The eternal conflict - ink/water balance - the tale of the tones

An AM/XM halftone screen has a builty-in conflicting ink/water balance requirement on press. The highlight dot and quarter tone range from 1-35% requires minimal water and maximum ink in order to prevent those dots from being washed away. The three-quarter tone range from 65-99% requires the opposite - a larger volume of water in order to prevent the shadow dots from filling in and disappearing. On the other hand, the mid-tone range from 35-65% is more of a balance between ink and water.

Halftone dots and the tones range they represent are affected differently by the condition of the ink on press - assuming of course, that the plate, press, and chemistry are set up correctly. Unfortunately, if the press operator attempts to fix tone reproduction in some areas, that built-in difference in ink/water requirement can exaggerate the inherent conflict and cause problems in other parts of the tone range.

1 - 1-35% This tone range is primarily affected by the body/viscosity of the ink. If the body is too soft the highlight area will print too full which may cause the press operator to decrease solid ink density in order to reduce the dot size. Alternatively the fountain solution may over-emulsify this tone range causing poor ink transfer and loss of highlight detail. If the ink body is too heavy the dot may print too sharp causing the press operator to increase the density or blanket pressure.

2 - 35-65% This tone range is primarily affected by the strength (pigment load) of the ink. If the ink is too weak the press operator will increase solid ink density which will cause increased dot gain and result in presswork that appears too full. If the ink is too strong the midtones may print too light. Also, the strength of the ink also impacts how well the inks trap, which in turn affects the color gamut the press should be able to achieve. Varying the strength and stiffness of the ink to achieve good tone reproduction in presswork is a method press operators, who don't have good communication with prepress, often employ. It's almost always better to use tone reproduction curves applied in plate imaging than to modify inks.

3 - 65-95% This tone range is most strongly affected by mechanically induced dot gain or chemistry issues i.e. (poor ink water balance). If the tone range from 1-65% is evenly balanced then excessive gain in the shadow tones is usually caused by running excessive water, too much blanket pressure, and/or mechanical slur.

How AM and FM screening equivalencies are measured

I'm often asked about what AM/XM halftone screens are equivalent to a certain FM screen - i.e. "What AM/XM screen is equivalent to a 20 micron FM screen?"

There are two ways that this halftone screening equivalency is usually measured.

One is equivalency of detail rendering - the ability of the screening to render image detail. The other is lithographic equivalency - how they perform on press lithographically. Note that in both cases, because the respective screening technology is so different, equivalency can only be an approximation.

Equivalency of detail rendering
Since halftone dots form the printed image - more dots per linear inch translates into more detail that can be rendered.

With an AM screen the detail rendering ability is specified in lpi (or lpc) - i.e. halftone dots per inch (e.g. 175 lpi or 60 lpc).

Since an FM screen has no "lines per inch" determining the equivalent detail rendering equivalency is usually done by drawing a line through the FM screen and counting how many dots are intersected (crossed) in a distance of one inch.Measuring the relative lpi of an FM screen.
The above example shows an FM screen enlarged. The distance measured is 1/16th of an inch. In that 1/16th of an inch approximately 36 dots are intersected. So, in one inch about 576 dots would be intersected (16 x 36). Put another way, there are 576 dots per linear inch - 576 lpi - to render detail, i.e. this FM screen is equivalent to a 576 lpi AM/XM screen.

Lithographic equivalency
Lithographic equivalency is a bit more complicated to figure out. It is usually measured by counting the number of edges (transitions) in a square inch.Measuring the number of edges of an AM/XM screen.
Measuring the number of edges of an FM screen.
Halftone screens with a similar number of edge transitions will have similar lithographic properties.

AM/XM equivalents of some popular FM screens.
Keep in mind, these are approximations only, however they do give a good indication as to screening performance.

Esko Concentric screening - some observations

Esko Concentric screening is at heart an AM screen which uses a unique halftone dot where solid AM dots are divided into thin concentric rings.Click on the above image to see it enlarged.
Concentric screening and color gamut
Chroma in press work derives primarily from the ratio of light being filtered by ink carried on halftone dots vs light reflected off the paper that hasn't been filtered by the ink. Light that is unfiltered by the ink effectively contaminates the color reducing the potential gamut of the inks. If one compares Concentric halftone dots with conventional AM/XM halftone dots at the same lpi - e.g. 175 lpi. what is clear is the difference in ink coverage area through which light can be filtered.
At left is a micro photo of Esko Concentric and on the right is an AM/XM screen (Esko Paragon). Both are imaged at 175 lpi.
Note that dividing the dot into rings actually lessens the area of ink and increases the area of unprinted paper. Effectively it increases the contamination of color by light reflected off of the unprinted substrate which can actually reduce, rather than increase, the potential gamut.

In the below plot, the CIEL*a*b* values of the same tone values for 175 lpi AM/XM/Paragon screening (in green) is compared to 175 lpi Concentric (in red). If the Concentric had a larger gamut the red dots would be significantly above the green dots indicating a greater chroma. Instead they track at, or are below, the chroma for 175 lpi AM/XM/Esko Paragon screening.
What this means is that, as far as I can determine, Concentric screening offers no additional gamut, and possibly less of a gamut, when it is compared with AM/XM screens at the same lpi.

Concentric screening and image quality
Since it is still an AM screen there is still the opportunity for screening and subject moiré - although the finer the screen (AM/XM or Concentric) the less likely that will be a problem. Because it's still an AM halftone screen it has rosettes - just like any other AM/XM screen - formed by the screen angles.At left Esko Paragon AM rosettes. At right Esko Concentric rosettes. Both screens are 175 lpi. (Squint your eyes or move a few feet away from the screen to make the rosettes more prominent.)
From a print buyer point of view there will likely be no visible difference between a 200-300 lpi conventional AM/XM screen and Concentric screening - even if viewed under a loupe.

Concentric screening and ink reduction
The two primary causes of the reduction in ink usage with high lpi screens are the thinner ink films and the need for tone reproduction curves for plate imaging to bring the press tone response in line with the standard 175 lpi AM/XM screening. Ink reduction with the use of Concentric screening should be similar to the ink reduction enjoyed by high lpi conventional AM/XM as well as FM screens.

Concentric screening and on press color stability
Greater color stability when solid ink densities naturally vary during the press run is a characteristic of high frequency screening (i.e. smaller dots) whether AM/XM, FM or Concentric. The actual ink film thickness of Concentric vs conventional AM/XM screening at the same lpi is actually very similar. Projecting dot density to height in 3D one can see this quite clearly (Concentric is left of the black line - AM/XM is right of the black line.)Of course, if the Concentric screening is run at a very high lpi it will acquire a stability that is similar to conventional screens (AM/XM and FM) that are run to the same high frequency.

Concentric screening and imaging system resolution
Concentric screening is effectively an AM screen ruling multiplier. What this means is that the resolution of the imaging system needs to be able to image the minimum specified ring width. Put another way, if the ring thickness called for is 10 microns wide then the imaging system (plate and press) must be capable of consistently imaging a 10 micron pixel/dot even though the actual final halftone dot size may be almost five times wider (e.g. 48 microns wide a 50% dot at 250 lpi).Concentric halftone dots that depend on 1-2 pixel width imaging integrity can be problematic for most imaging systems
As a result, using Concentric screeing can push the effective screen frequency so high that process stability and imaging may be compromised and it can be difficult to support their use on plate let alone find a way to implement them in the press room. The problem is that some concentric screen settings can drive rulings way over what plate imaging can support - on the order of 1-2 pixel widths for the rings, which is understandably problematic. For example, a 200 lpi screen with 2 pixel ringwidths = 600 lpi which is finer than, for example, a 10 micron FM screen.

Coarser ringwidths are easier to support but at that point it is probably more effective to use an AM screen of equivalent lpi.

For printers contemplating the adoption of Concentric screening
Since Concentric screening is a conventional AM screen using a unique halftone dot design, I suggest that when you are evaluating this type of screening that you "compare apples to apples". That means that you should compare the on press performance of Concentric against a conventional AM/XM screen imaged at the same lpi. Use a combination of subjective (pretty pictures) as well as objective measurable targets (single and two color step wedge gradients and IT8 profiling targets).

N.B. The data that I used as the basis for this post is derived from published promotional samples printed by Esko. I have contacted Esko as well as members of public prepress/press forums asking for press profiles and/or printed test samples of Concentric vs conventional AM/XM screens run at the same lpi under the same press conditions. Despite the product being in the market for over four years I have been unable to acquire such a basic color profile or press samples. If you have that data I would appreciate hearing from you by email ( pritchardgordon @ gmail (dot) com ).

Halftones as you've never seen them before

I use a variety of image analysis tools when investigating how different halftone screening solutions perform. These tools are normally used in the medical field to do image analysis of microscope acquired imagery. However I press them into service to analyze various aspects of halftone dot structures.

Here is a microscope view (200x) of a conventional AM/XM printed halftone dot (175 lpi elliptical):
And here's a microscope view (also 200x) of a 20 micron FM screen:
One of my favorite tools is to use image analysis software to project the pixel density values in the images into height - creating a 3D image that shows the relative ink density (ink film thickness) differences between the two screens. The thicker AM/XM:vs the thinner FM:Using color mapping instead of the actual ink color makes the difference in ink film thickness even clearer (yellow = greatest - blue= lowest ink film density):Lowering the viewpoint and warping the perspective of the 175 lpi AM/XM screen begins to turn the image into a kind of landscape: However, using terrain mapping software on those original microscope images of the AM/XM and FM screens really makes the transformation of the images into proper landscape views a reality.

175 lpi elliptical dots:
Sunlight across a deep FM canyon:
A low flight over a a barren land where FM and AM screens meet.
Sunrise over an AM screen mesa.
Moonrise over an FM peak.
FM screen hits the wall.

Planet Round Dot.

And if you have a pair of these:
You can add a bit of dimension to your halftones:
Of course, this is all very serious work - not fun at all. Really. ;-)

How to calculate halftone dot sizes in microns

When working with screen rulings, particularly those above 200 LPI or FM screening, you may need to know the size of dots in microns. This is to make sure that the plates, plate imaging system, press, and ink pigments are all capable of delivering the minimum printing dots through the process. For example, if the dot size is 10 microns but the ink pigment size is 25 microns there may not be enough dot surface area for the pigment to stick to and hence that tone will be dropped out on press. Or, if the screen calls for a 10 micron dot but the plate can only hold a 20 micron dot then, again, that tone will be lost or have to be compensated for by employing hybrid screening techniques.

Dot diameter in microns can be calculated using the following formula.
Where:
D = Dot area in percent (e.g. 1% dot equals .01)
F = Screen frequency in lines per millimeter (LPM)

To convert the screen frequency from lines per inch use the following formula (2,540 dpi device on left and 2,400 dpi device on right):
While this formula is not absolutely exact, it gives a close enough approximation for most practical purposes.

A few things to keep in mind about halftone dot size. The formula applies to the size of dot that is generated by the halftone screening algorithms in the RIP that will be sent to the imaging device. It does not calculate the size of the dot that appears in the final presswork which may have been affected by dot gain or loss. Also, a RIP will only image full individual pixels to form a halftone dot. So, in the case of a 2,540 dpi device, each pixel is 10 microns in size (10.6 microns for a 2,400 dpi device). Therefore, if, for example, the formula says that the final diameter of the dot is 15 microns what will happen is that the RIP will alternate between 1 pixel dots (10 micron) and 2 pixel dots (20 micron) which results in an effective 15 micron dot average for that tone value. You can see that happening by watching the dot formation in the lighter tones at the start of the video located HERE.

Below is a quick reference comparison chart showing the dot diameters for tone values of 1% and 2% for various screen rulings from 10 LPI to 400 LPI on a 2,540 dpi platesetter.Highlighted areas are where the required dot is smaller than a single pixel at the device's resolution and therefore will not be imaged.

The principle of dot gain compensation plate curves

In a film workflow the industry standard was to create film output that was linear. This meant that a 25% tone request in the original Postscript file would create a 25% dot on the film, a 50% request would create a 50% dot, and so one for all requested tone values. However, in a CtP workflow controlling tonality in the print reproduction process, allows you achieve the presswork quality you want without adjusting the press. It also provides the flexibility to tailor the print characteristic to meet different customer expectations.

Dot gain, or tone value increase (TVI), is a normal part of the print reproduction process. Controlling tones using calibration means that you can manipulate the exact size of the dots on the printing plates so that tone saturation and gray balance are controlled on the press sheet.

Tonal calibration can account for:
• type of plate or film used
• type of paper stock used for printing
• type of dot shape used
• type of screening used—for example, FM/Stochastic or AM/XM conventional, and frequency (lines per inch (lpi))

(Note: Adjusting CtP laser exposure is not tonal calibration and will affect the run length and performance of the plate.)

You cannot use tonal calibration as a substitute for stable operating conditions. Operating conditions must be controlled as a separate process. In fact, without a stable operating environment, you cannot achieve accurate tonal calibration let alone reliable press output.

What Is Tonality?

Printers are used to being concerned with dot gain/TVI. Indeed dot gain values are often included in printing specifications. However, for the purposes of calibration - tonality or dot area, rather than dot gain, is the key metric. It does not matter what dot gain you have. What matters is whether you achieve the required final tone values or dot areas at each originally requested tone.On the left is the desired "correct" tone reproduction and on the right is incorrect tone reproduction.
Tonality in printing is the progression of tints from blank paper to solid ink for each requested tone value in a printing job. It is measured with a densitometer, and reported as either dot gain/TVI or dot area:Dot area and dot gain - two ways of charting the same data.
The target print characteristic tone curve

Building dot gain compensation plate curves always begins with a target print characteristic, i.e. what you want to achieve on press. This is called the target curve - the current tone reproduction that you wish to achieve. It could be your current press work, a proof, or it could be an industry supplied set of tone values. You measure the target sample and enter the dot area (tonal value) for the tints achieved on the target curve graph. If the target is a press sheet, for example, your current 150 lpi AM/XM presswork, the graph will represent your current tone print characteristic:Target print characteristic tone curve - what we want our presswork to look like.
If you change your screening, for example going to FM screening, higher solid ink densities, or higher lpi AM/XM screening, etc. then, if nothing else changes, the tonal response on press will change due to the difference in dot gain:New print characteristic tone curve caused by a change in screening method being used - what the presswork now looks like after changing the halftone screening.
The goal of implementing dot gain compensation plate curves is to make the new press work mimic the original target press tone response. In the above example, the boy's face should appear the same as the original image despite the dot gain caused by changing the halftone screening.

Creating the dot gain compensation plate curve

Building a dot gain compensation plate curve starts with comparing the current target tone response with the tone response of the new presswork. In this case run to the same solid ink densities, on the same paper and press - only the screening has been changed:On the left is the current target tone curve and on the right is the new tone response resulting from the change in screening.
The graphs are then examined by looking at the original requested Postscript tone and the target response (left chart) and comparing it with the new tone response (right chart):In the current target tone curve a 50% tone request resulted in a 68% tone in the presswork. That same target 68% was delivered in the new presswork from a requested tone value of 30%.
Put another way, we are looking for what requested tone value in our new presswork delivered the same final tone value in the target presswork. In this example a 30% tone request in the new presswork delivered the same tone value as a 50% request in the old while a 50% request in the new gave the same tone as a 70% request in the old.

Here's another way to visualize it:Target 150 lpi compared with FM tone response.
Remapping the tones is simply doing this:Find the tone in the new presswork that delivers the required tone response in the old target presswork.

The comparison between target curve and new current curve is made for each 10% change in tone.

The idea is then to map these values so that a tone request in the original file gets changed to a new value that produces the same final tone as the same tone request did in the old target presswork. The result is a lookup table for tone swapping.

In this example:

The requested 10% tone is remapped to request for a 4% tone
The requested 20% tone is remapped to request for a 10% tone
The requested 30% tone is remapped to request for a 18% tone
The requested 40% tone is remapped to request for a 24% tone
The requested 50% tone is remapped to request for a 30% tone
The requested 60% tone is remapped to request for a 40% tone
The requested 70% tone is remapped to request for a 50% tone
The requested 80% tone is remapped to request for a 65% tone
The requested 90% tone is remapped to request for a 80% tone

The lookup table creates the dot gain compensation plate curve.
The lookup table is applied in the workflow to remap the requested tones to the actual tones on plate that will deliver the desired final tones in the presswork. The result is tonal alignment of the presswork despite differences in dot gain.On the left is the original target 150 lpi tone response. On the right is the "normalized" tone response of the FM screen.
Some points to keep in mind

1 - It does not matter if the plates are initially run "uncalibrated" or linear for the target presswork.
2 - A dot gain compensation plate curve is not usually applied to the tone range from 0%-5% and 95% to 100%.
3 - One dot gain compensation plate curve is usually applied to all process colors.
4 - There may be a need to apply a specific dot gain compensation plate curve to one of the process colors to maintain gray balance.
5 - Dot gain compensation plate curves cannot compensate for differences in gamut between FM/Stochastic screens and conventional AM/XM screens.

Why use halftone screen angles?

Go back in the history of halftoning and you'll discover that for multi-color presswork the halftone screens were always angled relative to one another. Why not just have all the screens at the same angle and be done with it? Well, when halftone dots are at the same frequency and angle then they will either print on top of each other (dot on dot)or they will print partially overlappingor they will print beside each other (dot off dot).
When dots print on top of dots you have wet ink sticking to wet ink (wet trapping) and white spaces between the dots. When dots print beside dots you have wet ink sticking to dry paper (dry trapping) with less white spaces between the dots. What does that mean? If you look at the images above you'll see that the dot on dot color looks darker and less vibrant than the dot off dot. The result is that the final blue hue, in this example, of dot on dot will also be different than the dot off dot blue because wet trapping inks reduces the ink's efficiency at filtering light. The white paper surrounding the dots also contaminates the perceived color by adding a graying effect and therefore the dot on dot printing will have less chroma (vibrancy) than the dot off dot.

The biggest issue though is that when there is slight misregistration on press the screen will shift from dot on dot to dot off dot causing the presswork to shift color and tone dramatically. However, by rotating the screens relative to one another, this wet trap/dry trap effect is randomized and therefore the color becomes more consistent when slight misregistration occurs.

The second major issue that occurs if all the screens in an AM/XM halftone have the same angle is that of moiré. Here, one of the colors has been slightly rotated - perhaps because of a small imaging problem, or because of a small press problem.
When this happens a very strong moiré appears when all colors have the same angle. However, by rotating the screens so that they are 30 degrees apart, there is some tolerance for small angle errors and moiré will not appear.

Using rotated screen angles for AM/XM halftones overcomes the dot on dot/dot off dot issues.
With FM/Stochastic screening, the same problems are overcome by using a different screen pattern for each of the process colors.

Choosing the right screen angle for over-printing spot colors

To decide which screen angle to use when a screened PMS/spot color overprints a 4/C process image you will need to look carefully at the image that you will be overprinting.

The basic rule is to use the screen angle of the least prominent (or missing) screened process color that will be underneath the screened spot color.

For example, if there's no screened black under the spot color - use the Black angle, if there's no Cyan use the Cyan angle, etc.

Try to avoid using the Yellow screen angle because in standard screen angle sets yellow is only 15 degrees away from C or M. As a result moiré is always there but it is usually not visible because the yellow is so pale. The moiré can become visible however, if the yellow becomes contaminated - or if it is used for a dark spot color.

For example, in the graphic below, the left image is Cyan (at 105°) overprinted with process Yellow at the standard 90°. The moiré is barely visible. However, in the center image overprinting Cyan with PMS 144 – a very dark yellow/orange color – using the same Yellow screen angle results in the existing moiré becoming very visible. On the right, PMS 144 uses the Magenta screen angle (75°) instead which eliminates the halftone moiré seen in the center image.

Alternatively, you could try running the 5th color using a second order FM screen. If you're using a 175 lpi AM/XM screen then the FM should be about 35 micron because if it's any finer you'll need to create dot gain compensation curves for the FM. Because 35 micron is a fairly coarse screen it is best used for fairly light colors otherwise you may find that the screen is too visible.

Because spot/PMS colors are typically formulated to be printed solid and not halftone screened, make sure that your ink vendor knows that you are going to be screening the ink and the dot size range as well (either in microns or lpi) so that they can formulate the inks accordingly.