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.

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.

---

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.)

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 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.

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.

AM and FM gamuts compared - part 1 of 2

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.

AM and FM gamuts compared - part 2 of 2

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.

Pantone Colors and CMYK gamut

This short video (click on the "play" arrow) shows 1113 Pantone colors (excluding metallics and fluorescents) plotted against the GRACoL7 CMYK gamut. It shows what region of the CMYK gamut the PMS colors exceed. Only 376 PMS colors (33.8%) are actually within gamut for four color process (at 175 lpi) printing. A higher resolution video is available - contact me at pritchardgordon@gmail (dot) com for a link to download it.

The Wayback View – From 1940 (video post)

Preview images from the video
Today's prepress folks often complain about how many different job functions they're expected to fill in their daily work. Well, perhaps things haven't really changed that much - as this 1 minute long video about the work of a country newspaper editor reveals.

Please press the play arrow to view the video. Note that it may stop for a moment while the video buffers in the background.

Enjoying the green on St. Patrick's Day

The issues of Optical Brightening Agents in paper and ink - part 5 of 5

The use of OBAs in paper has a significant impact on the reliability of proofing and alignment of presswork to the proof. This is an issue where the use of a black light really "shines" (see part 2 post). When the proofing paper contains OBAs the hue of pastel colors can shift depending on the amount of UV being emitted by the viewing light source as illustrated below:Although they cannot control the light under which their customers evaluate proofs, many printshops will use UV blocking filters to cover the D50 bulbs in their viewing booths. The notion is that, most of the time, the proof will be looked at by the customer under lighting with little UV content. The UV block filter helps the press operator to "ignore" the presence of OBAs in the proof/presswork.
Another strategy is for the printshop to try and align the OBA content of their proofing paper and press sheets. A black light can provide a qualitative measure of the OBA content of the media as illustrated here using a vendor's swatchbook of their proofing media:Selecting pairs of press and proofing papers according to their OBA content helps in the alignment of presswork and proofs and thereby enhances the printer's ability to set expectations correctly with their customers.

Part 1 on this topic can be viewed by clicking HERE

Part 4 on this topic can be viewed by clicking HERE

The issues of Optical Brightening Agents in paper and ink - part 4 of 5

The inks that are typically used in four color process printing block, to varying degrees, the fluorescence in papers containing OBAs. Black and magenta block the greatest amount, yellow a lesser amount, and cyan ink least of all. What this means is that when an image is printed using a halftone screen, lighter/pastel tones allow more more of the brightening and color shift of OBAs (towards blue) than the shadows. Color is effectively skewed towards the blue from shadows to highlights – but only when the paper being printed on has a high OBA content.In daily presswork this disconnect usually appears in midtones and pastels – sometimes the color matches the proof in those areas and at other times the color doesn't – depending on the OBA content of the paper being run. FM screenining will lessen this effect. The effect on an AM screen is emphasized in the image below to illustrate the issue.

Part 5 on this topic can be viewed by clicking HERE

Part 3 on this topic can be viewed by clicking HERE

The issues of Optical Brightening Agents in paper and ink - part 3 of 5

OBAs are sometimes use as additives in ink. In the example below it has been used in the yellow ink in a process set – while the paper itself contains little if any OBAs.Using OBAs in the yellow ink is a common strategy with newspaper printers as a way to add brightness to imagery and compensate for the poor whiteness of newspaper stock. The fact that the OBAs fade and cause a color shift over time is typically not a concern in that market. It is important to be aware of the OBA content of your process ink set particularly if the print specifier is concerned with the longevity of their printed materials - especially if they will be exposed to sunlight.
OBAs are also used in so called Hi-Fi inks, notably those used in the Pantone Hexachrome process, to add vibrancy and expand the gamut beyond conventional four color process. However the OBAs can make the inks more problematic in the pressroom as well as result in presswork that does not have a long shelf life (due to fading and color shift).

Part 4 on this topic can be viewed by clicking HERE

Part 2 on this topic can be viewed by clicking HERE

The issues of Optical Brightening Agents in paper and ink - part 2 of 5

While it is not practical for printers to quantitively measure the OBA content of the materials that they use, it is quite an easy matter to qualitatively see the OBA content. All it takes is an inexpensive (less than $15 USD) "black light" such as the one illustrated below (do not bother with incandescent black lights):For example, with the black light it is easy to see that the paper used for the Pantone Goe system swatch book (on the left in the image below) contains more OBAs than the conventional Pantone spot color swatchbook on the right. Also, it's clear that the uncoated paper section in the Pantone spot color swatchbook contains more OBAs than the coated section.The bottom line – the significance of which will be more apparent in the next parts of the blog on this topic – is that, although you may not be able to do anything about it, just being able to be aware of OBA content can help solve issues related to their use.

Part 3 on this topic can be viewed by clicking HERE

Part 1 on this topic can be viewed by clicking HERE

The issues of Optical Brightening Agents in paper and ink - part 1 of 5

As ICC color managed workflows become more prevalent in the graphic arts, so do the difficulties caused by OBAs (Optical Brightening Agents) that are encountered in the pressroom, prepress, and their customers. OBAs are used to increase the apparent brightness and whiteness of papers and their use is becoming more prevalent in paper manufacturing. They increase brightness and whiteness by absorbing energy in the ultra violet and emitting (fluoresce) the energy in the blue area of the visible spectrum. Because, to the eye, blue/white looks "whiter" than yellow/white OBAs are not really whiteners, but bluing agents. OBAs are also used in ink to expand gamut or brighten 4/C image printed on poor substrates - e.g. newsprint.
When fluorescence is present, the light coming from the sample is the combination of the light that is reflected and the light that is fluoresced. ISO 12647-2 printing paper grades specifies low OBA content, however, there is no specification as to the amount of OBA content. And, although they typically use light sources with little or no UV radiance, there is no specification describing the UV content of light sources in measurement instruments such as spectrophotometers. The addition of fluorescence to either the inks or the substrate greatly increases the level of uncertainty in instrument readings of the optical properties of printed images. This, in turn, may lead to a significant lack of reproducibility between two imaging centers that attempt to apply color management principles to their individual measurements of the same image printed on various substrates.

Part 2 on this topic can be viewed by clicking HERE

Print as Manufacturing?

From the print customer’s perspective, buying print is unlike any other buying experience. Imagine if cars were manufactured and purchased the way print is.
The buyer would describe in general terms what sort of car they would like to have and make a request for a car manufacturing quote from a variety of car makers who may not be equally qualified to build it. The buyer then makes the purchase decision, perhaps based on price – "not too high, not too low" or perhaps based on a relationship with the car manufacturer's sales rep. Once the the selection of manufacturer is made, most of the raw materials and specifications needed to build the car are provided by the customer who, typically, has only a vague idea about how cars are built. The car manufacturer purchases a few required materials – however, they are provided with no specifications as to their manufacturing suitability. Then a car is mocked up as "proof" of concept using materials that only partially resemble the materials that will be used to build the final car. Then, after customer approval of the proof, the car buyer may opt to watch their car being manufactured so that they can make adjustments to the look of the car while it's being built.
Now, that’s manufacturing!