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SCREENING ALTERNATIVES -- Screen Out the Competition

March 2005
BY MARK SMITH

Technology Editor

Few things are as fundamental to the color offset printing process as screening. Add to that the experience built up with conventional screening, general resistance to change and some technical issues, and it's easy to understand the industry's caution when it comes to alternative screening solutions.

The category of enhanced screening technologies now has been expanded to include what are referred to as "hybrid" systems, along with true stochastic or frequency modulated (FM) screening. Hybrid technology claims to combine the benefits of conventional, or AM (amplitude modulated), and FM screening. One group of specialty screening solutions that won't be considered here are the new spot color replacement systems that belong in a product class by themselves.

To help printers gauge the merits of alternative systems, the Graphic Arts Technical Foundation (GATF) has conducted several independent tests. The basics are covered in the "Stochastic and Hybrid Screening Printability Study" report that GATF released in 2003.

According to the researchers, this study was designed to show the differences between currently available conventional and stochastic/hybrid screens, particularly the products from the seven suppliers that participated in the study. Measurements were collected for density, tone value increase, print contrast and gray balance, then the data were compared to show the differences between conventional and stochastic/hybrid screening solutions.

Raise Your Tone Value

The major differences seen with stochastic screens were in tone value and print contrast, notes the report. Tone value, on average, was 10 percent higher compared to conventional 150- to 200-lpi screened work. This can be attributed to the fact that there are many more dots in a stochastic image, which introduces more area for tone value increase, the researchers explain.

However, samples printed with 10 micron stochastic screens were found to exhibit the same degree, or even less, of tone value increase as 20 to 28 micron work despite having more dots. This was because a curve had been applied to the plates, the report continues.

Another factor cited as possibly influencing the amount of tone value increase associated with stochastic screening is spot shape. Spot shapes associated with stochastic screens vary by manufacturer and, in some cases, vary within a manufacturer's screening options.

Hybrid samples, on the other hand, had tone value characteristics similar to the sheets with 175- to 200-lpi conventional screens, the study found. This is said to be due to the fact that hybrid screen dot patterns vary throughout the tonal range, and in midtones are very similar to a conventional screen.
 

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