Screening / Halftoning - Part 3

Attributes of AM (conventional) screening

There are four attributes of a conventional screen which must be understood

if halftoning is to be commissioned or approved. They are:

❑ dot percentage,

❑ dot shape,

❑ screen ruling,

❑ screen angle.

Dot percentage

The term ‘dot percentage’ is the means by which a fixed tonal value can

be described. In a given area, such as one of the sections of the scale , if the whole area is taken to be 100%, the dot percentage

describes the proportion of the square that is covered by black image. If,

as you would find in the highlight end of the scale, only a small part of

the square is covered by the halftone pattern, the dot percentage value for

the square will be low, perhaps five or ten per cent. Conversely, at the

shadow end of the scale the percentage coverage will be far higher, perhaps

80 or 90%. If the paper is unprinted it will have zero coverage; if

there is complete coverage the halftone value is 100%.

The dot percentage always refers to the image coverage on either the

film or the printed result. A magnifier with an enlargement of about 10

is an essential tool for checking dot percentages on film. However, if individual

dots need to be studied for shape and formation then a dot microscope

with a minimum magnification of 30 should be used. So, 10 for

dot area coverage (per cent) or 30 for individual dots.

By using a densitometer, a measuring device that calculates how much

light is absorbed by a given area of film or image, it is possible to translate

light absorption into a figure which can be expressed as an equivalent area

of halftone dot. The densitometer does not actually measure the area of a

halftone image but expresses darkness, calculated using either the Murray

Davies or the Yule Neilson equations, in terms of halftone area coverage.

It is the principal method by which halftone printing is controlled.

Dot shape

The overall shape of a halftone dot determines some of its visual and

printing characteristics. There are three main dot shapes in common use

for printing – square, elliptical and round. The reason for choosing one

shape, as opposed to another, is a combination of considerations including

the purpose of the picture, the printing process and the substrate (paper,

metal, film etc.).

Square dots are considered the most suitable for general purpose work

in that they provide a compromise between rendering fine, sharp detail

and smooth tonal transitions. However, they do suffer from the problem

that at a 50% value all four corners of a square dot link, simultaneously,

to all the four dots surrounding it. This sudden link is visible as a step in

what should be a smooth tone change.

Elliptical dots are more able to represent smoothly changing values in

the mid-tones than are square dots, because their links to the surrounding

dots do not happen in a single tone level. Across the long axis of the

ellipse the dots will join at about 30%, but the short axis will not link until

the coverage is up to 70%. The trade-off is that elliptical dots are more

troublesome to control in difficult printing conditions and can produce

visible ‘chains’ through the printed image. This is why another name for

an elliptical dot is a chain dot.

The most stable of the main dot shapes, particularly in relation to dot

gain, are the round ones. Round dots would be the natural choice for

newspaper printing because of the inherently high dot gain associated

with coldset web offset printing on newsprint. But there is a trade-off in

that it is difficult to keep detail open above 75% dot area coverage.

Screen ruling

The term ‘screen ruling’ refers to the number of halftone dots per linear

measurement of the pattern. The ruling can be expressed either as lines

per centimetre (lpc) or lines per inch (lpi). The term is a throwback to the

days when screens were made from ruled glass plates, and referred to the

number of cells per linear centimetre of the screen. A 60 lpc screen ruling

would produce 60 dot centres per linear centimetre of the image.

The screen ruling of any particular halftone can be determined by counting

the dots over a measured length, using a microscope or a tester made

specifically for the purpose. The most common type of screen ruling tester

is a small piece of film, on which is reproduced a line pattern and a scale.

By rotating the tester in contact with the halftone and noting the interference

patterns generated, the ruling frequency can be read directly from

the scale.

The decision about what screen ruling is the most appropriate for any

given job will be based upon the printing process and the materials used.

Simply stated, the finer the screen ruling, the more lines per centimetre

and the finer the detail which may be reproduced on a high-quality paper

surface. There is, however, a trade-off. The finer the screen ruling, the

more sensitive the image to dot-gain, the greater the likelihood of significant

tone changes occurring when the job is printed and the more difficult

the job to control on the press.

In any situation where the risk of the halftone dots spreading is

increased, due either to the process, as in the case of flexography, or to the

materials used, as in the case of web offset newspaper printing, a coarser

screen ruling will be chosen to minimise the effect of the dot change on

the detail and tone range of the printed image.

Screen angle

The purpose of using a halftone pattern

is to simulate various levels of grey with a system which is only capable

of reproducing one tone – usually solid black. The pattern of the screen

itself should not be readily visible to the viewer. Early in the development

of the use of halftone screens, it was recognised that the pattern of the

halftone was less noticeable and there was an improvement in the perception

of the detail of the picture when the screen pattern was angled at

45°. It is for this reason that any single ink printing, regardless of colour,

which is relying on amplitude modulated halftones for the representation

of tones, should be reproduced with the pattern at 45°.

Screening / Halftoning - Part 2

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.

For example 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.

Color Halftone And Screen Angles

In reproducing colour pictures that were originally continuous tone, printing

requires us to overlay the CMYK images as halftones. If the halftones are

AM (amplitude modulated), they will be formed from regular patterns of

fixed frequency. The need to rotate the angles of the halftone patterns comes

from the fact that printing cannot precisely place down one halftone dot on

top of a previously printed other-coloured dot. Commercial-quality colour

printing typically has 60 dots per cm in both the horizontal and vertical

direction. Slight inaccuracy in placing four identical patterns on top of each

other will result in an unpleasant moiré or screen clash. By rotating the

screen patterns at 30° from each other it is possible to reduce the frequency

of the moiré to a level which is below the visual threshold – it becomes too

small to be obtrusive. That reduced moiré is the pattern which is commonly

called the printing rosette.

As the halftone patterns are crossed lines at 90°, we only have 90° in

which to rotate the screens before we return to the

starting position. If we only printed with three

colours we would be OK, since there are three 30°

angles in 90°. But we print with 4 colours, and there

are not four 30s in 90, so we have to compromise.

The cyan, magenta and black are printed at 30°

apart, but the yellow, which has a low visual contrast

(it is difficult to see against the white paper) is

rotated to only 15° difference between the cyan and

the magenta.

The angle relationship between the four colours,

then, is usually fixed at 30:30:30:15. However, the

colours in this relationship may be switched. Most

usually this is done to overcome clash between the

yellow and another colour in a particularly important

picture or tint area.

Some more explanation:

When screens of cyan, magenta, and black are overlaid at their respective angles (105º, 75º, 45º) they form a moiré pattern called a "rosette." If the printer is required to use a fairly coarse AM/XM halftone screen (e.g. 85-150 lpi (newspaper & magazine work) ), then, depending on the image color content, the rosette pattern can become visible enough to be objectionable.

One way to reduce the visibility of the rosette structure is to move to a finer AM/XM screen which makes the rosette smaller and hence less visible. However, if that is not possible, then changing the separation method might be a viable option.

The majority of RGB to CMYK image conversions use "GCR" as the method (it is the default separation technique in Adobe PhotoShop). This ensures that wherever C, M, and Y inks are used black will be introduced.

To reduce, and even eliminate most rosettes, a better strategy is to use the UCR separation method on problematic images. UCR separations unlike GCR separations, primarily introduce black only in neutral and near neutral color areas. Since very little, if any, black is introduced in C and M screen tint areas – no rosettes are actually formed in those areas and hence no rosettes are visible. The result is smoother, less grainy appearing color.

While the UCR separation technique can reduce or even eliminate rosettes, there is a downside in that there will be a slight increase in ink usage as well as a slight reduction in color stability through the pressrun. That is why it should be used only for images with problematic colors - primarily dark blues and purples as well as dark skin colors/areas.

Rosettes:

Halftone dots are built inside halftone cells. Those cells have to fit together seamlessly. In order to rotate the screen, you have to rotate the cell – and there are only certain frequency/angle combinations at a given resolution where this seamless tiling is possible. The result is that at screen angles other than zero and 45 degrees, like cyan and magenta, the angles are not exactly as requested. As a result, the rosette can drift from being clear-centered to being dot-centered.

A well designed halftone screen will usually be able to maintain a clear-centered rosette across the largest diagonal plate that will be used. A less well designed screen may see "rosette drift" occurring over a distance of a few inches.

Rosette drift can also be caused by slight press misregistration caused by issues such as back sheet flare, web growth, or "waggle" (lateral sheet movement in the press). In this case rosette drift is not localized but occurs in the entire press sheet area.

Rosette basics

Printing depends on halftoning to simulate shades of gray, color, and image detail. In four color process printing, four halftones – one for each of the cyan, magenta, yellow, and black inks are overlaid to produce the image. Unfortunately, overlapping two or more halftone grids can create an objectionable pattern called a "moiré" which, interestingly is the basis of the rosette.

The greater the difference in angle between overlapping grids, the smaller the resulting moiré and the less apparent it is.

Once the grid has been rotated to 90 degrees, the moiré pattern is at its smallest and at a sufficient viewing distance seems to disappear.

Because a halftone screen is a quadratic grid (e.g. 90 degrees appears the same as 0 degrees, 135 degrees is the same as 45 degrees) the largest angle difference possible between two screens is 45 degrees, while the largest angle offset between three screens is 30 degrees (90/3=30). As a result, the defacto standard in four color printing has the three most visible process colors 30 degrees apart (C at 105 degrees, M at 75, and K at 45). Since Yellow is the least visible color it is angled at zero degrees – just 15 degrees from cyan. To further reduce moiré, the yellow screen is usually run at a higher frequency – typically about 108% of the other process colors.

The two kinds of rosettes

When screens of cyan, magenta, and black are overlaid at their respective angles (105, 75, 45) they form a moiré pattern called a "rosette."

Note that the yellow screen is not included since, because of its higher frequency, it does not form part of the rosette.This type of rosette is called a "dot-centered" or "closed-centered" rosette because each of the patterns has a dot in its center.

The second type of rosette is called a "clear-centered" or "open -centered" rosette. It is created by shifting one of the process colors one half row of dots from the other two colors.

In general, dot-centered rosettes:

• show a less visible pattern than clear centered ones

• have individual dots that land on top of one another - reducing chroma/gamut slightly

• produce color slightly differently than clear-centered rosettes

• tend to lose shadow detail

• with slight misregistration cause significant color shift

• are more popular with low screen frequencies - 100 lpi and lower

In general, clear-centered rosettes:

• show a more visible pattern than dot centered ones

• look slightly lighter due to more paper showing between dots

• produce color slightly differently than dot-centered rosettes

• tend to preserve shadow detail better

• resist color shifts better when slight misregistration occurs

• are more popular with high screen frequencies - 150 lpi and higher

MOIRE

A moiré pattern is an artifact that occurs in the print reproduction process when any two, or more, repeating patterns overlap each other.

Moiré in the print reproduction process is similar to the distortion effect on television.

The most common types of moiré encountered in the print production process.

Scanning/sampling moiré

These artifacts are caused by the frequency/angle of the scanner sensor (flat bed and drum scanners, or digital camera sensor) harmonically beating with a pattern in the object being scanned. In this case the artwork ends up having the moiré embedded in it and is part of the image.

Subject moiré

These artifacts are caused when the halftone screen that is being used to reproduce the image on press harmonically beats with a pattern in the image being reproduced.This artifact is sometimes referred to as "screening moiré" since it is the halftone screen that is causing the problem.

Screening moiré

Screening moiré, which is a term that is sometimes confused with subject moiré, is actually an artifact caused by either an inappropriate or incorrect halftone screen angle within a CMYK image. With modern screening systems this is rarely a problem. What is most likely to happen is that a screening moiré that is already present is somehow made more visible.

Another cause of screening moiré can occur if a prescreened (bitmap) graphic is imaged on a device that has a different resolution than the original art.

Resampling moiré

Moiré artifacts can be introduced when images are resampled (have their resolution changed) somewhere in the production process.Moiré, caused by resampling, usually occurs if the image is resized in a page layout program, or when the document is exported as a PDF, or as a result of the RIP settings when the document is processed by prepress.

Demosaicing moiré

On rare occasions you may encounter a "demosaicing" moiré. These occur when images with small-scale detail near the resolution limit of the digital sensor in a digital camera sometimes cause the demosaicing algorithm to produce repeating patterns, color artifacts or pixels arranged in an unrealistic maze-like pattern.

Single channel moiré

The standard photomechanical screen angles do not work best with digital screens. As a result some output device, halftone dot shape, screen angle and frequency combinations can result in moiré within one screen resulting in "single channel moiré."

One solution to avoid this problem was the development of shifted angles. The angular distance between screen angles remains more or less the same however all the angles are shifted by 7.5°. This has the effect of adding "noise" to the halftone screen and hence eliminating the moiré. For that reason, some individual screen sets may vary the requested screen angles slightly in order to overcome the potential for single channel moiré.

Paper related moiré

During the paper manufacturing process the side of a sheet paper that is in contact with the wire or forming fabric of the paper machine is the wire side (also called the reverse or bottom side). The wire side is usually not quite as smooth as the top or felt side and may carry a subtle impression of the wire pattern. If that pattern harmonically beats with the halftone screen pattern a subtle moiré will appear in the presswork. It often appears, and is confused, as a mottle. The difference is that mottling appears as random splotches while wire side paper related moiré appears as splotches that form a periodic pattern.

Screening / Halftoning - Part 1

Inks used in color printing presses are semi-transparent and can be printed on top of each other to produce different hues. For example, green results from printing yellow and cyan inks on top of each other. However, a printing press cannot vary the amount of ink applied to particular picture areas except through "screening," a process that represents lighter shades as tiny dots, rather than solid areas, of ink. This is analogous to mixing white paint into a color to lighten it, except the white is the paper itself. In process color printing, the screened image, or halftone for each ink color is printed in succession. The screen grids are set at different angles, and the dots therefore create tiny rosettes, which, through a kind of optical illusion, appear to form a continuous-tone image. we can view the halftoning, which enables printed images, by examining a printed picture under magnification.

Traditionally, halftone screens were generated by inked lines on two sheets of glass that were cemented together at right angles. Each of the color separation films were then exposed through these screens. The resulting high-contrast image, once processed, had dots of varying diameter depending on the amount of exposure that area received, which was modulated by the grayscale separation film image.

The glass screens were made obsolete by high-contrast films where the halftone dots were exposed with the separation film. This in turn was replaced by a process where the halftones are electronically generated directly on the film with a laser. Most recently, computer to plate (CTP) technology has allowed printers to bypass the film portion of the process entirely. CTP images the dots directly on the printing plate with a laser, saving money, increasing quality (by reducing the repeated generations), reducing lead-times, and saving the environment from toxic film-processing chemicals.

Screens with a "frequency" of 60 to 120 lines per inch (lpi) reproduce color photographs in newspapers. The coarser the screen (lower frequency), the lower the quality of the printed image. Highly absorbent newsprint requires a lower screen frequency than less-absorbent coated paper stock used in magazines and books, where screen frequencies of 133 to 200 lpi and higher are used.

Types of screening/halftoning

1.An amplitude modulated (AM), or conventional, halftone is one where

the frequency or periodicity of the pattern is fixed throughout the whole

illustration. The detail is carried by the change in size of the individual

dots in the pattern. Amplitude modulated halftones can be generated by

photographic means such as contact screens or by electronic methods.

2.A frequency modulated (FM) halftone pattern, often referred to as

stochastic or random screening, is one where the size of the individual

elements of the pattern (dots) are fixed at the outset and the tonal representations

are made by changing the numbers of the dots placed in any

given area of the image

BACK TRAP MOTTLING

Back trap mottling

Backtrap Mottling is mostly frequently seen in the ink printed in the first or second printing unit of a multicolor offset printing press. The cause lies in the partially absorption of ink into the paper. The partially variable immobilization of the ink leads to a variable splitting of the ink on the blankets.

An uneven (thicker/thinner in various areas) ink film remains on the paper surface, which the human eye sees as mottled. The partially variable ink absorption is configured differently on each of the following sheets, as a result of which the mottling effect increases rather than diminishes as experience shows.

Backtrap mottling can be triggered by an uneven formation which could be in the base paper stage and/or by a binding agent migration during the applied coating.

Another cause for backtrap mottling can be the surface energy on the paper. If this energy is too high it can influence the ink setting in a negative way.

There is a similar situation as far as ink is concerned. A higher surface energy value changes the rheology and the tack value. Faster ink setting with different tack values also has an influence on mottling. Faster ink setting and faster setting behavior of the paper influences mottling.

Solution to the problem.

The uniform base sheet formation is important. For uniform coating application and even binder/functional additive migration, which control open and close coating, one must provide an even coating thickness on base paper surface. Also, it minimizes the base paper influence on final coating.

Written By: Mr. Vijay Patharkar,

GM (QA, R&D), Bilt, Pune

Taken From: Technical Update in Print Week Magazine.

April 2011