Quality printing often goes unrecognized

The annual Premier Print Awards (a.k.a. "The Bennys") are the print industry's equivalent of "The Oscars" and will be handed out at a gala affair during Print 09 in September. Having been a judge for this award (that's me in the blue shirt top row center in the above photo) made me realize just how much good presswork is never even entered for award consideration. So, I'd like to highlight two pieces whose quality have always impressed me – especially considering that you would not expect such quality used for these applications.

First is the Behr series of paint catalogs which, in North America, may be picked up at any Home Depot hardware store. Over the past ten years or so, hundreds of thousands of these catalogs have been printed by Creative Press in California using 10 micron FM screening – a process that is extremely demanding of the entire print production process. The fineness of the halftone screening provides true photographic print fidelity even when examined under a loupe. It's a level of quality that one rarely sees in even the finest corporate brochures, let alone in such a modest application as paint catalog printing.Cover of Behr paint brochure
At left is a paint can image as reproduced in the brochure at 100% and shown enlarged at right

Next are the newspaper flyers produced by Transcontinental Press.Grocery store newspaper flyer insert
Here, 25 micron FM screening is used on relatively poor quality paper to provide near-photographic reproduction of their client's products. Again, even when examined with a loupe, the level of detail is astounding - especially considering, not only the paper that's being used, but the high speed web presses, and the enormous volume of flyers produced.
At left is a product image as reproduced at 100% in the flyer and shown enlarged at right

Using FM Screening for ink savings

The average ink savings by switching from, for example, a 175 lpi AM/XM halftone to a 20 micron FM halftone, is about 10-15% (depending on the range of tones in the original art). Since the average sheetfed printer will spend about 3-4% of their gross earnings on ink, as an example, for a $10,000,000 dollar a year printer the ink savings using FM screening will range from about $35,000 to $52,500 a year.
The reduction in ink usage for the same final presswork appearance, comes primarily from the use of a cut-back curve applied to the FM screened plate, the thinner ink film characteristic of FM screening, and the higher percentage of the tonality being created optically (optical dot gain) .

How higher dot gain saves ink

The smaller the halftone dot is, the greater its perimeter to area ratio is.
Since dot gain occurs at the perimeter of the dot this results in smaller dots having more dot gain. And since FM halftone screens are made up of very small dots they will initially have a higher dot gain compared to a conventional AM/XM halftone. That higher dot gain compared with, for example, a 175 lpi screen, must be “normalized” by using a tone reproduction curve applied to the plate to achieve the same final presswork appearance. As a result, to get the same final tone on press that a 50% dot at 175 lpi gives - you might only need a 40% dot with an FM screen. Because a 40% dot area carries less ink than a 50% dot area - the result is a reduction in ink usage.

For example, here is a 4/C image rendered at 175 lpi:Left bottom is a 50% tone, center bottom is the 50% tone on plate, right bottom is the final result: a 65% tone on the press sheet. The look of this presswork will be the target for the FM screen presswork.

Next is the same image but this time rendered with a 20 micron FM screen:Left bottom is a 50% tone, center bottom is the 50% tone on plate, right bottom is the final result: an 80% tone on the press sheet. The presswork is now too dark compared to the 175 lpi AM/XM target.

However, by applying a tone compensation curve to the plate, the extra dot gain can be factored out:Left bottom is a 50% tone, center bottom is the 50% tone mapped to 40% on the plate, right bottom is the final result: a 65% tone on the press sheet. Now the FM presswork is tonally aligned to the AM/XM target presswork. And since lighter tones on plate carry less ink area, the result is a reduction in ink consumption.

FM's thinner ink film also helps save ink

Small FM dots cannot carry as thick a film of ink as larger AM/XM dots can because there is not as much dot area to carry the ink. This characteristic also contributes to ink savings.
The below image shows the thickness of ink on a 175 lpi AM/XM halftone. Ink density has been mapped to height to show the thickness (depth exaggerated for illustrative purposes).
Here is the same 3D projection - this time with a 20 micron FM screen:
In North America, approximately 80% of telephone directory printers and 60% of newspaper flyers (as well as 90% of WalMart flyers) are printed using FM screening in order to take advantage of ink savings for cost and well as environmental impact reductions.

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.

FM Screening Halftone Dot Shapes/Patterns

The choice of FM halftone dot shape is important because it has an impact on plate choice, plate imaging, the aesthetics of the final presswork as well as on the lithographic performance of the press. Because there can be a wider variation of FM dot shapes than AM, this post will just illustrate several representative dot shape options. Each vendors' offerings can then be evaluated by using the information in this post as a guide.

In an FM screen each dot is formed in a halftone cell, typically based on a grid of 16 by 16 pixels. The pixels within the cell are "turned on" in pseudo random fashion in order to form the FM dot shape or tone area. The cells are then stitched together, like a mosaic, in order to form an area of dots or tone area.
On the left (enlarged) is a single halftone 16x16 pixel cell with several pixels turned on. On the right (reduced) is a tone area defined by a mosaic of sixteen individual 16x16 halftone cells.
Following are some basic FM screening dot shapes/patterns and their performance characteristics in use.

Click on the images to enlarge.

First Order FM: Dots are all the same sizeBenefits:Image has a photographic quality even when viewed under a loupe. Often used for fine art reproductions.
Issues:Grainy as well as mottled looking flat tone areas, small dots are more difficult to image consistently on plate and hold on press. Because the issues usually outweigh the benefits, this FM dot pattern is seldom used today except for specialized work.

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Second Order FM (a.k.a. Hybrid FM): Dots grow in the tone scaleSecond order FM screens are the de facto standard in today's print production. With this type of halftone screen the dots grow in size through the tone scale. Dot growth can be in one direction – perhaps forming worm-like features as in the first example (Kodak Staccato), or grow in both directions – forming more conventional looking dots as in the second example (Screen Spekta).
Benefits:Depending on the specific vendor's implementation, graininess in flat tone areas is eliminated.
Issues:Thin one or two pixel wide worm features, as in the first example, may demand higher resolution plates, and/or imaging in order to maintain consistency. Dots shapes, as in the second example, may cause a propensity for shadows to plug on press.

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Second Order directional FM: Dots grow directionally in the tone scaleDots have a strong directionality. Sometimes this dot shape in a vendor's FM offering is used for only one of the process colors in order to reduce "clumping" or secondary patterns when process colors overprint.
Benefits:Eliminates secondary patterns when process colors overprint.
Issues:Directionality of the dot shape can exaggerate directional issues, such as slur and doubling, on press.

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Nasty FM: Dots are plain uglyThis graphic is just intended to emphasize the fact that there is a great variety in FM screen patterns. Therefore one should not apply general statements such as "FM screening is grainy" – quality will vary according to each vendor's implementation.