The Design Division

Print Quality Explained – High definition print (FM Screening)

Not all print is the same. Different printers have different standards and capabilities, specifications that may sound similar can be very different in practice. This article discusses the difference between Stochastic (FM) and Conventional screening (AM).

AM Screening

AM screening (Amplitude Modulation,) utilizes a fixed linear dot pattern with various sized dots to emulate the tonal range in photographic images and colour builds. Standard AM line screens vary in resolution depending on the reproduction process and equipment quality. In commercial offset printing, these line screens are typically recognized as 100, 133, 150, 175, and 200. These numbers represent the number of dots per linear inch. The larger the dot the darker the image area, and the smaller the dot the lighter the image area.

Colour images utilize a separate AM screen for each of the primary process colours, Cyan, Magenta, Yellow and Black, or CMYK, much like a computer screen utilizes the primary spectrum of light, Red, Green, and Blue, or RGB. These screens are printed on top of one another, which gives the range of colour we see on paper. The colour we see in a printed image is more of an illusion, because our eyes can only discern so much detail at a given distance. If we used a high powered magnifying glass to enlarge an area of a printed photograph the image would become unrecognizable.

Problems With AM Screening

    • Limited Miniumu Dot Size – In printing, we are limited to a minimum dot size for ink to adhere and transfer back to the sheet of paper we’re printing on. We’re also limited at the other end of the tonal spectrum, because we can only go so large with the dot before a printed area becomes a solid. This results is an inherent flaw in the process called posterisation, and we have to adjust the photographic image before printing to reduce the problems it creates on press. When we make these adjustments, we are actually degrading the quality of the original photograph slightly overall, so we lose detail, colour, and contrast. It’s a necessary step in the AM screening process, and much of this is automated with today’s technology.

Sizes of the Dots

      –  AM screening uses a fixed dot pattern, and the tonal range is achieved by amplifying the size of the dot within that fixed pattern. Printing presses can only print so small of a dot, and they have to have a printing range between the smallest possible dot and largest possible dot to achieve a tonal range. So the peak resolution in an AM Screen is set by the largest or coarsest dot, not the smallest one. For a 175 line screen, the smallest possible dot ends up being approximately 10 microns, and the largest dot ends up being approximately 200 microns.

Appearance of a Visible Pattern In The Image

    – Sometimes this pattern conflicts with the actual subject matter in the photo itself, and amplifies the negative visual effects of the process. The human mind recognizes patterns fairly easily, so anytime we incorporate a fixed pattern into a process we naturally detect that pattern. As we said earlier, colour images are built on a series of screens printed over top of one another, and these screens are turned at specific angles to reduce the inherent negative effects. The flaws are still there though, like moire patterns, and rosette patterns, but we try our best to reduce the obviousness of the technical limitation. This is one big reason companies are insisting on FM screening, especially in the apparel industry where subject matter is all about patterns. Needless to say, there can be a plethora of adverse pattern conflicts with the AM screening method.

 

Stochastic (FM) Screening

Stochastic or FM screening provides another way to screen an image for printing is to break it into a pattern of dots that have the same tiny size but are distributed according to image density. This is called stochastic or FM (frequency modulated) screening. It is the opposite of conventional or AM (amplitude modulated) screening, in which dots of varying sizes are spaced at regular intervals on an invisible grid. In an image that has been stochastically screened, shadow areas contain more dots than highlight areas. The “clumping” of the dots, not their size, determines the density of the image in any given area. Compared with AM screening, stochastic screening will produce smoother tone gradations, sharper detail, and printed colour that looks much closer to the colour of the original. This makes it desirable for high-end product catalogs, art reproductions, and other work with strict quality requirements.

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 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 the black-and-white image below, stochastic screening is seen on the right, and conventional screening is seen on the left. The colour image shows FM screening on the left and AM screening on the right.stochastic vs am printing

In the black-and-white image below, stochastic screening is seen on the right, and conventional screening is seen on the left. The colour image shows FM screening on the left and AM screening on the right.

AM vs Stochastic Printing

 

Problems With Using Stochastic (FM) Screening

  • Expensive
  • Technically Challenging

 

We believe that Stochastic FM Screening is the superior technology, and we believe you will too when you see the results! Call now on 0845 230 4810 to discuss this with our team.