Reducing, or Eliminating, Objectionable Rosettes

When screens of cyan, magenta, and black are overlaid at their respective angles (105º, 75º, 45º) they form a moiré pattern called a "rosette." If the printer is required to use a fairly coarse AM/XM halftone screen (e.g. 85-150 lpi (newspaper & magazine work) ), then, depending on the image color content, the rosette pattern can become visible enough to be objectionable.
One way to reduce the visibility of the rosette structure is to move to a finer AM/XM screen which makes the rosette smaller and hence less visible. However, if that is not possible, then changing the separation method might be a viable option.

The majority of RGB to CMYK image conversions use "GCR" as the method (it is the default separation technique in Adobe PhotoShop). This ensures that wherever C, M, and Y inks are used black will be introduced. The result is a very visible rosette structure as seen in the left image below.GCR----------Click image to enlarge----------UCR
To reduce, and even eliminate most rosettes, a better strategy is to use the UCR separation method on problematic images. UCR separations (see image on right above) unlike GCR separations, primarily introduce black only in neutral and near neutral color areas. Since very little, if any, black is introduced in C and M screen tint areas – no rosettes are actually formed in those areas and hence no rosettes are visible. The result is smoother, less grainy appearing color.
While the UCR separation technique can reduce or even eliminate rosettes, there is a downside in that there will be a slight increase in ink usage as well as a slight reduction in color stability through the pressrun. That is why it should be used only for images with problematic colors - primarily dark blues and purples as well as dark skin colors/areas.

Creating Custom Halftone Dots

When we think about halftone dots we're usually thinking in the traditional terms of Round, Elliptical, Square, etc., however, halftone dots don't have to be restricted to such simple shapes. For specialty projects you might consider using a custom halftone dot (click on images to enlarge).

Perhaps a "Star" dot to reflect the iconic status of the subject:
Or you could even use the subject itself as the dot shape:
How to create a custom halftone dot

A halftone screen is built using several components. The two that are needed to create a custom halftone are the "spot function" which defines the shape of the dot and the "threshold array" which determines how each dot is created.
The spot function can be any graphic, including a photographic image. It should be visually simple, made up of 256 levels of grey and fit into a square shape.
For this example we'll use the Apple logo for the spot function - making it our custom halftone dot.To convert it into a threshold array we'll use the blend tool in Adobe Illustrator:On the left is the original logo in Illustrator. Next is the logo at two sizes - the small black apple in front of the larger white logo. The third graphic is a 256 level Illustrator blend of the small black apple and large white one. The graphic is then imported into PhotoShop and cropped to minimize the amount of white in the graphic. This will become our threshold array: The Apple logo threshold array.
To apply the custom halftone dot:

1) In Photoshop, reduce the apple image to make a "dot" the size desired for the final image. For example: an image that is 75 pixels wide would make 8 dots per inch for a 600 pixel wide image. Use "Image"--> Image Size to change the entire image as required.

2) Select the apple image with the Rectangle Selection Tool. Then choose Edit--> Define Pattern. Give it a name (in this case "Apple logo") and press OK.
3) Halftone the original image by choosing Image--> Mode--> Bitmap. For "Method", choose "Custom Pattern" and then choose the "Apple logo" pattern. Then enter an Output Resolution. The amount entered will determine how small the halftone dot will be in the final image. The higher the number, the smaller the dots. Choose a resolution that is a multiple of your target output device's resolution. For example, if your output device has a resolution of 2,400 dpi, choose 2,400, 1200, 600, or 300 dpi for the bitmap.
Click OK.

Voila! Steve Jobs is now rendered with a custom halftone using the Apple logo as the dot shape.
While black and white images are the easiest to do as well as the most effective, it is possible, with a bit more experimentation to do a 4/C image. This one uses the Star dot. Click on image to enlarge:

Because you can't rotate the halftone screens for each of the process colors - the trick is to take each process color channel and rotate it to the correct angle, screen it and then rotate back so that they overlay correctly and recompose the image. Here is the step by step process: Open each channel as a new document. Rotate each channel to the correct angle: C +15º, M+75º, Y 0º, K +45º.
Convert each channel to a bitmap using the pattern/threshold array. Convert each channel back to greyscale. Rotate each one back to its original 0º state, C -15º, M-75º, Y 0º, K -45º. Finally, return each bitmapped channel into a composite CMYK image and align the channels.

Halftone screen angles

With the exception of FM (stochastic screening), all screens consist of dots arranged in a regular pattern or matrix. The vertical and horizontal distance between successive dot centers is constant and is a function of the screen frequency. When the screen is aligned parallel to the paper edges, the screen angle is said to be 0° or 90°. The rotation angle away from the vertical axis is known as the screen angle. The screen can only be rotated up to 90° before it repeats itself. For example, a screen rotated 15° is at the same angle as 105°, 195°, and 285°.
A black and white halftone image consists of a single screen. The screen pattern is very noticeable when positioned at 0° and is least visible when rotated 45° as illustrated below.For that reason, black and white halftones are usually printed with 45° angled screens – particularly with coarser screens.

When two (or more) screens are printed on top of each another, a visually objectionable pattern known as moiré may occur. The most serious moiré patterns occur at very small angles between screens. Below are two overlaid halftone grids angled at 5 degrees and 10 degrees apart with the resulting moiré pattern:
The best angle between two screens that is least likely to cause moiré, and is most forgiving to small degrees of error, is 45°. However, in four color process printing, four different screens must be superimposed and all four screens must be angled within the 90° limitation.

A set of standard screen angles has been established that is based on a combination of theory and experience. First the least visible color, yellow, is placed at the most visible angle 0° (90°). Then the most visible color, black, is placed at 45°. The cyan and magenta are then placed between these two. Cyan at 15° (105°) and magenta at 75°. These angles represent a best all around compromise for most pictures and represent the standard, most commonly used screen angles. They also form the least objectionable moiré – the rosette pattern (more on rosettes here).
Because the Yellow printer is only 15° from the Cyan printer it produces moiré. The visibility of the moiré can be exacerbated if the Yellow becomes contaminated by ink traveling into it from previous press units. To help reduce the visibility of the Y/C moiré, most screening systems run the Yellow at a slightly higher frequency (lpi) – typically 108% of the frequency of the C, M, and K printers.Left: Yellow at the same frequency as Cyan. Right: Yellow at a higher frequency to help reduce visible moiré.

These standard screen angles are based on analog photomechanical screens and do not work best with electronic screens. At angles other than 0° and 45° a type of moiré patterning within one screen "single channel moiré" may occur. To avoid this problem, some vendors utilize shifted angles of 7.5° to introduce "noise" around the edges of the dots in order to break up and eliminate the visibility of single channel moiré.

Most printers will have a standard screen angle set that is used for all their jobs. However, if certain jobs have images where two of the process colors predominate and where those two colors are less than 30 degrees apart, then that screen set should be avoided and a different one used instead.

The following screen angle sets are all valid and are in common use. The sequence for the screen sets listed below is C, M, Y, K (i.e. the first screen set on the list is: 15C, 45M, 0Y, 75K). Remember that screen angles have quadratic symmetry so 0 degrees is the same as 90, 180, and 270 degrees.

Standard 4/C U.S. screen angle set:
15, 75, 0, 45 (possible moiré in greens since C and Y are only 15º apart)

Standard 4/C European screen angle set:
15, 45, 0, 75 (possible moiré in greens since C and Y are only 15º apart)

Other usable screen angle sets: Keep in mind that when two colors are less than 30º apart there is a risk of moiré
15, 45, 0, 75
15, 75, 0, 45
15, 45, 30, 45
45, 15, 0, 75
45, 75, 0, 15
75, 15, 0, 45
75, 45, 0, 15
75, 15, 60, 45

For 2/C jobs (e.g. duotones): Other angles can be used, however, the guiding principle is that the angles should be 30º or 45º apart and that the darkest color should be at 45º to reduce its visibility and lessen "sawtoothing" (see below)
Dark color at 45
Light color at 75

For 3/C jobs (e.g. tritones):
Darkest color at 45
Medium color at 75
Lightest color at 15

For 5, 6, or 7/C jobs (e.g. Hi-Fi color):
Use the angle of the unused color
Violet/Blue uses Yellow or Black angle
Green uses Magenta angle
Red/Orange uses the Cyan angle
Note that, depending on the original CMYK separation, the Black screen angle may be available to be used for one of the extra colors - V/B, G, or R/O.

Dealing with the Yellow printer moiré issue
Interscreen moiré becomes more visible when the angles of any two screens are less than 30 degrees apart. Yellow is usually allowed to be less than 30 degrees because it is such a light color that the moiré is usually not visible. Also, the frequency of the yellow printer is usually made higher than the other three colors (typically around 108% higher) to further minimize the visibility of the moiré. However, the moiré can become more visible if the yellow printer becomes contaminated/dirtied by the preceding process colors, or if its density is too high.

So, when skin color predominates:
15, 45, 0, 75 (avoids M/Y conflict/moiré but introduces C/Y conflict)

Or when light greens predominate:
45, 75, 0, 15 (avoids C/Y conflict/moiré but introduces M/K conflict)

Some printers use a coarse FM screen instead of a conventional AM screen for the yellow printer.This eliminates the moiré issue completely since FM screens do not have a fixed frequency or angle. For a 175-200 lpi AM screen an FM screen of about 35 microns would be used since that dot size will have a dot gain similar to the AM screened colors.

Other screen angle considerations
In certain circumstances, depending on the size of the graphic and the frequency of the halftone, the selected screen angle can distort the accurate rendering of images.

In the below graphic, the halftone screen angle is the same (45º) but the angle of the gray lines have been changed.Note how the screen has affected the rendering of the gray lines at different angles. The artifact at 1, 2, 3, and 4 is referred to as "ribboning" and is fairly common in automobile images.

In the below graphic, the halftone screen angles have been changed to the standard 4/C process angles (K 45º, C 15º, M 75º, Y 0º) but the angle of the three gray lines have been kept the same (0º).Ribboning has appeared in the Cyan and Magenta angles while the Black and Yellow angles have caused the appearance of different dotted line effects.

In the below graphic, the halftone screen angle is the same (45º) but the angle of the gray box has been changed in 10º increments.Note how the smoothness of the edges of the box changes as its angle relative to the halftone screen angle changes. The ragged appearance of edge of the last box is referred to as "sawtoothing."

Screen angles for more than four color - i.e. "Hi-Fi" printing (5, 6, or 7 colors)

Four color CMYK process printing is a good compromise that achieves a wide enough color gamut for most applications while using the minimum number of inks to achieve it. However, sometimes the printer needs to go beyond 4/C in order to achieve a satisfactory rendition of the image. Typically the gamut deficiency will be in the overprint colors - Red/Orange, Blue/Violet, Green.

Here is an original RGB image:And here is the CMYK version of it:To restore some of the original color impact, the printer may choose to use "bump" or "touch" plates to boost color back into areas where it was lost. However, adding extra colors causes problems since all possible screen angles have already been used by the C, M, Y, and K printers.
In this example, these are the four channels that make up the image:Note that there is virtually no Cyan in the Red/Orange areas, or Yellow in the Blue/Violet areas, or Magenta in the Green areas. Therefore, those screen angles become available for the extra bump inks. So the trick is to use the screen angles of these unused colors.
In this example Violet, Green, and Red:In short, the Violet ink would take the unused Yellow angle, the Green ink would take the unused Magenta angle, and the Red ink would take the unused Cyan angle. Note also that, depending on the original CMYK separation, the Black screen angle may be available to be used for one of the extra colors - V, G, or R.

Rosettes – everything you didn't realize you needed to know

Rosette basics

Printing depends on halftoning to simulate shades of gray, color, and image detail. In four color process printing, four halftones – one for each of the cyan, magenta, yellow, and black inks are overlaid to produce the image. Unfortunately, overlapping two or more halftone grids can create an objectionable pattern called a "moiré" which, interestingly is the basis of the rosette.
Here, the overlaid halftone grids are 5 degrees and 10 degrees apart:

Here, the overlaid halftone grids are 15 degrees and 20 degrees apart:
As you can see, the greater the difference in angle between overlapping grids, the smaller the resulting moiré and the less apparent it is.
Here, the overlaid halftone grids are 30 degrees and 45 degrees apart:

Once the second grid has been rotated to 45 degrees, the moiré pattern is at its smallest and at a sufficient viewing distance seems to disappear.

Because a halftone screen is a quadratic grid (e.g. 90 degrees appears the same as 0 degrees, 135 degrees is the same as 45 degrees) the largest angle difference possible between two screens is 45 degrees, while the largest angle offset between three screens is 30 degrees (90/3=30). As a result, the defacto standard in four color printing has the three most visible process colors 30 degrees apart (C at 105 degrees, M at 75, and K at 45). Since Yellow is the least visible color it is angled at zero degrees – just 15 degrees from cyan. To further reduce moiré, the yellow screen is usually run at a higher frequency – typically about 108% of the other process colors.

The two kinds of rosettes

When screens of cyan, magenta, and black are overlaid at their respective angles (105, 75, 45) they form a moiré pattern called a "rosette."
To make the structure easier to see, here is the same graphic but with C, M, and K all black. Note that the yellow screen is not included since, because of its higher frequency, it does not form part of the rosette.

This type of rosette is called a "dot-centered" or "closed-centered" rosette because each of the patterns has a dot in its center.

Here is a gradient using the dot-centered rosette:
The second type of rosette is called a "clear-centered" or "open -centered" rosette. It is created by shifting one of the process colors one half row of dots from the other two colors.
Here it is in color:
And in black only for clarity:
And as a gradient:
In general, dot-centered rosettes:
• show a less visible pattern than clear centered ones
• have individual dots that land on top of one another - reducing chroma/gamut slightly
• produce color slightly differently than clear-centered rosettes
• tend to lose shadow detail
• with slight misregistration cause significant color shift
• are more popular with low screen frequencies - 100 lpi and lower

In general, clear-centered rosettes:
• show a more visible pattern than dot centered ones
• look slightly lighter due to more paper showing between dots
• produce color slightly differently than dot-centered rosettes
• tend to preserve shadow detail better
• resist color shifts better when slight misregistration occurs
• are more popular with high screen frequencies - 150 lpi and higher

Halftone dots are built inside halftone cells. Those cells have to fit together seamlessly. In order to rotate the screen, you have to rotate the cell – and there are only certain frequency/angle combinations at a given resolution where this seamless tiling is possible. The result is that at screen angles other than zero and 45 degrees, like cyan and magenta, the angles are not exactly as requested. As a result, the rosette can drift from being clear-centered to being dot-centered.

In this image the cyan is off by just two degrees and you can see the rosette going from dot-centered in the upper left to clear-centered in the middle and back to dot centered in the lower right:

In black only for clarity:
And reduced in size for clarity:
As it can appear in an image:
A well designed halftone screen will usually be able to maintain a clear-centered rosette across the largest diagonal plate that will be used. A less well designed screen may see "rosette drift" occurring over a distance of a few inches.

Rosette drift can also be caused by slight press misregistration caused by issues such as back sheet flare, web growth, or "waggle" (lateral sheet movement in the press). In this case rosette drift is not localized but occurs in the entire press sheet area.

In register - clear-centered rosettes:

Out of register by one half row of dots - now dot-centered rosettes with a subsequent tone and color shift:

With either cause of rosette drift, the problem can appear in presswork as:
• a moiré. Since a rosette is itself a high frequency moiré it is very sensitive to angular shifts.
• as "noise" or a grainy appearance in flat screen tint areas. This is because as the rosette drifts it has the effect of lowering the frequency of the halftone.
• as a shift in tone as the clear-centered rosettes are filled with a dot and then cleared again.
• as a color shift as the overprinting colors change their relationships with the shift from clear-centered rosettes to dot centered rosettes.

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