AM-FM-Hybrid-Screening

 **AM / FM / Hybrid Screening**

__**Introduction:**__
We all see hundreds of printed signs, advertisements, books, magazines and pamphlets every day. Have you ever //really// looked closely at the text? How about the colorful images?? How does the print make it from the designers pen/laptop to the printed page?

Printers are constantly looking for new ways to add value for customers through quality improvements. Screening technology brings value to print because accurately screened high-resolution print yields a visibly better printed result. It can also differentiate a printing company from its competitors and provide new business opportunities.

__History of Screen Printing:__
Screen is one of the earliest methods of print. Almost 2000 years ago, the Chinese screen printed with human hair stretched on a wooden frame with shapes created using leave stencils. The Japanese then followed this example, but used woven silk with laquer stencils later referred to as silk screening.

In 1907, Samuel Simon of Manchester, England; was awarded patent on the first industrial screen printing process. Just a few years later, John Pilsworth of San Fancisco, California; developed the Selectasine Method (better known as [|Serigraphy] ) which allowed for images printed in multiple colours.



Shown are some early screen print tools.

__** What is A **____** M Screening? **__
The idea of //(also called AM screening)// was first conceived in 1850 by [|**William Fox Talbot**]. He suggested using photographic screens or veils to produce photographic images in the [|**intaglio printing process**].

By 1881, the first successful commercial halftone process was patented by [|**Frederic Ives**] of Philadelphia. Additional improvements followed in the decades to come, but the technique has largely remained the same since first conceived in 1850. This technique is still widely used by today’s printers, but there are many disadvantages which lead to a loss in quality of photographic images.

AM screening (Amplitude Modulation,) utilizes a fixed linear dot pattern with various sized dots to emulate the tonal range in photographic images and color builds. Standard AM line screens vary in resolution depending on the reproduction process and equipment quality. In commercial [|**offset printing**], these line screens are typically recognized as 100, 133, 150, 175, and 200. These numbers represent the number of dots per linear inch. The larger the dot the darker the image area, and the smaller the dot the lighter the image area. __ 1. __



__** What is FM Screening? **__
Although it is widely recognized that [|**Stochastic**] FM [|**(Frequency Modulation) screening**] was first conceived in 1965 by [|**Karl Scheuter**], at the Technical University of Darmstadt in West Germany, there is also a rich history of contributors that helped make this technology possible.

The core technology involves high level [|**computational algorithms**], which can be traced back to as early as the late 1800′s, by a man named [|**Ladislaus Bortkiewicz**]. We can also consider [|**Floyd-Steinberg dithering**], developed in 1976, or a technical paper “Frequency Modulated Picture Recording with Random Pixel Distribution” published by Scheuter and Gerhard Fischer in 1984, then subsequently publicized by [|**GATF**] in 1985. However, most of the core mathematics appear to have their roots in the [|**“Monte Carlo method”**].

The term “Monte Carlo method” was coined at [|**Los Alamos National Laboratory**] in the 1940′s, while physicists were working on the US nuclear weapons program, or [|**Manhattan Project**]. This method used complex computational algorithms that rely on repeated random samplings for their results. __ 2. __

= = == = One thing does become clear; the fundamental mathematical understanding of the FM screening process proceeded the physical technology by a solid 50 plus years. It wasn’t until the development of affordable compact [|**laser diodes**], and cheap [|**high speed microprocessors**] , that we were able to apply these complex algorithms directly to [|**printing plates**] commercially in the late 80′s and early 90′s. This technology is referred to as [|**CTP (Computer-To-Plate)**] in the printing industry. This breakthrough in technology virtually [|**eliminated an entire process**] in the manufacturing of printed material. =

Even though most, if not all of these CTP devices are capable of printing an FM dot on a printing plate, many printers have been apprehensive, and still opt for AM methods. The reason is fairly straightforward, because we have to have our shops in order, and expand our learning to be able to apply the new technology. Even printing the dots correctly and consistently on the plate becomes a challenge, because it’s a very small image, so regular [|**calibration**] and cleanliness is an essential part of the process. A printer has to change the way they operate, and educate themselves, because printing an FM screen is an unforgiving process on press with very little margin of error. Once a printer makes the transition to FM screening though, it becomes more efficient, and proofs tend to be easier to match with less unforeseen screening surprises on press as in the traditional AM screening method. The end result in the finished product is also a substantial improvement over the traditional AM method, so customers benefit greatly. Ultimately, printing companies that continue to drag their feet will become relics, and go the way of the VHS.

Although FM screening utilizes the same concept of CMYK printing as in AM screening, any other similarities end there. FM screening is not a fad technology, it’s an upgrade technology for conventional offset printing. And while this new technology isn’t without challenges, overall end results yield significant improvements over the old method. Most casual print buyers probably won’t understand the intrinsic differences between the two methods, but they will understand, “this print looks much better than that print.”

The FM screening process utilizes very small micro-dots, about 10-20 microns, which are placed randomly, but the frequency of those micro-dots increases or decreases depending on the tone of the image area. The lighter the area the fewer random dots, and the darker the area the more random dots. __ 3 __



__So what is Hybrid Screening?__
Learning from their experiences with first-order FM screens and accepting their limitations, engineers and programmers have continued to look for ways to improve screening technologies. Could the two types of screens be combined in a way that would accentuate their strengths while avoiding their weaknesses?

One way to avoid visible rosettes is to increase the screen frequency in the AM screens. This is tempting when using CtP, since the dots are placed more precisely and come out sharper than with a film-to-plate process. But there is a limit to how small a dot can be and still be safely transferred to paper. This limit is often said to be the equivalent of a 1% dot at 175 lpi conventional AM, depending on press type, paper used and, of course, the plate and CtP used. A 1 % screen dot at 175 lpi is roughly 10 microns (a millionth of a meter, or a thousand of a millimeter if you prefer) and should ideally be created using several exposure dots.

The hybrid screening technologies all ensure that the dot size isn't smaller than what can be safely reproduced in the printing process at hand. In the highlights and deep shadow parts, the hybrid screens use FM (or FM-similar) technology to render the screen. In the midtones, most of the hybrid screens use conventional AM (or AM-like) screening technology, thus avoiding the risk of a grainy look. In this way, the printing characteristics will also be very similar to conventional AM-based screening.

While hybrid screening is attractive for most printing applications, its benefits are most obvious in newsprint and flexo. In flexo (and newsprint), disappearing highlights is a constant problem. This is made worse in flexo with the unstable behavior of the dot gain in the near-highlight areas.

In sheetfed offset printing, it's possible to use 300- lpi screen rulings with hybrid screens or even higher. While this would mean very small &mdash; even too small &mdash; screen dots with conventional AM screen, remember that hybrid screens use an FM-similar screen in the highlight. While keeping the dots to the set minimum size, they are "thinned out" in the highlights. It&rsquo;s therefore possible to address tone values around 1% and lower without losing the screen dot. This is unthinkable with most conventional AM screens, depending on which CtP device and plate is being used. With some of the hybrid screen technologies, the screen dots are still arranged in the AM pattern, while in others thedots are removed randomly.

In this way, the hybrid screen is supposed to combine the strengths of both the AM and FM worlds. Does this mean they are poised to immediately replace the "old" screening technologies? It seems as if hybrid screens can replace the conventional AM screen in many production environments, while the secondorder FM screens still have one clear advantage: They can avoid moire in most print production. The hybrid screens reduce the tendency toward moire by using higher screen rulings than conventional AM screens, but most hybrid screens are still AM screens "at heart." FM screening remains a safer choice to avoid moire for printers that experiment with multicolor printing. But high-resolution hybrid screens are as good as FM screens in rendering images with high image details and sharpness.

__So...The Breakdown!!!__
FM Screen or Stochastic screen is the halftone process using pseudo-random distribution of dots varying in density (as related to the desired gray level) but of fixed size producing a contone image.

AM screen is a halftone process of fixed shape and spacing but that varies in size.

XM Screen or Transitional screen combines the two in attempt to optimize the benefits of both but unfortunately, has been somewhat unsuccessful.

In offset print, due to the desire to meet customer expectation with lower manufacturing cost, FM screen has recently become more prevalent in the market even though it has been available since the 70’s. Due to the FM screens random dots, there is no longer the issue of moire and low size means less visibility and greater image detail. Dots as fine as 10 micrometers enable a print quality close to that of photographic prints. Limited ink thickness reduces the amount of ink used, offering press stability and a greater gamut of colour. Also, the effects of mis-registration have all but been eliminated in the last few years.

FM screen does have its disadvantages as well. Small dots require some special care and cleanliness and FM has proven difficult to work with in prepress and on press with little colour adjustment possible.

AM is preferred in the mid-tone range for less dot gain and clean tint. In the highlights and shadows, the dots become too small for easy reproduction and therefore FM is prefferred. One of the reasons XM screen has been attempted. There are still some blending issues involved with the transition from AM to FM at the crossover. This has been resolved by maintaining a minimum dot size but reducing the number of dots used. Transition screen, typically used in flexography, is easier to print than FM and requires less stringent process controls but at increased cost.

__Summary__
The choice for AM with FM properties, or FM with AM properties, in the XM screen, depends mostly on the flexability required on press. FM is most common in the market because of cost and the recent development in quality of print.

__Additional Learning:__
__[|Check out this offset printing press in motion!]__

__[|WOW! This is amazing!]__

__[|A look at the inside of a printing location]__

[|Multi-colour screen experience!!]

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1. Copyright © 2011 · All Rights Reserved · Professional Printing Center 2. www.coloursplash.eu/Benefits%20of%20FM%20**screening**.pdf 3. www.packageprinting.com/article/industry...am-**fm**...**screening**