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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Electrographic Plates

Electrographic devices are the ultimate in simplicity. Similar to a laser printer, these devices deposit toner onto a substrate via a charged imaging drum. A special ink-receptive toner enables offset printing, but many devices can also serve as high-resolution laser printers. Manufacturers offering such devices include Printware (St. Paul, MN). The Printware 1440 can image both inexpensive “paper” plates as well as the more durable organic photoconductor (OPC) aluminum plates. In addition to being daylight safe, both options offer the advantage of being totally processless — when the plates emerge from the laser printer, they are ready to hang and run.

Xante's (Mobile, AL) recently introduced Platemaker 4 is its latest device for electrographic CTP, and extends the legacy of its workhorse Platemaker 3.

Xante users can choose between the Myriad 2 (a dry-toner plate with a polyester substrate) and the Myriad 4, which has a paper substrate.

“We made the decision to purchase the Platemaker 4 based on the positive experience we've had with Xante products,” says Floyd Black, president of Personalized Printing (Carrollton, TX). “In four years of operation, there's never been a moment of downtime on our Platemaker 3.”

Personalized Printing doesn't rely exclusively on paper plates. According to Black, it's easy for press operators to switch between aluminum plates and plates output from the Xante platemaker. “Our press operators print the jobs in the order they were received,” says the exec. “All day long, they go from Myriad 2 to metal and back, so they aren't forced to separate the paper-plate jobs from those running on metal plates.”

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Computer-to-Plate (CTP)

In the Computer-to-Plate or CTP process the image of the page from a digital files recorded directly from the file to the printing plate instead of creating film and making the plate from the film. Although CTP is a printing process, in order to insure the best possible output it is important that the designer discuss CTP with their printer. The printer's familiarity with the process, their equipment, the type of plates, and file format and preparation all play a role in the success of the computer-to-plate process.

When you eliminate film in favor of bits and bytes, your digital infrastructure must be able to keep up with production. You'll need up-to-date hardware and software and an internal workflow designed to avoid production bottlenecks. Most printers will require one or more digital proofing devices, as well as upgraded desktop publishing workstations, networks, scanning capabilities and archiving procedures.

Until recently, printers implementing low-cost CTP solutions might have opted for dye-sublimation or phase-change proofing devices; today, nearly every new CTP installation features some form of inkjet proofing. While inkjet has won broad acceptance for its strong price/performance balance, remember that not all devices are equally adept at creating contract-quality color proofs and outputting press-sheet-size imposition proofs.

• The direct-to-plate process, known as CTP (computer-to-plate), has eliminated the film output step of plate-making. Printing plates are now made directly from computer files with the use of lasers.

Eliminating the need for film means less cost to you

as well as a reduction in turnaround time.

• In the CTP process, the computer file used to make hi-resolution colour proofs for your job is the same file used to produce the printing plates for the press. This generally eliminates problems such as font or image variations between proof approval and the printed job.

• Another more important benefit of the CTP process is the quality of the “dot” that is achieved on new plate system. The direct-to-plate dots are now sharper and have less “dot-gain” than the traditional film-to-plate process.

• Craigs Platesetter constantly delivers high quality plates for printing presses. The accuracy of the plates enables shorter make-readies and the enhanced dot definition results in superbly sharp print quality.

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