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.

The Wayback View – Rust in Peace

Just 7km/4.5 miles south of the city of Duncan and 53km/33 miles north of Victoria on Vancouver Island is Whippletree Junction - a grouping of specialty shops and restaurants set within 14 restored buildings from the early 1900s.Behind one of those buildings are the rusting remains of a typographic classic. It's an Intertype hot metal typesetting machine, most likely the very popular model C.
In 1911, New York newspaperman Hermann Ridder collected $4,000,000 in capital and organized the International Typesetting Machine Company. Several Linotype engineers joined the effort since it is believed that some Linotype people were disgruntled and felt they could produce a better machine. Since the early years, Intertype boasted a simpler design with fewer, but more standardized, parts than the Linotype. In fact, it was advertised that any standard Intertype part would fit any standard Intertype machine regardless of its age.

By late 1912, the company expanded their operating space to 80,000 square feet and employed in the neighborhood of 750 persons. The first machine was installed for a price of $2150 at the New York Journal of Commerce.
An Intertype ad from 1913
Intertype machines were so well engineered that they are still in productive use today as this one in Holland demonstrates.
Click play arrow (and maybe wiggle the play head), to view the video.
They are mainly used to produce people's names on slugs which are then used to personalise pencils, pencil cases, bookmarks, etc. with gold blocking.

In the UK, Express Gifts, part of Findel PLC, also uses Intertype C4s to produce slugs of people's names to manufacture personalised gift items. However these are computer driven by Decitek Floppy Disk Drives operating a Fairchild Teletypesetting unit.
Click play arrow (and maybe wiggle the play head), to view the video.
For more information regarding the early days of typesetting please visit "Metal Type" by clicking HERE

Special thanks go out to Trevor Bruckshaw for his help with research for this post.

Avatar, iPad, and the future of the book

Amazing, but, in the future world of Avatar circa 2154 way out 4.4 light years from Earth, according to James Cameron, with all the high technology - when it comes to the important things and to knowledge transfer - it's not an iPad, it's still the printed (very thick) book - ink on paper - that is used as the key reference source as is shown in this very short scene from the movie:
Click play arrow (and maybe wiggle the play head), to view the short video clip from the movie.

Putting a glow in your presswork - OBAs and black light posters

Most printers who are faced with a printing job that's intended to glow under black light tend to think only about the issues of printing with fluorescent inks (see the post HERE). However, there is another important item that needs to be considered and that is the effect that the amount of, or lack, of OBAs (Optical Brightening Agents) in the paper will have on the final result.

For example, viewed under a black light this poster "glows" in an expected way:And it will look similar, except for the glow effect, under normal room and black light conditions.

However this poster which has large areas of white, i.e. unprinted paper, looks very different under normal house light:compared with being viewed under black light:
The difference in appearance is caused by the amount of OBAs in the paper. In this example, the paper contains low/no OBAs. Under normal home lighting the paper looks white and the fluorescent inks fairly bright. However, under black light the inks glow brightly but the paper which contains low/no levels of fluorescing agents goes dark - there is effectively no light for the paper to reflect.

This difference in color response can be used creatively or it could destroy the intended look of the poster as envisioned by the artist.
If a paper with high levels of OBAs is used instead, then the appearance of the image would be preserved under black light:
So, when quoting a "black light" job consider the amount of OBAs that the paper contains because it can have a profound effect on the final result. If the paper will have 100% ink coverage then choose a paper with high OBA content to assist in the final glow effect. If the design includes areas of white, then discuss the issue with the original designer and choose a paper that has high, or no OBA, content according to the final result the designer is trying to achieve.

To determine the relative amount of OBAs in the paper, view a sample using an inexpensive fluorescent type black light:

The future of newspapers - camouflage?

Once upon a time hollowed-out books were used to keep items hidden and safe from prying eyes.
Today, this idea has been resurrected to safeguard laptops from thieves by camouflaging the laptop as a newspaper.

MiteMite Unnecessary Objects Lab (http://www.mitemite.es/st/laptop.html) produces vinyl laptop sleeves that look like folded newspapers. At the present moment, you can choose from four different European newspapers.
The bags are designed for the MacBook Pro, but they can accommodate other laptops in their 15" X 11" interior.
Because the sleeve is designed to resemble a folded newspaper it won’t look like you are carrying a laptop around and, after all, thieves are not interested in worthless items such as newspapers.

How a knowledge of printing could have prevented major embarrassment and a retraction.

Recently the TMZ.com website announced the discovery of a never-before published photograph which appeared to show John F. Kennedy on a boat filled with naked women.
TMZ had multiple experts examine the photo - and all said there was no evidence that the picture had been Photoshopped. The original print was scanned and examined for evidence of inconsistent lighting, photo composition and other forms of manipulation. The experts all concluded that the photo appeared authentic.

Professor Jeff Sedlik, a forensic photo expert, said the photo is printed on paper consistent with what was used in the 1950s. The emulsion on the surface of the print has numerous cracks - the result of aging and handling.

Forensic analyst Sedlik superimposed an image of Kennedy taken at the Democratic National Convention in August 1956, just days before Kennedy went on a Mediterranean cruise.Sedlik says the features from the two pics almost precisely sync up. TMZ also had two Kennedy biographers examine the photo - they also believed JFK is in the picture.

Unfortunately it appears that everyone, including Sedlik, may have missed the importance of a distinctive pattern seen around "JFK's" face and clearly visible in this closeup of one of the young (now a pensioner) lady's legs:
Oooops! Those aren't Suzettes - they're Rosettes!

It's now confirmed that the photo was part of a Playboy magazine story published in 1967. The JFK doppelgänger was just a male model. Below is the original image that was published in the magazine:
Hopefully, in the future, TMZ.com will consult with a printer in matters related to print.

RE:Print - "Mail Power"

Click image to enlarge

FM/Stochastic Screening

The Benefits
Below are listed some of the reasons why printers adopt FM/Stochastic screening for some, or all, of their presswork. In the next posting I'll be going into more detail on some of the benefits that have not been covered in past posts on this topic.

1 - No screen angle moiré
2 - No subject moiré
3 - No rosettes
4 - Photographic/contone look Some examples are mentioned HERE
5 - Greater tone and color stability as SIDs naturally vary during press run
6 - Larger color gamut The reason for the increased color gamut is explained in the post HERE
7 - Faster drying
8 - Reduced ink usage - About 10-15% depending on tone content of the presswork. The reason for reduced ink usage is explained HERE
9 - Tonal and color stability when misregistration occurs
10 - Halftone dot structure stability when misregistration occurs
11 - Competitive differentiator

It's important to understand that, when it comes to the press, how halftone dots are organized is not as important as the size of the actual dot. As a result, some of the benefits of FM/Stochastic screening are available to conventional AM/XM screening if the lpi (halftone frequency) is fine enough. Specifically benefits 6, 7, and 8 in the list above can be delivered by AM/XM screening if run at an equivalent lpi to the FM halftone. As a general guide, the lithographic AM/XM/FM equivalents for a "second order" FM screen at various dot sizes are shown below.
Color stability through the press run

Printing presses are stable - but not consistent. Solid ink densities will naturally vary through the press run, and when they do two things happen; dot gain varies, and ink density on the halftone dots varies. The result is variation in tone and color reproduction through the run.
When printers measure dot gain they measure it at the 50% tone value rather than at the 1% or 2% point because at around 50% dot gain variation is at its greatest.The tone variation happens because the large midtone dots in an AM/XM screen can accept more ink than the small dots in the highlights and deep shadows. Put another way, you can’t build a column of ink on a small dot, but the large dots around the 50% tone can take on the ink so they will grow the greatest. So, the midtone color gets darker and lighter as solid ink density naturally varies through the press run.

However, the halftone dots in an FM screen have a different ink profile.


FM screens effectively have small dots throughout the tone range. They act much like the highlight dots of an AM/XM screen and do not take on as much extra ink as SIDs naturally vary - so they remain more stable and hence the color remains more consistent through the run. For example, the below chart shows the dot gain profile (solid line) of a 133 lpi AM/XM screen and a 20 micron FM screen calibrated to match that dot gain profile. The dashed lines show the dot gain response when the solid ink density is raised by 30 points (i.e. 1.40 to 1.70) to magnify the difference. The curve with the small dashes shows the response of the FM screen, while the longer dashed line shows the response of the AM/XM screen.
The color and tone stability that FM screening provides is similar to that provided by heavy GCR color separations or ink reduction re-separation applications.

In some cases this on-press stability can be a two edged sword since, like heavy GCR separations, it does not allow the press operator the ability to move color quite as easily on press should the print buyer decide to be creative during a press approval. It also means that if the press operator makes a SID change, the region of color/tone shift will not be the same as that of an AM/XM screen.

That being said, consistency in color is usually the goal of presswork so the stability of FM screening is appreciated by printers and their customers as an excellent complement to their “print -by-the-numbers” goals.

Faster Ink Drying
One of the little known benefits of FM screening is that it helps the presswork dry faster. With FM screening, the tones are built with many small dots - effectively a “mist” rather than puddles of ink. The small FM dots carry a thinner ink film, and are distributed in a finer pattern that allows the ink to flash off its volatiles quicker causing the ink to dry faster than their AM/XM counterparts.

To illustrate how this benefit is achieved, looking at the same final tone value in the below image, on the left is a microphotograph of the large AM/XM dots while on the right is magnified the smaller FM dots.
In the below image I've taken those microphotographs of the dots and passed them through a 3D rendering program that translates density into height to see just how thick the ink film is. As you can see on the right, for the same final tone the FM screen has a thinner film of ink than the AM/XM dot.
The faster drying can also be demonstrated with a simple test. Below are three images printed on uncoated book stock, the lefthand image is 133 lpi, the center is 200 lpi, and the righhand image 20 micron FM. I then used a coin to scratch across the 3/C gradient as soon as the sheet came off the press:
It's very easy to see the marks made by the coin in the 133 and 200 lpi gradient - while the FM gradient is almost completely dry and hardly shows any marks from the coin at all.

The faster drying of FM screening brings other benefits, namely:
1- It's easier to align presswork with proof - since they are both dry one doesn't have to account for dryback
2 - Reduced setoff
3 - Prints better on uncoated, newsprint, foils and plastics
4 - Less spray powder is needed
5 - Less distortion on perfecting presses
6 - Jobs are faster to bindery
7 - Heatset presses may be able to lower their drying ovens a few degrees and thus save on energy costs

LPI Integrity
Even though most printers have experienced the problem, few have understood it. LPI integrity is a benefit of FM/Stochastic screening that may be a bit difficult to understand at first.

AM/XM screens generate a rosette pattern when all the screens are at the correct angles. A rosette is a high frequency moiré and as such is very unstable. If there is a slight misregistration, what happens is that the rosette pattern of the AM/XM screen changes - typically from clear-centered to dot centered. The next few images show the results of a half row shift in mis-registration:As the rosette shifts from being clear-centered to being dot-centered - the visibility of the rosette changes. Effectively, the rosette shifts from being high frequency to low frequency. Put another way, the effective lpi of the halftone can shift from a fine 175 lpi to a coarse 85 lpi as a result of slight misregistration. So, a rosette pattern that occurs in important skin colors may not be visible when in register, suddenly becomes very visible with slight misregistration. However, FM screens do not produce rosettes and as a result they maintain their lpi image integrity when slight misregistration naturally occurs.

Color Integrity When Misregistration Occurs

You can have the best presses and the best plate imaging available, however, because you are covering a sheet of paper with a mix of oil and water, squeezing and pulling it through the press - some amount of misregistration is an inevitable fact of press life. The impact of that misregistration on color and tone reproduction will depend on whether you are using an AM/XM or FM screen.

Here is a full sized image:

And here, enlarged, is a section of the image using an AM/XM screen on the left and an FM screen on the right. Both are in register.

And here is the same section but with the Magenta for both screens out of register by a half row of dots (1/300th of an inch) - considered within normal printing tolerances.
Notice how the dot structure in the rosette visibly changes in the AM/XM screen (175 lpi in this case) when it's out of register but the FM screen (20 micron in this case) remains stable since it does not have a rosette.

More importantly note the change in AM/XM rosette structure from clear-centered to dot-centered and back to clear-centered as it drifts in and out out of register. It’s like turning a light on and off and back again - so the color goes darker and lighter and darker and lighter through the run.

Also, misregistration changes the overprint and ratios of wet and dry trap. A blue made from Cyan and Magenta dots beside each other is not the same blue as when Magenta dots overprint the Cyan. The result is a color shift. So while the AM/XM presswork is going darker and lighter it is also shifting color from bluish to redish and back again as slight misregistration occurs. This can be particularly troublesome in, for example, car brochures where there are large expansive areas of neutral grey made up of 4/c process. Again, since FM screens don’t have a rosette the color and tone are better preserved with natural slight misregistration in presswork.

The bottom line - FM screening can help deliver more presswork color consistency - which, after all, is one of the main goals of printing.

Potential Issues
So far it's all good - but what about the potential downsides of FM/Stochastic screening?

A printing press or plate does not "know" how a pattern of halftone dots is arranged - it does not know whether the screening is AM/XM or FM. So, the problems related to FM screening are not usually the result of the halftone pattern itself, but by the size of the dots that make up the halftone screen. In that respect, FM screening shares the same problems associated with any high frequency halftone - FM or AM/XM.

This old Planeta press prints breakfast cereal boxes using FM screening for press runs that can last a week.
This new KBA press prints daily newspapers, inserts/flyers and commercial work using FM screening exclusively.
Here are some of the issues that can arise using FM and AM/XM screening.

1 - Grainy appearance in flat tint areas. This usually doesn't occur with AM/XM. There are several possible causes:
1) Poor screen design and/or using a first order FM screen pattern (see HERE for explanation). This problem has been overcome by modern second order FM screens.
2) Graininess can also result from a lack of plate imaging integrity e.g. plate doesn't have a high enough resolution for the dot size being used, and inconsistent plate imaging. Again it generally occurs with older first order FM screen designs. Graininess can also occur when screening back a spot color. This is because spot colors are normally not formulated to be halftone screened and hence their pigment grind may be too coarse for the dot size being used.

2 - Inability to align presswork color to an industry standard like SWOP, GRACoL7, etc. The higher the halftone screen frequency, AM/XM or FM, the larger the color gamut becomes (this is explained HERE and HERE). The extra gamut appears in one and two color screen tint builds making them appear more vibrant than the same screen tint build using a lower lpi AM/XM screen (e.g. 175 lpi). It usually only effects vector artwork and typically does not show up in raster images. If this is a problem it can be dealt with using color management and re-separating images to contaminate color builds to reduce the gamut. Unexpected color shifts can also occur when screening back a spot color. This is because spot colors are normally not formulated to be halftone screened and hence the impact of using a high frequency screen, with its increased gamut can lead to an unexpected final color appearance.

3 - Piling. Piling may occur on heatset web presses and is usually caused by not following proper blanket washup schedules or by poor ink rheology - i.e. using inks formulated for the large dots of a coarse AM/XM screen rather than formulated for the smaller FM dots. Piling is rare on sheetfed presses - typically an ik that works with a 200 lpi AM/XM screen will work with a 20-25 micron FM screen.

4 - Mottle. Mottle (splotchy appearance of flat tone areas) can appear with any high frequency screen. It is usually the result poor ink transfer (incorrect ink formulation), fountain solution condition, or paper surface/coating characteristics. Mottle can also occur when screening back a spot color. This is because spot colors are normally not formulated to be halftone screened and hence their pigment grind may be too coarse for the dot size being used.

5 - Poor image reproduction. The higher the lpi of the screen, AM/XM or FM, the higher the detail rendering and hence the better the detail, flaws, and artifacts of an original image are reproduced. Another problem can be a "softness" to the image reproduction. This is usually caused when high dpi images are resampled to a lower resolution when the PDF is created or resampled when the image goes through the RIP. The solution is to not resample images.

6 - General failure. The most common reasons that cause printers to fail with FM and high frequency AM/FM screening that I've encountered are:
1) Poor process control in the shop. Halftone screening in the range of 65-175 lpi is very tolerant of process variations however as the dots get smaller, process controls need to be tighter. That being said, the process controls that need to be in place for FM screening to succeed are no different than those needed for an efficient 175 lpi print manufacturing process. If there are any issues - high frequency screening will make those issues more visible.
2) Lack of buy-in. If the frontline people (e.g. prepress and/or press operators) have not bought into the idea of using FM screening - it will fail. Without buy-in the change merely represents a disruption to what they see as a smooth running process. "Why fix it if it ain't broke?"
3) Lack of use. If FM screening is only used for "special jobs" then press operators never get used to how it performs on press and as a result they will struggle with it. As with most activities, the more you do them the better you get at doing them. It is best if the shop makes the commitment to change to FM and make traditional AM/XM screening the special case exception.

In future posts I will be covering success strategies for implementing FM screening as well as general process control proceedures.

About 80% of all the directory printing in North America is done using FM screening.