The Wayback View – The pressman's hat

One thing you notice very quickly when rummaging around old photographs of print shops is that most press operators are wearing stylish headpieces made out of a sheet of newspaper.
Here's publisher Eugene Meyer in March 1954, proudly wearing his newspaperman's hat, as he presses the "Print Button" to run the first issue of the Washington Times Herald.
Records show that these hats were worn by press operators as early as 1748 as protection against grease, oil, and paper lint which otherwise might get in their hair.

Here, in 10 steps, are instructions for making your very own.

1. Start with the largest 4 page newspaper sheet you can find. Fold it once along the center line.
2.With the crease at the top, fold the top left and right corners in until they meet at the center of the page. Crease the new folds.

3. Fold a single thickness of the open edge up about an inch and crease. Then fold the same flap again as far as it will go and crease again.
4. Flip the entire hat over so that the top points away from you.

5. Fold the outside left and right edges towards the center and overlap. The greater the overlap, the smaller the hat will be. They do not need to overlap if you need a larger hat size - but not overlapping will make construction a bit more difficult.
6. Now fold the lower corners up to the hat band and crease.

7. Fold the bottom piece up and over the band and crease. Then tuck the end into the band to form the brim of the hat.
8. Fold the triangular peak down to the bottom of the hat band and tuck the tip into the brim.

9. Pick up the hat. Open wide and flatten out the inner area to create a rectangular shape. As you do so you will see two triangular shapes form - not quite as large as in the drawing though. Fold them so that you can tuck their tips into the brim.
10. Crease the corners square on all four sides, flip the hat over and try it on for size:Oooops!
For this post I started with the largest newspaper I had - a Japanese broadsheet with a whopping 32" x 22" sheet size.
Either newspaper sheet sizes have gotten smaller over the years - or pressmen's heads have gotten bigger!

Update January 27, 2010: Here is a video showing how to make a pressman's hat courtesy of the Fort Wayne Newspapers press project:
Click "play" arrow to view video.
A slightly different method is shown here courtesy of the Palace Press New Mexico History Museum. The main difference occurs with the last few folds:
Click "play" arrow to view video.

GCR Reseparation for ink savings and color stability in offset printing

Special note: This posting on ink savings via reseparation was republished by Print Action magazine in December of 2009 and subsequently became the basis for the IPA Ink Optimization RoundUP technical evaluation of ink optimization software presented at the IPA Technical Conference in June 2010.Ink storage vats at a magazine printing facility in Australia
Reseparating customer supplied files is rapidly gaing popularity with printers and publishers as a way to lower print manufacturing costs.
Although the application is not limited to specific print market segments - these are the ones that are most quickly adopting this technology:
• Newspaper publishers
• Insert and flyer printers
• Magazine publishers
• Catalogers
• Directory printers

Calculating ROI

Although there are many benefits to reseparating customer supplied files, the most promoted and fairly easy to justify is in regards to reduced ink usage - typically suggested as a savings of around 20% in CMY inks with an increase of about 6% in K ink used while maintaining the same visual appearance in presswork.

Based on that figure, calculating a return on investment seems fairly straight forward. For example - based on the industry average of ink consumption for a sheetfed printer being about 2% of their gross earnings, a $10 million dollar a year printer will spend $200,000 a year on ink. If they reduce their ink usage by 20% they will save about $40,000 a year in ink costs. Theoretically, if the printshop spent $10,000 on a reseparation solution their payback time would be just three months and they will have saved $30,000 in the first year of implementation - a very good investment.

However, the promoted savings are typically based on the ink saving resulting from converting a "UCR" separation into a "heavy GCR" separation (see part 4) and then extending that savings to total annual ink usage - a method which doesn't really reflect the actual print production experience. One reason is that the majority of separations are already done using GCR since it is the default setup with the most popular separation profiles used by Photoshop. For example, this image was separated using Photoshop's defaults:while this:was separated using heavy GCR. In this case the resulting CMY savings is about 15% with an increase in K ink consumption of about 6%.

Another consideration is the amount of CMY used in daily production vs the amount of black only presswork. For example, if your typical color ratio usage is low on CMY vs K as in this example:
you will not benefit as much as if this is your typical CMY to K ink ratio:
So, to get a better idea as to what your ink savings ROI might be, you should also consider taking a representative sampling of materials that you are currently receiving and examine the separation techniques used (part 4 will have some tips for determining separation technique used). You might also look at your current presswork to determine the ratio of CMY ink usage (where the savings should occur) relative to black ink.

Benefits of GCR in the press room

Ink usage
The difference in ink usage of GCR vs a UCR separation, for the same final visual appearance on press, can be substantial. While ink usage, and therefore ink savings, is image content dependent, it is not unusual to see an average reduction of 15-25% in ink usage with GCR separations compared with UCR separations (see art 1 for ROI). Note that the savings occur primarily with the often more expensive chromatic inks.

Color stability
Because a GCR separation uses a non-chromatic color – black – throughout the tonal range and reduces the proportion of C, M, and Y in the mid- and quarter tones, the color in GCR separated images is more stable as solid C, M, and Y ink densities naturally vary through a press run. Note, however, that the added stability means less ability for the press operator to move color if required. For many printers, the increased color stability is a perfect compliment to the industry trend for a “by the numbers” print manufacturing process.

Other benefits
• Reduced make-ready times/faster start-ups/less wastage
• Harmonized separations enhance press form printability
• Reduced fan-out or web growth
• Dramatic improvement of image appearance when slight press misregistration occurs
• Reduced drying times
• Higher printing speeds
• Improved repeatability of print jobs
• Grey balance within images is more stable

Key basic concepts

Converting raster images from an RGB colorspace into a print CMYK colorspace has two significant impacts:

1) Typically a compression and alteration of colors as the image is transformed from the original RGB gamut to the different gamut used for CMYK presswork.The pixels of an RGB image (left) plotted against the color gamut of a CMYK press (right) shows that much of the original color is outside of press gamut and therefore cannot be accurately reproduced.
2) The on-press printability of the imagery in terms of color stability, press performance/runnability, and ink usage (i.e. cost).The CMYK image on the left uses more ink than necessary for reproduction and will cause ink drying and offsetting issues. The image on the right does not use enough ink and has compromised color gamut and stability on press.
Converting images from one CMYK separation condition into a different CMYK separation condition by reseparating files is primarily intended to enhance the printability of the imagery while maintaining the appearance of the original CMYK imagery. Put another way, reseparating CMYK files is effectively a way to optimize press forms.

The principle of RGB to CMYK separation

In order to go to press, RGB color images must be converted to their process color counterparts; cyan, magenta, and yellow. An achromatic black channel is added because if the color black in presswork is just made from CMY it can often appear “muddy” or "patchy." Also, making dark colors from the three chromatic process colors can lead to a higher than desirable volume of ink on the press sheet. Neutral colors made up of three process colors are also more difficult to maintain consistent on press as solid ink densities normally vary through the run compared with a neutral made primarily of a single black ink. The net effect of introducing black ink in process printing is a reduction of ink usage/costs, stabilization of color (especially gray tones), and and better printability.

The conversion process is done by taking the 3 channel RGB image, passing it through a 3 channel device independent CIEL*a*b* profile connection color space where the RGB is converted to CMY and the black channel added, and finally outputting the result as a 4 channel CMYK image.
The two approaches to replacing chromatic colors with black

The colors in an image can be made up of a variety of different percentage combinations of the process colors while still delivering the same final visual appearance. As a result, there are a several ways to introduce the black component in a color separation.

The first separation method is typically referred to as UCR (Under Color Removal) Click image to enlarge.Whenever large percentages of three process inks are together to form a color, there will be a substantial neutral or grey component. That is to say, past a certain point, adding more of one of the C, M, or Y process colors simply darkens the result. So, instead of simply adding more color, the UCR formula uses black ink to replace a partial amount of the other process colors in the shadow areas and in neutral colors. For example, in a screen tint build of 50% cyan, 40% magenta, and 45% yellow, magenta acts as the greying process. So, the 40% magenta value indicates the largest potential amount of black that can be added in place of a percentage of some of the other colors.

The second separation method is typically referred to as GCR (Gray Component Replacement) Click image to enlarge.GCR is a specialized form of under color removal which involves a more general replacement of chromatic inks - not just the close to neutral ones as with UCR. In a GCR separation, black is substituted for a partial amount of the process colors in all areas where the three chromatic colors are present – even in lighter tones. That added range of color where CMY can be replaced with black is the key to how GCR reduces CMY ink usage.

Both UCR and GCR separation techniques use a defined parameter to limit the maximum amount of ink that will be used in the darkest part of the image. This maximum total dot percentage is referred to as either TIC (Total Ink Coverage) or TAC (Total Area Coverage) and is determined according to the type of presswork and substrate that the images will be used for.

The difference in the range of the black channel makes it fairly easy to determine which method was used to create a particular CMYK separation. With a UCR separation the black printer appears as a “skeleton” image usually missing tones around the 0%-30% range while the CMY looks very colorful. In contrast, the black printer in a GCR separation looks more like a full range greyscale image while the CMY looks washed out.

CMYK to CMYK transform – Reseparating documents for press

Reseparating CMYK makes it possible to optimize CMYK data targeted for one, or more, print conditions to work with the required single CMYK print condition. It further allows the changing of the maximum total ink area coverage as well as the application of GCR separation techniques to maximize ink savings and on-press color stability. Reseparating images also renders all separations in multi-page and multi-file source input projects such as magazines and newspapers to a common, optimized, color separation format which ultimately enhances the print-ability of the job.

The reseparating of documents is typically done using a sophisticated DVL (DeVice-Link) profile which may be embedded within the primary prepress workflow/RIP or housed in a dedicated system linked to the main prepress workflow. Unlike ordinary ICC profiles, DVL profiles do not describe a specific color space, but instead define the conversion from a specific source color space to a destination color space.The basis for creating a DVL profile is always an ordinary ICC profile. The use of a DVL profile allows the integrity of the color separations to be maintained so that pure C, M, Y, and K screen tones remain pure and that solid (100%) C, M, Y, and K values remain solid.

Potential issues with GCR separations in the press room

Maximum GCR separations may create problems on press depending on the type of work that you do. Overall though, the benefits by far exceed any issues encountered that might occur in daily print production.
Here are the major concerns to watch out for:

• A GCR separation makes extensive use of black ink throughout the tonal range. This places a greater emphasis on the integrity of the black ink and press unit, particularly in a black first down ink sequence. If there are issues of poor transfer, trapping, dot gain instability, or ink/water issues with the black printer the impact on presswork will be greatly magnified with a GCR separation. Also, because black is used throughout the tone range, there is a slight increase in the visibility of rosettes in large, flat, screen tint builds. There can also be a greater risk of black ink traveling down to and contaminating the next ink units. Lastly, because of the greater amount of black ink, there is a possibility of pastels and skin tones to have a grainy appearance.

• Reseparating images using heavy GCR can introduce subject moiré (black screen angle conflicting with dark pattern in the original) where there was no moiré in the original separation.

• Ink savings can be quite dramatic with furniture and other dark imagery, however, if the GCR program is too aggressive and removes all of the Cyan the furniture can simply go red.

• Heavy GCR separations are problematic if the print buyer is the type that prefers to go to press runs and adjust color rather than match the signed-off proof. Since there is little process color to adjust, solid ink density moves to adjust color will not have much impact.

• Poorly implemented GCR algorithms may remove all of the color near a natural trap line in an image - effectively negating the trap. Then, if the press gets a little out of register the reseparation process has created a press issue.

• There is an increased disconnect between gray balance targets in the color bar (CMY no K) and gray balance within the live image area (minimal CMY heavy K). This may cause issues with some closed loop press control systems.

• In coldset newspaper printing where yellow is first down, there will be less yellow ink laid down and hence, a reduced sealing effect of the paper prior to the other process inks being overprinted. This can lead to less saturated appearing color in image reproduction.

• If there are any concerns expressed by the print buyer about the quality of the color in the final presswork, the fact that the supplied separations were not used but substituted with reseparated images may increase the printer's liability for reworks or charge backs.

Considering the various options and offerings

This post is intended to help you narrow down a list of requirements and features (and understand their impact), and provide questions to consider when evaluating a GCR reseparation solution. In general, there is no right or wrong answer, but rather a weighing of values and impacts to determine a best fit for your shop. As a general rule (which is not always true), the simpler the implementation, the less expensive, flexible, and accurate it is while delivering the least amount of ink savings. The more complex the implementation, the greater the expense, flexibility, and accuracy it has while delivering the most amount of actual ink savings.

1) Device Link Profile creation/validation

• Is the DVL profile created/supplied by the vendor, created by the customer, or supplied by a third party?
In general customer created DVLs require greater customer sophistication while supplied profiles equal greater simplicity.

• What is the DVL profile created from? Generic industry reference characterization data/profile (e.g. SNAP, GRACoL 7, SWOP), or customer specific data?
In general, customer specific data provides the best potential for quality since industry data may not reflect actual shop conditions even though the shop may be running to an industry standard. Industry derived DVL profiles provide a simpler implementation.

• Who supplies the customer specific characterization data - the customer themselves, the vendor or third party measuring customer supplied press sheets, the vendor or third party doing on-site press runs?
In general, customer supplied characterization data is the least reliable, vendor or third party conducting on-site press tests the most reliable.

• How will the press run that will be measured to provide the characterization data, be validated to ensure that it is operating within normal parameters and hence will provide reliable data?
In general, it is worth having the press checked for mechanical and chemical condition by the vendor or third party prior to any characterization run

• How will the DVL/or solution be evaluated in order for customer acceptance and sign-off of delivery of the solution?
In general, there should be specific performance criteria (e.g. maintain single color colors, no contamination of solid colors, etc. (subject will be covered in next post.

• Will validation be based on a proof (pre- and post-reseparation), press run (pre- and post-reseparation), or data (pre- and post-reseparation)?
In general, a soft on screen and hard physical proof (pre- and post-reseparation) are sufficient to show color integrity and ink usage reduction.

• Can the reseparation parameters of the DVL profile be changed by the customer or does this require vendor intervention?
In general, allowing the customer to adjust the parameters themselves makes for a finer tuned process that can be quickly adapted should press conditions change. However, it requires a degree of customer knowledge to be successful. If the vendor needs to be involved, then fine adjustments will take longer and likely be more expensive. However, it does place responsibility on an organization that likely has greater resources and knowledge which, in the end, may lead to a better final result.

2) Reseparation implementation

• Is the GCR reseparation done inside the main prepress workflow or is it run on a dedicated workstation?
In general, GCR reseparation done inside the main prepress workflow provides a simpler, faster process. GCR reseparation done on a dedicated workstation provides a more customizable and updatable solution. Some vendors offer both, or allow the customer to scale from one to the other as circumstances change.

• Does reseparation occur during the initial PDF refine stage, after the PDF refine stage (perhaps at a dedicated workstation) or during the rendering of the files to plate?
In general, early stage reseparation means that print buyers can see that their files have been altered/optimized if they proof the files using a remote soft proofing solution or if they receive the files back from the printer either during the production process or afterwards. This may, or may not, be a concern. Late reseparation at the render stage means that the altering/optimization would be hidden.

• Can the solution handle vector graphics separately from raster images?

• How does the solution handle documents containing a mix of RGB and CMYK images?

• How does the solution handle documents containing a mix of embedded profiles?

Testing the selected solutions

Once you've narrowed down the solutions that you think will provide the best fit for your business, it's a good idea to run a test to see how closely their promise meets their performance. To perform a test requires running a test form through the solution and examining and comparing the results.

This suggested press test form can be used as a basis to build your own.The following test elements are designed to provide an objective evaluation of the solution's performance.

Targets 1-4 and 18 are designed to test the ability of the solution to maintain the integrity of pure single and two color builds. The elements should be butted together and trapped to see how trapping is affected.
1 - CMY gray balance vignette and step wedge.
2 - CMY brown balance (CMY have the same tone values) vignette and step wedge
3 - C, M, Y, K vignette and step wedges should remain single color with tone values preserved.
4 - B (C/M), R (M/Y), G (C/Y) vignette and step wedges should remain two color only.
18 - Maximum TIC/TAC targets ranging from too low (e.g. 230%) to too high (e.g. 400%) to see how the solution brings TIC/TAC to a common requested value.

The following test elements are for subjective visual evaluation.

7 - Gray balance image separated GCR using PhotoShop defaults.
8 - Gray balance image separated UCR using PhotoShop defaults.
7 and 8 Are intended to test preservation of gray balance through reseparation
9 - Saturated and deep skin tone colors. Should be separated according to your existing standard.
10 - PDF file to test ability of solution to handle mix of images and text.
11 - GCR (medium) separation using black limit 100% and maximum ink 320%
12 - GCR (medium) separation using black limit 90% and maximum ink 280%
13 - UCR separation using black limit 100% and maximum ink 320%
14 - UCR separation using black limit 90% and maximum ink 280%
11 to 14 Are intended to reveal how different types of separations are brought to a common setting through reseparation and whether any color appears distorted as a result.
15 - Deep color with lots of black using UCR with a black limit 100% and maximum ink 320%. This is intended to show any loss of richness or any color shift in deep shadows as well as artifacts in the transition to shadow tones.
16 - High key pastel original, originally separated using UCR with a black limit 100% and maximum ink 320%. This is intended to show how pastels with heavy black GCR performs on press.
17 - Vector (e.g. Illustrator) graphic. To see how, and if vector graphics are affected by reseparation.

If you have any unusual image requirements, such as graphics that overlay spot colors on top of CMYK, or process color images that include a 5th or 6th spot color channel, be sure to include those as well.

Once the test elements are returned by the prospective vendor they should be imposed in line with the original test elements. Add elements:
5 - Gray balance bars which go the width of the press sheet
and
6 - Your preferred color bar.
The top half contains the test images/files as supplied by you to the vendor. The bottom half are the same images processed by the vendor's GCR reseparation solution.Create two proofs of this imposed form.
Cut the proof in half so that you have two pieces - one that contains the original images and the other containing the processed images. Take the proof of the processed images and cut it into strips through the center of the images. Then lay the strips on top of the proof of the original art.The images should fuse together looking no different where they cross over. Examine the objective elements 1-4 to determine whether they have maintained their integrity and 18 to determine how the different TIC/TACs have been handled.

The next step is to validate the increased color stability on press by going on press with the imposed form. The press operator should be instructed to run to the appropriate solid ink densities (SIDs) trying to even them out across the width of the sheet. The proof should not be available - the press sheet should simply be "run to the numbers."
Once the SIDs have been reached - pull out several sheets for later evaluation. Then raise the magenta SIDs by 10 points (i.e. 1.35 M becomes 1.45 M) and pull some sheets. Then raise the magenta SIDs by a further 10 points (i.e. 1.45 M becomes 1.55 M) and pull another few sheets. Then do the same increase, but with the cyan SIDs. Again, pull sheets at the appropriate increased SIDs.

Because the images are inline with each other, the increase in SIDs gets applied to both conventional and reseparated images. By cutting the form in half and comparing strips of the images, as was done with the proof, you should be able to see the difference in color stability between the processed images and their originals.

Hybrid AM Screening/XM Screening

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 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 (2% in this example) size, placed in pseudo-random fashion with their frequency (number) changed to vary the tone.
Here is a 4/C conventional AM screened image:
To compare with a Hybrid AM/XM image:Although these are at different lpis you can see the important difference which 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 offset 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.

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 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 metal plate CtP, if the CtP 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.

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.

Kodak is the top vendor as far as clarity and consistency of stating imaging capability is concerned. For example, for one of their CtP devices: 2400/1200 or 2540/1270 dpi. 450 lpi max linescreen 20-micron KODAK STACCATO Screening Optional: 10-micron STACCATO Screening.

A few of the vendors provided a bit more information about some of their devices - but not others (e.g. Agfa which gave more info for their :Palladio than their :Avalon series).

Here are the vendors that provided the least amount of information - just the "resolution" of the device (see addendum part 1 post regarding that metric).
Heidelberg Suprasetter, Lüscher XPose! thermal, Agfa :Avalon N series, Krause Smart ’n’ Easy Commercial, Screen PlateRite 8800.

Some of the vendors simply used vague meaningless terms: FFEI Alinte is "FM capable", Lüscher XPose! UV can do "FM Coarse".

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.)

Even with that, you may need to validate resolution – a topic which will be covered in the addendum part 3 post.

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Below, in alphabetical order, is the list of vendors and devices I checked, including the specifications they publish either on their web site or in their brochures. I did not list all the devices from a particular vendor if they shared common specifications (e.g. Agfa :Avalon N series).

Agfa :Avalon N series: Resolution: 2400 dpi
Agfa :Palladio II: Output resolutions: 1200, 2400, 3000 dpi. Screening technology :ABS 200 lpi- :Sublima 200 lpi

ECRM MAKO 8x: Resolutions: 1800 dpi to 3556 dpi. Maximum Line Screen: 200 lpi

FFEI Alinte 8 Page: Resolutions 1200 to 3657 dpi. AM screening up to 200 lpi. 1-99% dot reproduction (plate dependent) FM capable

Fuji Luxel V-8: Resolutions supported from 1,200 to 3,657dpi Line screens up to 200lpi
Fuji Luxel T-9800CTP S: Resolution 1200 / 2400 / 2438 / 2540 dpi

Heidelberg Prosetter: Resolution 2,032 / 2,400 / 2,540 / 3,200 / 3,386 dpi
Heidelberg Suprasetter: Resolution 2,400 or 2,540 dpi

Kodak Magnus 800: Resolution: 2400/1200 or 2540/1270 dpi. Up to 250 lpi max linescreen Optional: 25-micron STACCATO Screening
Kodak Magnus 800 Quantum: Resolution: 2400/1200 or 2540/1270 dpi. 450 lpi max linescreen 20-micron KODAK STACCATO Screening Optional: 10-micron STACCATO Screening
Kodak Trendsetter III: Resolution: 2400 dpi. Screening: 200 lpi max linescreen Optional: 25-micron KODAK STACCATO Screening
Kodak Trendsetter III Quantum: Resolution: 2400 dpi. 450 lpi max linescreen 20-micron STACCATO Screening Optional: 10-micron STACCATO Screening

Krause Smart ’n’ Easy Commercial – Platesetter: Resolution 1,016 – 2,540 dpi
Krause LS Precision V8: Resolution 1,016 – 2,540 dpi Spot size 25 – 10 μm

Lüscher XPose! thermal: Resolution 600 to 2540 dpi
Lüscher XPose! UV: Resolution 2,400 dpi 60 L/cm (150 lpi), 80 L/cm (200 lpi) and FM Coarse

Presstek Dimension Excel: Resolution: 2540 dpi / 200 dpi[SIC] 2540 dpi / 200 dpi [SIC]
Presstek Compass: Resolution: Continuous variable resolutions of 2032 to 3048 dpi. Screen Ruling up to 250 line screen
Presstek Dimension Pro 800: Resolution: 2400 dpi or 1200 dpi. Screen Ruling: up to 250 line screen

Screen PlateRite 8800: Resolutions (dpi): 1,200 / 2,400 / 2,438 / 2,540
Screen PlateRite Niagara: Resolution: 2400, 2438, 2540 dpi (Note: There are currently no halftone dots that can be used at 2,438/2,540 dpi.)

Now let's be look 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 5 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 combination 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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Below, in alphabetical order, is the list of vendors and plates I checked, including the specifications they publish either on their web site or in their brochures.

Agfa :Amigo: Resolution 1-99% with :Sublima 240, 200 lpi. 25μ FM
Agfa :Ampio: Resolution 1-99% dot rendering at 200 lpi
Agfa :Azura: Resolution Up to 2-98% at 200 LPI depending on imaging conditions
Agfa :Energy Elite: Resolution 1-99% at 200 lpi. FM and :Sublima 280 lpi capable depending on platesetter
Agfa Lithostar Ultra LAP-V: Resolution 1%-99% at 200 LPI

Fuji Brillia Thermal: Excellent tone and dot reproduction

Heidelberg: No information

KodakElectra XD: Resolution 1% to 99% @ 250 lpi with Kodak SQUAREspot Imaging Technology, FM capability 10 micron stochastic
Kodak Thermal Platinum: Resolution: 1% to 99% @ 400 lpi dependent upon capability of imaging device. FM capability 10 micron stochastic dependent upon imaging device capabilitites and screening algorithms.
Kodak Electra Excel: Resolution: 1% to 99% @ 200 lpi. Dependent on capability of imaging device. FM capability 20 micron stochastic. Dependent on screening algorithms.
Kodak Sword Excel: Resolution 1% to 99% @ 200 lpi. Dependent upon capability of imaging device. FM capability 20 micron stochastic. Dependent upon screening algorithms.
Kodak Thermal Direct: Resolution 1% to 99% @ 175 lpi; 1% to 98% @ 200 lpi. Dependent on capability of imaging device. FM capability 25 micron stochastic. Dependent on screening algorithms.
Kodak Thermal Gold: Resolution 1% to 99% @ 250 lpi. Dependent on capability of imaging device. FM capability 10 micron stochastic. Dependent on screening algorithms.
Kodak Violet Print: Resolution 2% to 98% @ 200 lpi, platesetter dependent.

Presstek Aurora Pro: Resolution 1% - 99% @ 200 LPI or FM
Presstek Anthem Pro: No information
Presstek Freedom Pro: Resolution 2%–98% @ 175 LPI

Southern Lithoplate Viper: Resolution 1-99% @ 200 lpi Screening FM Screening Certified
Southern Lithoplate Cobra: 1-99% @ 300 lpi FM Screening Certified

AM and FM gamuts compared

This short video (click on the "play" arrow) shows a 20 micron FM (Kodak Staccato) gamut (translucent) over top of a 175 lpi AM screen gamut. The characterization data used to build these two profiles came from press sheets run to GRACoL 7. The FM screen had a curve applied to the plate to align its tones to the AM screen.

Here is a view from the top looking down:
And here is a view from below looking up:What's important to note is that the FM gamut volume is larger (more chroma) than the AM screened gamut. The difference will be seen primarily in one and two color screen tint builds in the 10% to 90% tone areas. Since it is not usual for raster images to contain areas of only two colors, the difference in gamuts may not be always visible in image reproduction. The difference in chroma at specific tone values will also affect the hue of screened Pantone spot colors.
The increase in gamut is the result of the smaller dots of the FM screen covering more of the paper compared with an AM/XM screen at the same reproduced tone value, rather than how the dots are arranged. Hence, if the frequency of the AM screen is increased to about 350 lpi, its gamut will increase and will closely match that of the 20 micron FM screen.

A higher resolution of the gamut comparison video is available - contact me at pritchardgordon@gmail (dot) com for a link to download it.
Technically speaking, FM - or more properly microdot screening - does not actually increase the gamut (as demonstrated in part 1). Instead, it is more accurate to say that FM reduces the potential gamut less than the larger dots of an AM screen does. The function of ink in printing is to filter light, when that happens you see color according to what part of the spectrum is filtered by the ink.
So, how is it that FM screening increases the gamut (as demonstrated in part 1)?
Some light passes through the film of ink and is filtered by it as it is reflected of of the substrate. Some light scatters in the substrate under the dot of ink causing what is called optical dot gain - a colored shadow around the dot of ink. Some light passes between the dots and comes back through the film of ink. While some light is simply reflected off of the surface of the dot rather than actually passing through it.
However, some of the light is not filtered by the ink. Instead it goes between the dots of ink and is simply reflected off the surface of the substrate. This unfiltered light ends up mixing with the light that's been filtered by the ink and contaminating/greying it.
Because an FM screen distributes more dots of ink per tone area, and because the small dots have a greater perimeter to area ratio (more optical gain) the result is that more light is filtered by the ink. There is much less empty space between dots for light to reflect without being filtered.
For comparison here is a 15% and 40% tone in AM:
and here are the same 15% and 40% tones in FM (Kodak Staccato):
You can easily see that there is more ink coverage of paper with the FM screen hence more opportunity for light to be filtered. As a result, less white paper contamination and therefore less of a loss of gamut.

There is another factor at work which helps FM dots retain more of the possible gamut of the ink.
As ink thickness increases its becomes a less efficient filter of light. Instead light tends to reflect of the surface of the dot. FM screen dots have a more uniform film of ink that is thinner than that of an AM dot at the same tone value.
Here is a photomicrograph of a Magenta AM dot on the left and FM dot on the right both representing the same measured tone value on press:
Below is the photomicrograph transformed in 3D imagery that plots density to height:
Note the thickness of the film of ink for the AM dot. Note also the different densities across the surface of each AM dot. Those micro areas of increased density are effectively areas of reduced ink filtering ability.
Lastly, FM screens, because of their thinner ink films, dry slightly faster than the larger thicker ink AM dots. Hence overprint trapping in screened areas is slightly more efficient.
All of those factors contribute to FM screening delivering a wider gamut than AM screening.