink drying

Conventional screening (150 lpi, 175 lpi, 200 lpi) refers to AM screens, or amplitude modulation. This refers to halftone dots that are fixed on a grid, angled in 30 degree increments (except yellow: 15 degrees) and grow in size based on tonal value.

LPI = lines per inch

Stochastic screening (Staccato) refers to FM screens, or frequency modulation. This refers to micro-dots (20 micron, 10 micron) that are FIXED in size and tone values increase by adding more dots. The dots are rendered in a psuedo-random algorithm making them ideal for high definition details in photography and artwork. The micro-dots are rendered in a ‘weave’ to create very smooth tonal transitions.

Micron =1/1,000,000 of a meter

It’s important to note that stochastic printing produces a larger CMYK color gamut than AM screens. This occurs because light reflecting off the paper is filtered more efficiently, resulting in less ‘whiteness’ from the paper reflecting into the eye.

Also, reprints are less likely because of the stability in controlling color on press. FM screens are much less likely to be impacted by ink density variations on press. The ink film on press is much thinner and less likely to be affected. Notice in the enlargement photo above the ‘pooling’ of ink in the conventional halftone dots. This causes the press to use more ink than is necessary.

FM Frequency          AM Frequency

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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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Color Separations

In order to reproduce or prepare a full color image for printing using four process colors, the image must be divided into the the individual subtractive primary color components. The separation process can be accomplished photographically or electronically.

• Photographic Separations: Using a large process camera, a full color image is converted into halftone negatives which contain a series of dots of various sizes to represent shades of gray. The dots are achieved with the use of special screens placed over the negative material during exposure. When printed, the smaller dots create the lighter areas of the image, with the lightest appearing white, and the larger dots make up the darker areas of the image, with the darkest appearing black. This system of using the halftone screens directly with the film is known as the Direct Screen Photographic Color Separation method. A halftone negative is made for each of the separate subtractive color components of the image (cyan, magenta, yellow) and black. The four halftone negatives are records, in varying shades of gray dots, for each of the separate color components and are known as "Color Separations".

Another photographic technique known as the Indirect Photographic Color Separation method is also used. The original image is separated into the individual primary color components and continuous tone films are made for each of the separations. From continuous tone films, the halftone negatives or positives are created. The advantage in first creating continuous tone separations before creating the halftones is that the continuous tone separations can be reduced or enlarged and corrected before the actual halftone films are produced.

Color separations created from photographic methods are used to prepare analog plates for traditional printing methods. Each plate corresponds to one of the four colors, cyan, magenta, yellow, or black. The plates are coated with the corresponding ink colors during the printing process and the inked image from the plates is transferred to the substrate. The varying sizes of the halftone dots on the plates determines the amount of ink that will be printed on the substrate. The dots from each of the different colored images overlap to produce the full color image.

• Electronic Separations: The electronic separation of an image into its individual color components has become the most popular and labor saving method of producing color separations. The original image is placed in a scanner which stores the image as a digital record. The digital image can be imported into a software program such as Adobe PhotoShop™ where the color separations can be produced with the click of a mouse. The scanned image and/or the color separations can be quickly manipulated and edited in an infinite number of ways with the tools provided with the software. Color separations created electronically can be output to film to prepare analog plates or they can be used for Computer-to-Plate applications and digital printing.

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