Editor’s Foreword: This article is a consolidation of a series of four sequential blogs, each building on feedback and questions from readers of the previous postings. The original readership for the blogs at http://qualityinprint.blogspot.com are mainly in the commercial printing industry, and so some of the examples focus there, but the information and methodology explained are equally applicable to other print processes, especially flexo where hybrid AM screening originated.
Hybrid AM screening (a.k.a. XM screening) is a method to compensate for resolution issues in the print production process in either plate, plate imaging, processing, press condition, or a combination of those areas. When there is an issue with resolution, it typically reveals itself as an inability to hold small dots in highlights and/or shadows. For example, in this image:
Poor resolution has caused tone clipping—a loss of highlight dots on the girl's cheek and hair as well as plugging and loss of detail in the shadows.
This screening method began in flexography as a way to recover the loss of highlight and shadow dots resulting from the low-resolution rubber-like plates and plate exposure methods used in that process.
In flexography, small highlight dots either fail to image on plate, or if they do, they may not have the strength to hold up under pressure on press and simply bend over, creating "scum" dots and harsh tone breaks. This screening method has recently been marketed to offset printers as a way to recover highlight and shadow tones that might otherwise be lost.
The underlying screening technology is typically the vendor's conventional AM screen and is indistinguishable from it as this image shows—top gradient AM, bottom gradient Hybrid AM/XM:
The only differences occur at the extreme highlights (1%-3%) and shadows (97%-99%).
How Hybrid AM/XM screens overcome resolution issues
Here is an unscreened gradient:
If we look at just the highlights, this is what the 1%-3% dots should look like when it's screened (in this case at 240 lpi):
However, if the plate has low resolution, or the CtP device has problematic resolution, or if the plate processing has issues, or the press condition is not optimal, then there may be a loss of highlight dots. In this example, the 1% dots are lost:
Hybrid AM/XM screening recovers the lost part of the tone range by constraining the size of highlight and shadow dots so they never get smaller than a size that can be held through the plating/printing process. For example, if the smallest reproducible dot is a 2% or 98% dot, then that is the smallest the system will image. Recovering the 1% tone when only 2% dots can be used is done by imaging 50% of those 2% dots in the 1% tone area. The result looks like this:
Hybrid AM/XM screens are so called because they leverage a technique borrowed from FM screening (see February 26, 2009 blog entry). Dots are all the same size (2% in this example), placed in pseudo-random fashion with their frequency (number) changed to vary the tone.
Here is a 4/C conventional AM screened image:
Compare this with a Hybrid AM/XM image:
Although these are at different lpis, you can see the important difference that is at the extreme highlights of the gradient.
Possible issues with Hybrid AM/XM screening
The dots used in flat tone areas are discontinuous as is shown here with a 3% AM tone on the left and 3% Hybrid AM/XM tone on the right:
This can result in grainy appearing flat tone areas, pastels, and light screen tone values of black. The larger highlight dots are subject to more dot gain as solid ink density varies and, hence, may become more visible in the reproduction. Gradients may appear "noisy" at the transition from gradient to unprinted page.
Evaluating Hybrid AM/XM screening offerings
- Only compare AM and Hybrid AM/XM screening at the same lpi (i.e. 175 lpi AM to 175 lpi Hybrid AM/XM or 240 lpi AM to 240 lpi Hybrid AM/XM).
- A 1% dot a 240 lpi is a single pixel imaged at 2400 dpi (10.6 micron).
- A 1% dot a 150 lpi is two pixels (10.6 micron each) imaged at 2400 lpi (21 micron).
- Do not assume that the inability to print a single pixel is the fault of the press. A press in reasonable mechanical/chemical condition can print a 240 lpi AM screen. Separate the print production process to plate, plate imaging, processing, and press condition to determine where the resolution limitation is taking place.
- In printing, the tone scale is typically identical to the vendor's conventional AM screen offering - only the size of highlight and shadow dots are constrained. The range of tones that are constrained may be predetermined/preset by the vendor, or many be adjustable by the customer, depending on the vendor's implementation.
- Vendors differentiate themselves by how well their screens transition to the XM tone area and the smoothness of those tones.
- Vendors also differentiate themselves by whether they allow the printer to set the minimum dot sizes themselves or whether it is fixed at a certain value by the vendor.
Hybrid AM Screening/XM Screening -
Addendum Part 1
I've been asked to provide some guidance on how to go about isolating which of the problem areas (plate, plate imaging, processing, and press condition) might be the cause of the resolution limitation that creates the need for a Hybrid AM/XM screening workaround.
This is actually an important topic with broad-reaching implications, especially if you are considering a CtP purchase—no matter what halftone screening you use.
Background - Resolution vs Addressability
As one important feature, vendors describe their output device's (CtP, inkjet, etc.) capability in terms of "dots per inch" (dpi) output resolution. For example, the Fuji Luxel V-8 is listed as having "Eight multiple resolutions supported from 1,200 to 3,657dpi" while the Heidelberg Suprasetter family is listed having a "resolution 2,400 or 2,540 dpi.” Unfortunately, dpi does not define resolution. Instead it defines "addressability." In other words, dpi tells you how many locations a spot of energy can be focussed on—not the actual size of the spot of energy (or splat of ink).
Resolution vs Addressability
A CtP device uses a beam of energy to expose the plate:
The exposing beam of energy is guided by a grid—much like the grid of a city map. However, instead of locating streets using X/Y coordinates, the grid locates the target pixel location/address for the laser exposing energy:
In the above example, the addressability grid has 2,400 locations per inch ("2,400 dpi"). Therefore, each location is 1/2400th of an inch, or 10.6 microns in size—the same as a 1% dot at 240 lpi. The energy beam, following the grid, is then swept across the media to expose/image it.
This graphic shows the media being exposed at 2,400 dpi by six different CtP devices:
Note that they are all 2,400 dpi—that is that they all can hit the target location with their beam of energy. However, the exposing spots of energy are all different sizes, in this example ranging from about 2 microns on the left to about 30 microns on the right.
So, what's the big issue about using/needing a Hybrid AM/XM workaround?
For CtP, if the device is unable to image a single pixel (1% dot at 240 lpi/10.6 micron at 2,400 dpi) the argument can be made that it cannot image the rest of the halftone screen tone range consistently. This is because the halftone dots themselves are made up of individual 10.6-micron spots/pixels.
Left - Coarse AM screen. Center - High lpi AM screen. Right - FM screen
On large dots, or coarse AM screens below about 133 lpi, inconsistent dot edges due to an inability to reliably image 10.6 micron pixels will have little effect on the final presswork – the loss is within the "noise" of the system. However, as halftone dots become smaller and made up with fewer pixels, as with finer screen rulings (above about 175 lpi or FM screening), the impact in dot consistency, and therefore presswork, is much greater – one pixel lost when only 4 pixels make up the dot is a significant loss. With FM screens which may use only single pixels to make a tone, or draw "worms" as in the rightmost graphic above - the loss of a pixel or two can make a significant tone shift or contribute to a grainy appearance in flat screen tint areas.
Since the industry trend is towards finer, not coarser halftone screens, the ability to reliably image 10.6 micron pixels, in turn it is argued, becomes more important when making an investment in CtP equipment.
Hybrid AM Screening/XM Screening -
Addendum Part 2
A bit of resolution detective work
To determine if a particular CtP device might be the cause of the resolution limitation that creates the need for a Hybrid AM/XM screening workaround, one would imagine that the published specifications from the various CtP vendors would be the best source. So I began looking for an unambiguous statement of imaging capability in their imaging specifications/features. For example, for one of their CtP devices they state: "2,400 dpi, 1%-99% at 240 lpi using conventional AM screening (depending on plate resolution)." Unfortunately, many of the vendors don't appear to disclose information regarding their imaging capability.
The bottom line
To determine the imaging performance of most of the CtP devices on the market, you will need to engage a sales representative to provide you with a clear statement and specification in writing. At the least, the information needs to include: device "resolution" (dpi), maximum lpi, type of screening at that maximum (AM or Hybrid AM/XM), tone range using conventional AM screening (e.g. 1%-99%), and FM capability expressed in microns (e.g. 10 micron, 20, micron, etc.)
Hybrid AM Screening/XM Screening - Addendum Part 3
In the Addendum, part 2, I looked at the published specifications from the various CtP vendors to determine whether they provided information about their CtP device imaging capability and, hence, whether there would be a need for a Hybrid AM/XM screening workaround for specific devices. In general, the information they provide is quite vague. So, in this post I'll be looking for that information in the published specifications for plates.
A bit more resolution detective work
Once again Kodak (followed closely by Agfa) is the top vendor as far as clarity and consistency of stating their plate resolution capability is concerned. From the information they provide, one can tell exactly what the resolution limitation of their plates are (e.g., Agfa :Amigo supports a 21/25 micron minimum dot and requires a Hybrid AM/XM screen to go above 200 lpi . The same goes for Kodak Electra Excel.)
Some vendors provided either no, or vague information (e.g., Fuji Brillia Thermal: "Excellent tone and dot reproduction,” Heidelberg: No information provided)
So, as with the CtP devices, in order to determine the imaging performance of most of the CtP plates on the market, you will need to engage a sales representative to provide you with a clear statement and specification in writing.
However, you can also run some tests yourself to validate the vendor's CtP imaging and plate combinations. To do that you will need a test target such as the PIA/GATF Digital Plate Control Target.
The digital file provides a means of monitoring exposure level, checking imaging resolution, diagnosing directional effects or image inconsistencies.
Validating CtP device/plate resolution capability
The Digital Plate Control Target should be imaged at five locations on the plate—the center and four corners. After the plate is processed, the targets are checked under a loupe to determine the resolution capability of the CtP/plate combination. There is an informational box in the test target that lists, among other things, the horizontal and vertical resolution as well as direction of travel through the imaging device, which is helpful in interpreting information provided by the various targets
Horizontal and vertical microlines
These are examined visually and provide a quick indication of the exposure level and resolution capability of the CtP/plate combination. If the CtP device images at 2,400 dpi then each 1 pixel microline will be 10.6 microns thick (1% dot at 240 lpi). Proper exposure is indicated when the positive and negative microlines are imaged at the same width. If the one pixel lines are not rendered this indicates a resolution limitation with that particular CtP/plate combination. In that case, check the two or three pixel lines instead to determine the resolution threshold. Also note if the vertical and horizontal microlines are rendered equally well. Inconsistencies with imaging vertical and horizontal microlines indicate directional differences in the output system.
One pixel through four pixel checkerboard
This target is extremely sensitive to the resolution capability of an imaging device. Nearly all CtP/plate combinations will have trouble rendering the 1x1 pixel checkerboard sharply. If the overall appearance of the checkerboard is indistinct with soft edges between the positive and negative pixels, then the resolution of the CtP/plate combination has been exceeded. Due to their lack of resolution, many CtP/plate combinations cannot successfully image less than the three-pixel checkerboard at a 10.6 micron pixel size.
Put another way, they cannot resolve halftone dots made up of less than three pixels and as a result require a Hybrid AM/XM solution to recover highlight and shadow tones between 1% - 3% and 97% - 99% when the screen ruling is finer than about 175-200 lpi. They may also be restricted as to whether they can do FM screening and/or the level of fineness of FM screen they can reliably image. On a related note, it is argued by some vendors that because it is the consistency of imaging of the perimeter of the halftone dot - made up of 10.6 micron pixels - that determines the consistency of halftone dots throughout the tone scale, an inability to reliably and consistently image the 1x1 pixel checkerboard indicates a CtP/plate combination that is not optimal as far as delivering consistent plates to the pressroom is concerned.
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About Gordon Pritchard (a.k.a. Gordo)
Gordon Pritchard was formerly Print Quality Marketing Manager for eleven years at Creo/Kodak. He presented at print technical conferences and trained printers and buyers regarding print quality issues in Europe, North America, and Southeast. Asia.
Gordo has articles published in trade journals, co-authored TAGA paper on halftone screening, and authored BRIDG's guide to halftone screening. Previously, he was Technical Director of Western Canada's largest commercial sheet-fed shop.
For several years, Gordo was Professor of Digital Graphic Design at Emily Carr University. He was formerly Creative Director at McCann Ericksson Vancouver. and is currently a Freelance Graphic Designer, Photographer, Illustrator, Consultant.
Gordo can be contacted at: qualityinprint@gmail.com.
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