Sunday, January 17, 2010

FM/Stochastic Screening

The Benefits
Below are listed some of the reasons why printers adopt FM/Stochastic screening for some, or all, of their presswork. In the next posting I'll be going into more detail on some of the benefits that have not been covered in past posts on this topic.

1 - No screen angle moiré
2 - No subject moiré
3 - No rosettes
4 - Photographic/contone look
Some examples are mentioned HERE
5 - Greater tone and color stability as SIDs naturally vary during press run
6 - Larger color gamut
The reason for the increased color gamut is explained in the post HERE
7 - Faster drying
8 - Reduced ink usage - About 10-15% depending on tone content of the presswork.
The reason for reduced ink usage is explained HERE
9 - Tonal and color stability when misregistration occurs
10 - Halftone dot structure stability when misregistration occurs
11 - Competitive differentiator

It's important to understand that, when it comes to the press, how halftone dots are organized is not as important as the size of the actual dot. As a result, some of the benefits of FM/Stochastic screening are available to conventional AM/XM screening if the lpi (halftone frequency) is fine enough. Specifically benefits 6, 7, and 8 in the list above can be delivered by AM/XM screening if run at an equivalent lpi to the FM halftone. As a general guide, the lithographic AM/XM/FM equivalents for a "second order" FM screen at various dot sizes are shown below.
Color stability through the press run

Printing presses are stable - but not consistent. Solid ink densities will naturally vary through the press run, and when they do two things happen; dot gain varies, and ink density on the halftone dots varies. The result is variation in tone and color reproduction through the run.
When printers measure dot gain they measure it at the 50% tone value rather than at the 1% or 2% point because at around 50% dot gain variation is at its greatest.The tone variation happens because the large midtone dots in an AM/XM screen can accept more ink than the small dots in the highlights and deep shadows. Put another way, you can’t build a column of ink on a small dot, but the large dots around the 50% tone can take on the ink so they will grow the greatest. So, the midtone color gets darker and lighter as solid ink density naturally varies through the press run.

However, the halftone dots in an FM screen have a different ink profile.

FM screens effectively have small dots throughout the tone range. They act much like the highlight dots of an AM/XM screen and do not take on as much extra ink as SIDs naturally vary - so they remain more stable and hence the color remains more consistent through the run. For example, the below chart shows the dot gain profile (solid line) of a 133 lpi AM/XM screen and a 20 micron FM screen calibrated to match that dot gain profile. The dashed lines show the dot gain response when the solid ink density is raised by 30 points (i.e. 1.40 to 1.70) to magnify the difference. The curve with the small dashes shows the response of the FM screen, while the longer dashed line shows the response of the AM/XM screen.
The color and tone stability that FM screening provides is similar to that provided by heavy GCR color separations or ink reduction re-separation applications.

In some cases this on-press stability can be a two edged sword since, like heavy GCR separations, it does not allow the press operator the ability to move color quite as easily on press should the print buyer decide to be creative during a press approval. It also means that if the press operator makes a SID change, the region of color/tone shift will not be the same as that of an AM/XM screen.

That being said, consistency in color is usually the goal of presswork so the stability of FM screening is appreciated by printers and their customers as an excellent complement to their “print -by-the-numbers” goals.

Faster Ink Drying
One of the little known benefits of FM screening is that it helps the presswork dry faster. With FM screening, the tones are built with many small dots - effectively a “mist” rather than puddles of ink. The small FM dots carry a thinner ink film, and are distributed in a finer pattern that allows the ink to flash off its volatiles quicker causing the ink to dry faster than their AM/XM counterparts.

To illustrate how this benefit is achieved, looking at the same final tone value in the below image, on the left is a microphotograph of the large AM/XM dots while on the right is magnified the smaller FM dots.
In the below image I've taken those microphotographs of the dots and passed them through a 3D rendering program that translates density into height to see just how thick the ink film is. As you can see on the right, for the same final tone the FM screen has a thinner film of ink than the AM/XM dot.
The faster drying can also be demonstrated with a simple test. Below are three images printed on uncoated book stock, the lefthand image is 133 lpi, the center is 200 lpi, and the righhand image 20 micron FM. I then used a coin to scratch across the 3/C gradient as soon as the sheet came off the press:
It's very easy to see the marks made by the coin in the 133 and 200 lpi gradient - while the FM gradient is almost completely dry and hardly shows any marks from the coin at all.

The faster drying of FM screening brings other benefits, namely:
1- It's easier to align presswork with proof - since they are both dry one doesn't have to account for dryback
2 - Reduced setoff
3 - Prints better on uncoated, newsprint, foils and plastics
4 - Less spray powder is needed
5 - Less distortion on perfecting presses
6 - Jobs are faster to bindery
7 - Heatset presses may be able to lower their drying ovens a few degrees and thus save on energy costs

LPI Integrity
Even though most printers have experienced the problem, few have understood it. LPI integrity is a benefit of FM/Stochastic screening that may be a bit difficult to understand at first.

AM/XM screens generate a rosette pattern when all the screens are at the correct angles. A rosette is a high frequency moiré and as such is very unstable. If there is a slight misregistration, what happens is that the rosette pattern of the AM/XM screen changes - typically from clear-centered to dot centered. The next few images show the results of a half row shift in mis-registration:As the rosette shifts from being clear-centered to being dot-centered - the visibility of the rosette changes. Effectively, the rosette shifts from being high frequency to low frequency. Put another way, the effective lpi of the halftone can shift from a fine 175 lpi to a coarse 85 lpi as a result of slight misregistration. So, a rosette pattern that occurs in important skin colors may not be visible when in register, suddenly becomes very visible with slight misregistration. However, FM screens do not produce rosettes and as a result they maintain their lpi image integrity when slight misregistration naturally occurs.

Color Integrity When Misregistration Occurs

You can have the best presses and the best plate imaging available, however, because you are covering a sheet of paper with a mix of oil and water, squeezing and pulling it through the press - some amount of misregistration is an inevitable fact of press life. The impact of that misregistration on color and tone reproduction will depend on whether you are using an AM/XM or FM screen.

Here is a full sized image:

And here, enlarged, is a section of the image using an AM/XM screen on the left and an FM screen on the right. Both are in register.

And here is the same section but with the Magenta for both screens out of register by a half row of dots (1/300th of an inch) - considered within normal printing tolerances.
Notice how the dot structure in the rosette visibly changes in the AM/XM screen (175 lpi in this case) when it's out of register but the FM screen (20 micron in this case) remains stable since it does not have a rosette.

More importantly note the change in AM/XM rosette structure from clear-centered to dot-centered and back to clear-centered as it drifts in and out out of register. It’s like turning a light on and off and back again - so the color goes darker and lighter and darker and lighter through the run.

Also, misregistration changes the overprint and ratios of wet and dry trap. A blue made from Cyan and Magenta dots beside each other is not the same blue as when Magenta dots overprint the Cyan. The result is a color shift. So while the AM/XM presswork is going darker and lighter it is also shifting color from bluish to redish and back again as slight misregistration occurs. This can be particularly troublesome in, for example, car brochures where there are large expansive areas of neutral grey made up of 4/c process. Again, since FM screens don’t have a rosette the color and tone are better preserved with natural slight misregistration in presswork.

The bottom line - FM screening can help deliver more presswork color consistency - which, after all, is one of the main goals of printing.

Potential Issues
So far it's all good - but what about the potential downsides of FM/Stochastic screening?

A printing press or plate does not "know" how a pattern of halftone dots is arranged - it does not know whether the screening is AM/XM or FM. So, the problems related to FM screening are not usually the result of the halftone pattern itself, but by the size of the dots that make up the halftone screen. In that respect, FM screening shares the same problems associated with any high frequency halftone - FM or AM/XM.

This old Planeta press prints breakfast cereal boxes using FM screening for press runs that can last a week.

This new KBA press prints daily newspapers, inserts/flyers and commercial work using FM screening exclusively.

Here are some of the issues that can arise using FM and AM/XM screening.

1 - Grainy appearance in flat tint areas. This usually doesn't occur with AM/XM. There are several possible causes:
1) Poor screen design and/or using a first order FM screen pattern (see HERE for explanation). This problem has been overcome by modern second order FM screens.
2) Graininess can also result from a lack of plate imaging integrity e.g. plate doesn't have a high enough resolution for the dot size being used, and inconsistent plate imaging. Again it generally occurs with older first order FM screen designs. Graininess can also occur when screening back a spot color. This is because spot colors are normally not formulated to be halftone screened and hence their pigment grind may be too coarse for the dot size being used.

2 - Inability to align presswork color to an industry standard like SWOP, GRACoL7, etc. The higher the halftone screen frequency, AM/XM or FM, the larger the color gamut becomes (this is explained HERE and HERE). The extra gamut appears in one and two color screen tint builds making them appear more vibrant than the same screen tint build using a lower lpi AM/XM screen (e.g. 175 lpi). It usually only effects vector artwork and typically does not show up in raster images. If this is a problem it can be dealt with using color management and re-separating images to contaminate color builds to reduce the gamut. Unexpected color shifts can also occur when screening back a spot color. This is because spot colors are normally not formulated to be halftone screened and hence the impact of using a high frequency screen, with its increased gamut can lead to an unexpected final color appearance.

3 - Piling. Piling may occur on heatset web presses and is usually caused by not following proper blanket washup schedules or by poor ink rheology - i.e. using inks formulated for the large dots of a coarse AM/XM screen rather than formulated for the smaller FM dots. Piling is rare on sheetfed presses - typically an ik that works with a 200 lpi AM/XM screen will work with a 20-25 micron FM screen.

4 - Mottle. Mottle (splotchy appearance of flat tone areas) can appear with any high frequency screen. It is usually the result poor ink transfer (incorrect ink formulation), fountain solution condition, or paper surface/coating characteristics. Mottle can also occur when screening back a spot color. This is because spot colors are normally not formulated to be halftone screened and hence their pigment grind may be too coarse for the dot size being used.

5 - Poor image reproduction. The higher the lpi of the screen, AM/XM or FM, the higher the detail rendering and hence the better the detail, flaws, and artifacts of an original image are reproduced. Another problem can be a "softness" to the image reproduction. This is usually caused when high dpi images are resampled to a lower resolution when the PDF is created or resampled when the image goes through the RIP. The solution is to not resample images.

6 - General failure. The most common reasons that cause printers to fail with FM and high frequency AM/FM screening that I've encountered are:
1) Poor process control in the shop. Halftone screening in the range of 65-175 lpi is very tolerant of process variations however as the dots get smaller, process controls need to be tighter. That being said, the process controls that need to be in place for FM screening to succeed are no different than those needed for an efficient 175 lpi print manufacturing process. If there are any issues - high frequency screening will make those issues more visible.
2) Lack of buy-in. If the frontline people (e.g. prepress and/or press operators) have not bought into the idea of using FM screening - it will fail. Without buy-in the change merely represents a disruption to what they see as a smooth running process. "Why fix it if it ain't broke?"
3) Lack of use. If FM screening is only used for "special jobs" then press operators never get used to how it performs on press and as a result they will struggle with it. As with most activities, the more you do them the better you get at doing them. It is best if the shop makes the commitment to change to FM and make traditional AM/XM screening the special case exception.

In future posts I will be covering success strategies for implementing FM screening as well as general process control proceedures.

About 80% of all the directory printing in North America is done using FM screening.


  1. Hi Gordon. What are potential issues with using hybrid or stochastic screening on our 10-year old flexo press running UV inks?
    Thanks, Sarah

    1. AFAIK, no vendor has commercialized FM screening for flexo application. So that option isn't available. Hybrid AM, on the other hand is very popular with flexo printers as it allows you to recover highlight and shadow tones that would otherwise be lost. Check out my blog posts on Hybrid AM/XM screening.

  2. I recently ran a job stochastic, with nice results, but when it came to the cover, which was on a different press, the press manager had switched to a 200 line screen. I called this out, and he said this press—also a Heidelberg—would not print stochastic. The cover, printed on uncoated stock, ultimately had more ink gain than I liked, which I attribute in part to the line screen. How is it that a press would know whether the plate was stochastic or not? Is it that the electronics/densitometer expect to see a line screen in the test bars?

  3. The high ink gain for the 200 lpi cover was likely because the printshop hadn't built proper dot gain compensation curves for that screen/paper combination.
    A press and press controls like densitometers don't know anything about the type of screen being used. Press controls are not affected by the halftone screen being used. The press, on the other hand, may not be capable of imaging halftone dots (AM or FM) below a certain size. That being said, typically, if the press can do 200 lpi it can also do FM. I think there is something going on at the print shop that they are not fully explaining to you.

  4. Hi Gordon

    Thank you for a very interesting and useful blog.

    We are using FM 21 µm, from Esko for all jobs to one of our customers.
    Most of these designs we are printing with blue brand colors covering the whole body which only contain small elements in cmy and that works fine.
    This morning we were printing a new design with a wide stripe made with reddish gold simulation mixed 100%Y, 40%M, 15%C.

    For some reason the magenta is fading away after a few hundred sheets and it does not help to increase density.

    Printer started at D1.35 and before he stopped they were above D1.50 and the print was much too light in magenta.
    We checked the blankets and even put in a new blanket for magenta but same phenomena.
    There were no chalk or dust on the blankets when they were checked.
    After stopping and starting again the same thing happened again - good result in the beginning but then magenta is fading away.

    The printers are using the same settings for the dampening unit printing both AM & FM jobs - normally that is obviously working for them but my guess is this is part of the problem, what do you think?

    Process calibration have been done.
    Plates are ok +/- 1 %
    Press KBA -2008 in ok condition

    Best regards

  5. Blinding is a progressive loss of ink receptivity in the image or printing area while the background remains unaffected. It's usually a sign that the image has been attacked chemically.
    Blinding can be caused by precipitation of calcium salts either from the ink (especially from the Magenta you mentioned), the paper or the fountain solution. On negative plates, calcium carbonate crystals from the paper (two to three microns in diameter) may accumulate on small dots and cause blinding.
    In this case, change the fountain solution. Sponge and bucket should be kept free from detergents and wash up solution should be kept from the plate. Clean and rinse dampener rollers/covers.

    Other causes...
    A fountain solution which is too strong may bring about over emulsification (and as a possible consequence: plate blinding) due to the effect of surfactants or wetting agents. Has the source of the fountain solution concentrate changed?
    Check with the supplier whether it contains any different wetting agents, biocides, or preservatives.
    Check pH/conductivity. Does fount solution contain too much gum or been contaminated with detergent? Is the plate incompletely developed resulting in gum on image areas? Has a proprietary plate cleaner or abrasive scratch remover been used? Ensure that the plate has been thoroughly rinsed afterwards. Ensure that plate cleaner has not been allowed to dry on the plate. Check the dampeners. If they have recently been cleaned, ensure that no detergent has found its way into the dampening system. Covered dampeners must be thoroughly rinsed after being cleaned with detergent and before use on the press. Examine the incoming water supply, to see if the conductivity has changed. Harder water can be a source of minerals that can be deposited on the plate.

    Mechanical blinding can be caused by too much ink deposited on the plate, plate-to-blanket pressure too high, durometer of rollers too hard, and abrasive particles contaminating fount solution. solvents, ink, etc.

  6. What type of plates?

    UV ink?

    Is the Image OK on the Magenta plates or is there dot loss due to CTP exposure?

    Tend to agree with minimal info that it is related to calcium.

  7. You can use any kind of plate to do FM as long as the plate vendor warrants that it has the resolution to do the screen you want to use.
    UV ink is not an issue with FM.