An Introduction to Halftone Printing

Understanding Colour

Digital Repro

Mesh and Stencils

Inks and Colour Density

Printing

The Sericol Support Offer

Troubleshooting Guide

AZ of Terminology

Led by trends in design and marketing, advertisers are more frequently incorporating complexcolour graphics and photography in their screen printed advertising and point of salematerials. In printing terms, this has prompted a move away from solid colours and towardshalftone printing 4-colour halftones, in particular. As a result, screen printers now findthemselves competing in a changing marketplace where a greater emphasis is being placed onthe ability to produce good quality 4-colour halftone prints.

Some screen printers have been producing halftone work for many years and are perfectlypositioned to capitalise on the growing demand for this type of work. Indeed, the ability to print 4-colour halftones has traditionally been held up as a sign of a superior print operation. Otherprinters, however, are only just coming to grips with the new and greater demands of thisapplication of the screen printing process, having recognised the need to move with the times.

Whether you are an established halftone printer or entering the market for the first time, The SericolHalftone Printing Manual is intended to provide an invaluable source of information and advice.Derived from Sericols long involvement with halftone screen printing, it places a quarter century ofexperience and expertise at your fingertips.

For the newcomer, the manual is structured to provide comprehensive, step-by-step explanations ofall the key areas that affect the final printed result. It allows you to build up your knowledge of thedifferent processes at your own pace, in a logical, easy-to-follow manner. Plain English makes eventhe most complex concepts easy to understand.

The experienced halftone screen printer will probably prefer to dip in and out of the text to find theinformation and advice which is most relevant to their level of expertise. For you, the manualprovides useful information on how to adapt your current working practices to improve the qualityand efficiency of your operation and become even more successful in an increasingly competitivemarketplace. In particular, the manual spells out the considerable benefits to be gained fromadopting the latest digital repro technologies.

The expansion of halftone printing has been accompanied by a general move towards computer-generated art, coupled with major developments in digital pre-press technologies. Just as screenprinters are being asked to produce more and more 4-colour halftones, to ever higher standards, sothey are being asked to work in an increasingly electronics-based production environment. Thesedemands, together with advances in the speed and performance of hardware and software and thegreater affordability of even high end digital pre-press systems, are rapidly changing the workingpractices of the screen printing industry and the graphic arts industry as a whole. The recent, rapid development of digital pre-press technology means that there are new skills andnew opportunities for even the most experienced screen printer to master. The manual offers anauthoritative introduction to this increasingly important area of the industry.

The Sericol Halftone Printing Manual is, however, just a part of Sericols overall customer package.An extensive range of advanced ink and consumable products is complemented by an experiencedteam of expert staff, providing a wide range of specialist services designed specifically to meet thevaried needs of the halftone screen printer. This combination of products and service makes Sericolthe obvious partner for any printshop wishing to develop its own 4-colour halftone printingoperation.

CHAPTER 2

CHAPTER 3

CHAPTER 4

CHAPTER 5

CHAPTER 6

CHAPTER 7

CHAPTER 8

Foreword Index

GLOSSARY

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1.1 1.2

Before you consider printing halftone work, it isessential to have a clear understanding of thefundamental principles underlying the halftoneprinting process. This opening chapter explainswhat halftones are; why you would want to usethem; how they are made; and the variablesyou must control when producing halftonepositives

Look at a photograph, airbrush illustration, artistspainting or shaded pencil sketch and you willnotice that it contains smooth gradations of tone,from the lightest to the darkest areas. This type ofimage is described as a continuous tone image(often referred to as contone or CT). Nowcompare a continuous tone image with aconventional single- or multi-colour screen print:youll notice that the print contains flat areas ofcolour and no gradations of tone.

Like most other printing processes, screenprinting cannot be used to produce continuoustone images, as it is not capable of laying downcontinuously variable densities of ink. However,through the use of halftones, it is possible for thescreen printer to create the illusion of acontinuous tone print.

Fig. 1.1. A halftone is made up of varying size dots tosimulate continuous tones.

What is a halftone?

A halftone is the result of converting a continuoustone image into a pattern or grid of regularlyspaced dots (a halftone screen). The individualdots all have the same density, but vary in size.

On the final print, light reflected by the dotsmerges with light reflected from the substrate toaccurately recreate the smooth tonal gradationsfound on the original image.

The tone (lightness or darkness) of a printed areais dictated by the size of the printed dots. Thedots reflect less light than the white substrate,which means that larger dots, which cover moreof the substrate, reduce the overall amount oflight reflected by the print, resulting in a darkertone. Conversely, small dots cover less of thesubstrate, so more light is reflected, resulting in alighter tone.

The relative size of each halftone dot is describedas a dot percentage. The dot percentage scaleruns from 0% to 100%, where white (no dot)equals 0% and black (total coverage of thesubstrate) equals 100%. A midtone area is madeup of 50% halftone dots, which cover half the areaof the substrate.

Fig 1.2. The amount of the surface of each halftone cellcovered by a printed dot dictates the tonal effect thelightness or darkness of that area of the print.

Fig 1.3. Printing different dot percentages together allowsyou to create the impression of tonal gradation. The lessnoticeable the individual dots, the smoother the tonalgradation appears.

The actual size of the dots will be dictated by thelevel of definition required and will be influencedby the distance from which the finished print willbe viewed. Put simply, the less noticeable theindividual dots in a halftone screen, the moreoriginal detail can be resolved on the final print

Today, the photographic method has beensuperseded by electronic scanners andimagesetters, which do away with the need forprocess cameras, halftone negatives and thescreens themselves.

A scanner is used to capture a digitalrepresentation of the original image. A pattern ofred, green and blue light is reflected (ortransmitted, in the case of transparencies) by theoriginal image. This pattern is captured as a digitalimage and is subsequently passed on to animagesetter, which can be thought of as a veryprecise laser printer. An interpreter in theimagesetter converts the digital image into amatrix of halftone dots (arranged in regular rowsand columns) and then records each dot on asheet of light sensitive or thermal imaging film.The matrix of dots (also known as a raster)represents the halftone screen.

Digital techniques speed up the production ofhalftones significantly. More importantly, theyallow far more creative and technical control overthe many variables which affect the final result.The screen printer has the opportunity to modifythe digital image extensively before it is output tofilm, using a desktop computer or workstation tocontrol variables such as colour balance, imagesharpness, contrast, dot gain and tonalcompression. These variables will be discussedlater, but for now it is safe to assume that there isno longer any reason to produce halftones usingtraditional techniques.

Halftone parameters

When a digital halftone is output to film there areseveral key parameters which must be specified:the screen ruling; screen angle; and dot shape.Each can have a marked effect on the appearanceof the final print.

Screen ruling is a measure of the frequency of thelines of dots on the halftone screen that is, theactual size of the dots and how close togetherthey are. The ruling is measured in lines per inch(lpi) or centimetre (lpcm): thus, you will find 150lines of dots in each inch on a 150 lpi halftonescreen. In theory, if you wish to find out thescreen ruling of a single-colour print you can lay arule and measure the number of dots along aninch or a centimetre length. However, this is more

An Introduction to Halftone Printing CHAPTER 1

20% dot 80% dot

and the smoother the tonal changes will appear tothe eye. Ideally, the dots should be too small tobe noticeable, as this will ensure that there is astepless gradation of tones between light and darkareas. For example, the dots on a 48-sheet postermay be very noticeable if you view the posterfrom close range; but when you view the posterfrom a distance of several metres, the eye is nolonger able to make out the individual shapes ofthe dots and the smooth tonal effect is created.

Fig 1.4. Dot size percentage is a relative measurement bothof these dots cover 30% of their halftone cell.

How are halftones made?

Traditionally, halftones were createdphotographically, by producing a halftone filmnegative, which was then contact exposed toanother piece of film to obtain a positive. Thenegative was made by placing a screen a glassplate carrying a finely ruled grid made up of lineswhich cross each other at 90 between theoriginal continuous tone image and a sheet offilm. Light from a process camera was reflectedfrom the original image, passed through thesquare windows formed by the crossed lines onthe screen, and on to the film. This caused theoriginal continuous tone image to be rendered asa series of dots. The size of the dots depended onthe brightness of the reflected light, which in turnwas dictated by the tone of the original: forexample, highlight areas reflected the most lightand created the biggest dots on the negative and,after contact exposure, the smallest dots on thepositive film.

To reproduce a full-colour original, multipleexposures were made with the light being passedthrough red, green and blue filters (on separateexposures), to produce the film positives (orcolour separations).

STOCHASTIC SCREENS

In recent years, an alternative screening method has emerged, which offers certain benefitsover conventional halftone screening. Stochastic screening (also known as FrequencyModulated or FM screening) takes the opposite approach to halftone screening instead ofarranging different sized dots in a regular pattern, it uses dots of uniform size and distributesthem in random patterns. Variations of tone are achieved by adjusting the concentration ofdots in a certain area the dots are spaced further apart in highlight areas and clusteredtightly in shadow areas.

The stochastic approach can help to produce smoother tonal gradations, as the use of similarsize dots means that prints do not suffer from tonal jump (see Dot shape page 1.6). Therandom pattern of dots also eliminates the problem of moir (see sub-section dealing withmoir, page 1.7).

Another benefit is the use of very small dots (the equivalent of using very high screen rulings),which can reproduce finer details and smoother tonal transitions. However, until recently, thisproved to be a double-edged sword for screen printers, as the largest dot size available 20microns was too small to be supported by even a 150 threads per centimetre (tpcm) mesh,with its 25 micron opening. Today, stochastic screens can carry maximum dot sizes up to 100microns, so the problem no longer exists.

Stochastic screening is still relatively new. Some of the specialist software developed toproduce these types of screens is still not capable of producing top quality positives (althoughthis will change) and an extremely accurate (and expensive) output device is essential for bestresults. If you do want to print with stochastic screens, therefore, you will need to find a reprohouse which has high end software and imagesetting equipment and a staff who are qualifiedin stochastic screening for screen printing applications.

Fig 1.9

Conventional halftone

screening

Stochastic

screening

1.3 1.4

easily achieved using the screen ruling indicatorwhich is included at the back of this manual.

The higher the screen ruling, the smaller and lessvisible the dots. Thus a halftone screen with ascreen ruling of 133 lpi (54 lpcm) is finer than ahalftone screen with a screen ruling of 65 lpi (25lpcm). Generally speaking, higher screen rulingsgive sharper and more detailed prints withsmoother tonal changes. The caveat is that theyare harder to print well, being more susceptible todot gain (see Dot gain page 1.6) and darker areasmay well fill in completely, sacrificing shadowdetail. Also the smaller dots are not alwayssupported by the mesh. Higher screen rulings alsorequire more digital information to be captured bythe scanning device, which can slow down theprocessing of the digital image.

The type of substrate you are printing on to willinfluence the optimum screen ruling for a halftoneprint: higher screen rulings can be used withsmooth, coated, non-porous substrates.

Higher screen rulings can give sharper, moredetailed prints, but require more skill to print well.

Fig 1.5. 120 lpi halftone screen

Fig 1.6. 55 lpi halftone screen

Halftone angles (often referred to as screenangles) are not be confused with screen meshangles. (The latter will hitherto be referred to asmesh angles to avoid confusion.) They describethe angle of the halftone screen (the lines of dots)as measured from the horizontal axis.

Changing the angle of the halftone screen canmake the dots more or less noticeable to the eye.Since the aim of halftone printing is to reproducethe smooth tonal transition found on the originalimage, the less noticeable the dots are the better.

When printing a single colour halftone, thehalftone screen is usually produced at an angle of45 the angle at which the dots are leastnoticeable to the eye. The dots are mostnoticeable when the halftone screen is angled at90.

Fig 1.7. Individual dots are most noticeable when the halftonescreen is angled at 90. The dots are least obvious when thehalftone screen is angled at 45.

Halftone angles become especially important onmulti-colour halftone prints. Theoretically, if eachpositive is given the same halftone angle and eachdot on each of the positives is in exactly the sameposition, the printed image would be perfect. Inpractice, even a minute misregister results indistinct interference patterns, known as moir.You can see this for yourself by laying twopositives together and rotating the top one to seethe resulting patterns. You should find that themoir is least noticeable when the positives are30 opposed.

Fig 1.8. Theterm moirdescribes theunwantedinterferencepatterns whichare producedwhen regularpatterns, such ashalftone screensand screenmesh are placedover oneanother.

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1.5 1.6

CHAPTER 1

background at higher dot percentages. However,when you generate a halftone screenelectronically, using an imagesetter, you canchoose from several different dot shapes,including diamonds and ellipses.

The shape of the dot has little effect on theappearance of the printed image, especially at finehalftone screen rulings where the dots will not benoticeable to the eye in any case. (However,elliptical dots may help to minimise moir.)Where dot shape does have an effect is on thesmoothness of tonal gradation in the midtoneareas.

With a conventional halftone screen, as the dotpercentage increases and nears 50%, the cornersof individual dots just touch the corners ofadjacent dots, forming a checkerboard pattern.This gives the impression of an abrupt darkeningof the tone at that point and is known as a tonaljump. The visual effect is to destroy the smoothtonal gradation from light to dark. Tonal jump isparticularly noticeable in fleshtones where distinctsmooth gradations of tone give way to aposterised effect. The image may also have anoticeably grainy appearance in areas on eitherside of the 50% dot.

Diamond- and elliptical-shaped dots are a betteralternative to conventional round/square shapeddots as they reduce the impression of tonal jump.As the dot percentage increases, the edges of thedot join in two separate stages: first, the ends ofthe dots join; as the dot percentage continues torise, the sides of the dot join. In this way,diamond- and elliptical-shaped dots give asmoother tonal transition around the midtonearea.

Fig 1.13. At the 50% dot percentage, each corner of aconventional halftone dot (top) just comes into contact withits neighbour. This creates a tonal jump (an abrupt darkeningof the print) as dot values increase in size from 40% to 50%.

Elliptical dots (bottom) are joined in only one direction, sothe tonal transition is smoother and there is no noticeabletonal jump.

While you cant ever eradicate moir, you canminimise it by keeping a 30 angle between theseparate halftone screens. For a 2-colour halftone(duotone) you could position the dominant colourat 45 and the other colour at 15. When you tryto specify halftone angles for a 4-colour print,however, you will run into an obvious problem:because halftone dots are arranged at 90 to eachother on the halftone screen, maintaining a 30angle between each screen will mean that the firstand the last screens would be at the same angle,causing moir.

For this reason, the angle of the fourth screen isalways going to be a compromise. However,there are accepted sets of angles for both offsetand flexographic printing which are proven tominimise moir when printing four colours.Producing the four positives at these anglescauses the different coloured dots to be printed ina characteristic rosette pattern, with some of thecolours overlapping.

Fig 1.10. Halftone screens have 2 angles opposed at 90. If30 was maintained between each colur, the last angle wouldbe the same as the first.

As explained previously, the halftone pattern ismost noticeable when the halftone angle is at 90.For this reason, yellow the lightest and leastvisible colour is often positioned at 90.Magenta is regarded as being most visible to the

eye, so this halftone screen is set at 45 (37.5when using the Flexo standard) to render thehalftone dots less obvious. The cyan and blackhalftone screens are most likely to cause moir, sothey are set at 30 to the magenta screen.

Fig 1.11. When 4-colour process inks are printed together,they form a distinctive rosette pattern in the areas wherethey overlap one another.

Fig 1.12. Left, standard Offset halftone angles (DIN 16547a)and standard Flexo halftone angles (DIN 16547b), right.

While the use of correct halftone angles canminimise the risk of moir, there is another majorfactor to consider the interference pattern set upbetween the halftone screen on the stencil andthe threads of the mesh you are using. The closerthe halftone screen ruling to the mesh count, thegreater the risk of moir occurring. This problemcan be overcome by carefully controlling thehalftone angles/mesh count relationship, anglingof the screen mesh, experimenting with thehalftone angles and controlling the halftone screenruling/mesh count relationship (see, AvoidingMoir on page 1.7).

Dot shape When halftone positives are producedusing the traditional photographic process, dotsappear as black circles on a white background atlower dot percentages, squares at around the50% dot, and open white circles on a black

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C 67.5Y 82.5

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Dot gain

An important variable that must be controlled onany halftone print is dot gain. This describesincreases in the size of printed dots whencompared with the dots specified by the softwarewhich created the halftone screen. As its namesuggests, dot gain causes dots to grow larger, sothat they cover more of the substrate and reducethe intensity of the reflected light. The effect ofthis is to make the halftone appear darker,especially in midtone areas. Dot gain also lowerscontrast by darkening highlight tones, and it caneliminate shadow detail by causing shadow tonesto fill in.

There are two types of dot gain: optical andphysical.

Physical dot gain is the result of changes in dotsize caused by the pre-press and productionprocesses. When a halftone screen is output tofilm by an imagesetter, the individual dots mayincrease slightly in size. Similarly, transferring thehalftone screen to the stencil may also increasethe size of the dots. However, the biggestincrease in dot size is caused by the printingprocess itself. Ink viscosity, press parameters such as screen tension and squeegee pressure and the absorbency of the substrate will all affectthe size of the printed dot.

Optical dot gain describes the apparent increasein dot size caused by the scattering of light in thesubstrate. This causes the dots to cast tinyshadows, which have a darkening effect.

Total dot gain is the sum of both physical andoptical dot gain and is measured using adensitometer (see Chapter 3).

Dot gain occurs around the circumference of thedots, so the larger the dot, the greater the gainwill be. This is why dot gain is most noticeable inmidtone areas where the dots have the highestcircumference to surface area ratio. (Figuresquoted for total dot gain always refer to the 50%dot, precisely because dot gain is greatest at thisdot percentage).

As previously mentioned, dot gain also variesbetween different ink systems it can be as highas 20% with some conventional UV inks, and aslow as 3% with some water-based inks. This,together with the wide variety of substrates used,

makes it impossible to quote a standard dot gainfigure for the screen printing process. Thus, youhave to determine the dot gain for your own setup and compensate for it by reducing dot sizes onyour positives accordingly. (How to control dotgain is covered in Chapter 3.)

Fig 1.14. This image shows the effects of dot gain filled inshadows, dull highlights and an overall lack of contrast.

Fig 1.15. Compensating for dot gain brightens the highlights,reveals shadow details and boosts overall contrast of theimage.

1.7 1.8

Alternative AnglesAs explained previously, there is no reasonwhy you shouldnt experiment with halftoneangles to come up with the optimum result foryour own work. The halftone angles shown onthe right provide a starting point for yourexperiments. Experimentation may be necessary to counterspecific production parameters. For instance, ifyou have used gray component replacementduring the processing of your digital file (see,Chapter 3) the black screen will be muchheavier than usual, which may make the dotsmore obvious. In this case, you may find youachieve a better result if you switch the

How to select mesh angles

Included in this manual are two sets of positives for each of the following halftone screen rulings:

55 lpi/21.6 lpcm 75 lpi/29.5 lpcm 100 lpi/39.4 lpcm 133 lpi/52.4 lpcm60 lpi/23.6 lpcm 85 lpi/33.5 lpcm 120 lpi/47.3 lpcm

One set of positives was produced using standard Offset halftone angles; the other using standard Flexohalftone screen angles. Both sets include panels which simulate different mesh angles when printedthrough mesh fixed at 90 to the frame.Assume that you wish to establish a standard for printing 85 lpi halftones for a particular ink system. Thefirst step is to determine which set of standard halftone angles produces the least moir. Prepare ascreen, stretched with straight mesh of the recommended count for the ink system you wish to use;place the two 85 lpi positives adjacent to each other and square to the edge of the frame; expose thescreen and develop under your normal production conditions.Next, print through the stencil using your chosen ink at the normal thinning rate. Study the print to seewhich of the Offset or Flexo angles gives the best result that is, no moir pattern on the print. Youmay find, for example, that the Flexo angles produce the best result, giving a moir-free print for threeof the colours, with moir only present on the yellow screen. To eliminate this moir pattern, look at theother Flexo panels marked 5, 15 and 30. If the 15 panel reduces the appearance of the moir, thenhave the screen for the yellow printer stretched at 15.

Because the maximum width of screen fabric is 2.2 metres, angled mesh is not an option for printersusing very large format screens. One solution to this problem is to print one colour at a differenthalftone ruling to the others, this will eliminate the clash between that colour and the mesh. Thedifferent screen ruling is not usually detectable in the final print. For this reason, if you have room onyour test screen it is a good idea to include the screen ruling positives that are either side of the rulingyou intend to use. Alternatively you could experiment with different halftone angles (see boxAlternative Angles)

The standard Litho and Offset halftone anglesminimise moir occurring between filmpositives. However, the screen printer also hasto consider the possible interference effectcreated between the halftone screen and themesh.

To reduce the chances of moir occurring, youmust ensure that the mesh count you use isappropriate for the halftone screen ruling thatis, the mesh count which is least likely to causeinterference.

You can generally calculate which mesh countsare appropriate and which are not bydetermining the ratio between the mesh countand halftone screen ruling you are using. Tocalculate the ratio, use the following formula:

Ratio =

(Make sure that both figures refer to the sameunit of measurement either inches orcentimetres.)

If the ratio is 5.0 or above, you can be reasonablysure of producing moir-free prints, all otherfactors being equal. If the ratio is below 3.5, youshould choose a different mesh count as moir isa strong possibility.

It has also been shown that moir is more likely ifthe first decimal place in the ratio is an evennumber for example 3.4.

If you have to use such a combination, you caneither experiment with the halftone angles on thepositives, or change the angle of the meshitself

AVOIDING MOIR

halftone angles for the black and magentascreens. However, if you are trying toreproduce fleshtones, always try to keepyellow and magenta 45 apart. If green is thedominant colour in the image for example ina landscape photograph you may discoverthat it helps to have yellow and cyan 45apart.Although 45 has been shown to produce theleast noticeable halftone dots, it can sufferfrom interference with the mesh. In whichcase, use the Flexo angles describedpreviously or experiment with your own set ofangles.

Possible Alternative Halftone Angles

C M Y K

172.5 52.5 7.5 112.5

82.5 112.5 7.5 52.5

22.5 52.5 7.5 82.5

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CHAPTER 1

GEOMETRICSCREENINGA screening method, which is gaining inpopularity with screen printers, is Geometric.The positives in this case comprise parallellines, instead of dots, which vary in thickness toeffect tone changes (see fig. 1.16).

As with halftone dots, the lines must run atdifferent angles for each of the four printingcolours to avoid primary moir (see page 1.7).Because each geometric positive has only oneangle of direction (conventional halftones havetwo angles, see Fig 1.10. page 1.5), it is easier toproduce a set of angles, that avoid primary moir,and can be used on straight mesh with minimalsecondary moir being created.

Other advantages include smoother tonalgraduations and reduced tonal jump. This isachieved because there is no break point as withconventional dots. i.e. where a dot begins toseparate from adjacent dots below 50% tonevalues. Commonly used rulings are 55 - 75 l.p.i.,although some printers regularly use 85 l.p.i. Mostobservers comment that finished prints appear tobe a finer screen ruling than is actually used but,adversely, others comment on the grainyappearance of some images.

Can geometric totally eliminate moir?In the lightest areas, where the lines becomeextremely narrow, they can reach a point wheresome will print and others may dry in, appearingas breaks in the lines, possibly causing visibleinterference patterns on high key images (wherethe majority of colours are less than 50% tonevalue).

Some repro houses have worked very closely withtheir clients to achieve the optimum angles for thework they produce. However, most printers wouldagree that total elimination of moir is unlikelywith any halftone pattern but the tendency isgreatly reduced with geometric. Ultimately, toensure consistent and best quality halftone prints aprinter must fingerprint his press, type of workand production methods to establish optimumangles and other parameters such as print orderand dot gain values.

Dot Gain?

Although the halftone pattern is lines, the termdot gain still applies. Target areas can still bemeasured with a densitometer to record anygains, or losses, in tone value. As withconventional halftones, test prints are necessarywith any unfamiliar ink/screen ruling/substratecombination.

In the interest of good communication betweenprinter and repro house, we recommend having ageometric positive with agreed step valuesproduced by the repro house. The resulting valueson the print can be recorded and used to set up atone curve. This can be applied wheneverpositives are produced for the same screen ruling,ink, substrate and production equipment. As aguide a useful set of percentage values is 5, 10,20, 30, 40, 50, 60, 70, 80, 90 and 95.

Fig. 1.16.

Enlargement ofmagenta positive.

1.9 1.10

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Any screen printer who is contemplating theuse of digital repro techniques for 4-colourprocess work, must have a clear understandingof colour: its properties; how the eye perceivesit; how it is represented on digital displays andthe printed page; and the ways in which it canbe managed. This information is essential if youare to make the most of the powerful colourcontrols which digital pre-press systems offer.This chapter provides a guide to thefundamentals of colour theory; the RGB andCMYK colour models; and the relationshipbetween colours on the desktop and final print.

First things first: colour does not exist it ismerely an illusion created by a combination oflight, the perceptual mechanisms of the humaneye and brain, and the ways in which differentsurfaces affect the light that they reflect ortransmit. To understand basic colour theory, youmust appreciate how these factors combine tocreate the colours that your brain perceives.

Additive colour So-called white light is made upfrom various wavelengths of radiation emitted bya light source, such as the sun. These wavelengthsare what the human eye perceives as colours andare referred to collectively as the visual spectrum.(There are many other wavelengths that falloutside the visible spectrum and cannot beperceived by the human eye. Examples include:infra-red radiation; ultra violet radiation; and X-rays.)

Using a prism, you can separate white light intoits constituent colours. You will notice that themain bands of colour are red, green and blue.These are said to be the primary colours of light.The entire range, or gamut, of colour that thehuman eye can see can be expressed by addingthese three colours of light together in differentproportions and intensities. For example, addingall three colours together at full intensity produceswhite. Adding equal proportions but lowerintensities of all three colours produces shades ofgray. Adding any two of the primary colourstogether produces a secondary colour: green andred produce yellow; green and blue producecyan; and red and blue produce magenta.(Whenever you add two primaries together thesecondary colour they produce is brighter hencethe term, additive.) The absence of any coloursproduces black.

Fig 2.1. Additive colour notice how cyan, magenta andyellow (the subtractive primaries) are formed in areas wheretwo of the primary colours overlap.

Your computers monitor uses an additive coloursystem, known as the RGB colour model, todisplay colours. The screen is covered with tinyred, green and blue phosphors which glow (emitlight) when activated. So, the monitor reproducesthe yellow of a lemon by activating a combinationof red and green phosphors. The mixture of redand green light that they emit is perceived asyellow by the eye.

The brightness of the colours on screen isdetermined by the intensity of the light emittedby the phosphors and this is dictated by theintensity of the electrons which are fired at thescreen to make the phosphor coatings glow.

Subtractive colour A printed image does not emitlight in the same way as the phosphor on yourcomputer monitor. Instead, the light that reachesyour eye has been reflected from the surface ofthe printed material. Crucially, all surfaces act as afilter reflecting or transmitting certainwavelengths of light and absorbing (subtracting)others. For example, when light strikes a tomato,the pigment in the surface of the tomato filters outall the non-red light, so what you see is a redcolour. Printing inks act in the same way,absorbing some wavelengths and reflectingothers.

This explains why the primary colours of light donot produce an additive effect when printed

together. Put simply, each primary colour filtersout the other primaries, making it impossible tocreate secondary colours. To illustrate: supposeyou print an area with blue and red ink; the blueabsorbs all the non-blue wavelengths (red andgreen) and the red absorbs all the non-redwavelengths (blue and green). Hence, all theprimary colours would be removed, resulting inblack.

To reproduce colours on printed materials, youmust control the amount of red, green and bluelight which is reflected by the print. This can beachieved by printing combinations of thesecondary colours of light cyan (C), magenta (M)and yellow (Y). These are generally referred to asthe subtractive primaries, as each colour absorbsone of the primary colours of light: cyan absorbsred; magenta absorbs green; and yellow absorbsblue. For instance, if you want to reproduce a redcolour, you would print with a combination ofmagenta and yellow inks to filter out both thegreen and blue light.

Fig 2.2. Subtractive colour - Notice how the primary coloursare formed where two of the subtractive primaries overlap.

In theory, printing cyan, magenta and yellow inkstogether should produce black. However, this isnot the case the result is a dark brown colour.Adding black ink to the subtractive primariesincreases the possible density range that can beprinted, ensuring clean blacks, deep shadows andneutral grays. Black is known as the key colourand is denoted by the letter K - hence the CMYK

colour system and 4-colour process printing.

Colour percentages

By using different proportions of CMYK inks youcan effectively control the proportions of red,green and blue light reflected from the print andthe colour of light perceived by the eye. With4-colour process printing, the four inks are printedas separate, overlapping halftone screens. In thisway, the proportion of each subtractive primary isdictated by the size of the halftone dots for thatscreen. (Process colours are expressed in terms ofthe dot percentage for each CMYK ink.) Dot size,then, dictates colour as well as tone.

It follows that dot gain can have a significanteffect on colour also. If the size of a yellow dotincreases by, for instance, 5% when it is printed,then it will absorb 5% more of the blue light thatfalls on it. This will result in a warming of theimage, as there will be less cold blue lightreaching the eye.

The problem is exacerbated by the fact that dotgain values vary from colour to colour, dependenton the print order. This can have a marked effecton the colour balance of 4-colour process prints.For example a colour printed onto virgin paperwill have a slightly different dot gain value thanwhen overprinting because of factors such asstock absorpency, ink build etc. Added to this isthe fact that dot gain is not uniform, so midtoneareas are more likely to suffer a greater colourchange than highlights and shadows. This explainswhy the most common example of colour changedue to dot gain occurs in the magenta content offleshtones.

Colour fidelity is another reason why dot gainmust be measured and controlled.

2.1 2.2

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Understanding Colour CHAPTER 2

BLUE

MAGENTA

CYANGREEN

YELLOW

RED

WHITE

YELLOW

REDGREEN

CYAN BLUE MAGENTA

C0LOUR GAMUTS

Each colour system is said to have a colour gamut. This describes the range of colours thesystem can reproduce. As explained previously, white light contains all the colours that thehuman eye can perceive.

The RGB colour system, as used in computer monitors, is capable of reproducing millions ofdifferent colours, but it cannot reproduce the entire gamut of colours that the eye can see.This is due to the limitations of the phosphor coatings which emit the light.

The CMYK colour model has an even smaller colour gamut, due to inherent impurities in thepigments used to manufacture printing inks; the substrates they are printed on to; and thevery fact that it is a subtractive system the inks absorb light so light intensity (and colourbrightness) is reduced.

In order to print an image that was captured as a colour digital file, it must be separated converted from the additive RGB colour model, as used by scanners and monitors to thesubtractive CMYK colour model, as used in printing inks. This conversion process can giverise to problems in terms of colour fidelity. (Colour separation is covered in greater detail inChapter 3.)

The difference in colour gamut also makes it very difficult to predict how colours on amonitor will appear in print. Even after an image has been separated, it may still appear verydifferently on screen to how it will appear in print. (You can prove this by holding a printedcolour swatch next to the monitor and bringing up the same colours on the screen. Thechances are they will appear very differently on screen.)

The common solution is to never trust thecolours you see on the monitor. Instead, usethe softwares colour information tool(referred to as the Info palette in AdobePhotoshop) to measure colour rather thanjudging it by eye. You should also makesure that you use a reliable proofing system,which provides an accurate representationof what the printed result will look like.(Both topics are dealt with in later chapters.)

Another option is to use a monitor which iscalibrated to show colours as they willappear on the printed page. These arerelatively expensive and must be usedunder carefully controlled viewingconditions changing light levels, flare onthe screen and even the reflection of theoperators clothing can affect the on-screencolours. Similarly, different softwareapplications may represent coloursdifferently. For these reasons, so-calledsoft proofing (judging colours from amonitor by eye) cannot be relied on totallywhen colour fidelity is critical.

SUBSTRATES AND COLOUR

In 4-colour process printing, the CMYKinks are printed as overlapping halftonescreens. This requires that the inks aretransparent, so that the light can passthrough them, reflect off the substrateunderneath and bounce back towardsthe eye. In areas where the coloursoverprint, the colour of the reflected lightis determined by the combined filteringeffect of all the inks.

The fact that the light is reflected fromthe substrate, however, introducesanother colour variable namely, thecolour of the substrate. If the substrateis a colour other than white (whichreflects all colours equally) it will haveits own filtering effect on the light. Thismeans that generally acceptedcombinations of ink colours couldproduce markedly different colours tothose specified.

2.3 2.4

Fig 2.3. The CIE (Commission Internationale delEclairage) Yxy colour model indicates the colour gamut ofdifferent colour models. As you can see, CMYK inks canproduce only a small percentage of the total colour gamutperceived by the eye.

CMYKGamut

RGBMonitorGamut

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There are many screen printing parameters toconsider when producing 4-colour processwork, but thanks to recent advances in thedigital repro process, it is no longer necessaryto tackle all the key variables on press. A fullunderstanding of colour manipulation at therepro stage has enabled printers whospecialise in halftone work to move theemphasis away from the manipulation of inkformulations at the time of printing andtowards tighter control over the production ofhalftone film positives during pre-press.

By investing in the latest technology andadopting up-to-date working practices, screenprinters now have the necessary means tostandardise as far as possible the 4-colourhalftone screen printing process. The potentialbenefits, in terms of speed, quality, efficiency,cost and repeatability, are substantial.

This chapter opens with a tour of the digitalrepro process, providing an introduction to thevarious technologies and how they operate. Buthow something works, and how it is of

practical use, are two very different questions.The following section addresses the practicalapplication of digital systems to control keyhalftone printing variables, such as colourbalance, dot gain and tonal range.

The other question to consider is whether ornot to bring the digital repro process in-house.The final section provides valuable pointers onwhich parts (if any) of the repro process screenprinters should be handling themselves, andwhich should be left to specialist repro houses.

The digital repro process can be split into threestages: input; processing; and output. Thesestages are shown in figure 3.1. Whether you planto handle the repro process yourself, or use aspecialist repro house, a clear understanding ofeach stage is vital if you are to achieve the highstandard of film positives needed to producequality halftone prints.

ORIGINAL IMAGES

The first step in the digital repro process is toconvert original images, such as photographs andartists illustrations, into digital files. But beforeyou begin, you need to check whether the imagesare suitable for print reproduction.

Often, you will have little say over which imagesare to be used the client or their designer willhave made the choice before you becomeinvolved. However, by understanding thelimitations imposed by less than ideal images, youare better equipped to deal with the problemsthat are likely to arise.

If possible, ask for photographic images to besupplied as transparencies rather than prints.Transparencies possess a wider possible tonalrange and more saturated colours.

The rich colours used in some paintings areimpossible to reproduce using printing inks. Acommon solution is to photograph the work ontransparency film, along with a colour chart. Youcan then compare the colours of the chart in thescanned image with the colours of the originalchart and match the colours as closely as possibleusing image editing software.

Artwork which has been generated in computerimage editing and illustration programs can betransferred directly to the processing stage.

3.1

Digital Repro CHAPTER 3

THE INPUT STAGEOnce the appropriate images have beenevaluated, the next step is to convert them todigital image files, using a scanner.

During the scanning process, red, green and bluefiltered light is passed through or reflected fromthe original image and is sampled by an array oflight sensitive measuring devices either photomultiplier tubes (PMTs) in a drum scanner orcharged coupled devices (CCDs) in a flatbedscanner. The brightness and the colour of thetransmitted or reflected light is used to mapcolours and tones from the original image to agrid of small squares. These squares are known aspicture elements (or pixels) and the grid is knownas a bitmap (hence, bitmapped images). Pixels canappear as white; black; a shade of gray; or acolour.

Four closely related parameters are specified atthe time of scanning:

scan resolution

scan size

bit depth

colour model

Scan resolution

Scan resolution is a measurement of the numberof pixels in a given distance. It is most commonlyquoted as the number of pixels per inch (ppi) forinstance, 133 ppi. Resolution can also be quotedin terms of a Res number, which specifies thenumber of pixels per millimetre. A resolution offive pixels per millimetre, for example, would bequoted as Res 5.

The actual size of each pixel is determined by theresolution of the image, in much the same way asthe size of a halftone dot is determined by thehalftone screen ruling. (Incidentally, there is animportant relationship between scan resolutionand halftone screen ruling see, Scan resolutionand sizing). At high scan resolutions there aremany pixels per inch, so individual pixels aresmall: details are rendered sharply and tonaltransitions appear smooth. At low scan resolutionsthere are fewer pixels per inch, so individualpixels are larger. At very low resolutions, the

3.2

ORIGINATIONFlat copy artwork,

photographic prints& transparencies

DIGITAL ARTWORKCreated on PC or AppleMac

and supplied on disk(Jaz, CD etc.)

SCANNERMaps original and

converts image intodigital information

COMPUTERImages adjusted using

photograph manipulation softwareand combined with other artwork

in page layout programme

RIP(Raster Image Processor) creates halftone screens

and angles

IMAGESETTERProcesses informationand outputs separated

film

POSITIVESRight reading

emulsion side up(for screen printing)

DIGITAL PRINTERProofing results used toapply final adjustmentsbefore sending to RIP

CROMALIN PROOFProduced fromfilm positives

Fig 3.1. The digital repro flow path

STENCIL MAKING&

PRINTING

individual pixels are so large as to be visible tothe eye, causing images to look jaggy. This effectis most noticeable in areas of the imagecontaining straight, diagonal lines, which appearas sets of distinct steps hence the term,staircasing.

Fig 3.2. At high scan resolutions (left), fine details can beresolved and tonal gradations are smooth.

At very low scan resolutions (right), individual pixels may bevisible to the eye and staircasing is evident on straight,diagonal lines.

It would seem logical, therefore, to scan originalimages at the highest resolution possible.However, this is not necessarily the best solution,as the higher the scan resolution, the larger thefile size. Large files take longer to edit, slow downmonitor redraw and increase the time taken bythe imagesetter to output the final films.

Scan size

The dimensions of the scanned image aredetermined by the number of pixels it containsand the scan resolution. This can be illustratedusing the following example:

Suppose a scanned image measures 1,000 pixelsby 1,000 pixels and has a resolution of 200 pixelsper inch (200 ppi). The scanned image wouldmeasure 5 inches by 5 inches:

1,000200

Now suppose you were to scan the image at 500ppi. The pixels would be smaller and arrangedcloser together, so the image would be reduced insize: it would now measure 2 inches by 2 inches:

1000500

In practice, correct scan size and scan resolutionshould be determined before the scan is made, bymatching them to the intended size and halftonescreen ruling of the printed image. Altering theseparameters after the scan has been made can havea dramatic effect on the appearance of the image.For example, if you were to simply increase theoverall size of the image, each pixel would beenlarged until the bitmap fitted the newdimensions. In effect, you would be lowering thescan resolution the pixels would be bigger, sothere would be fewer pixels per inch. This mightspoil the smooth transitions of tone or even createnoticeable jaggies or staircasing, as describedpreviously.

Reducing the size of a digital image has theopposite effect: pixels become smaller, so thereare more of them per inch. This effectivelyincreases the resolution of the image and givessmoother tonal transitions. However, the file sizemay now be unnecessarily high for the dimensionsof the scan, as explained previously.

Resampling

Ideally, if you need to increase the size of ascanned image for any reason, you should rescanthe original to achieve the correct dimensions. Ifthis is not possible for one reason or another, youcan use computer software to resample theimage. This involves adding extra pixels to theimage, rather than changing the size of theexisting pixels. The most accurate method isknown as bicubic interpolation, whereby theprogram assigns the colour of a new pixel byaveraging the colours of the surrounding pixels.

You may also need to use interpolation if yourscanner offers a low maximum optical resolution(scanner specifications usually state the unitsmaximum optical and interpolated resolutions).You are most likely to need to use it whenscanning a small original such as a 35mmtransparency for output at a large size.

Resampling should be treated as a last resort as itdegrades the quality of the original scan andcauses a marked softening of the image. This iswhy it is worth investing in a scanner with a highmaximum optical resolution, so that you canensure the correct scan resolution without havingto resort to using interpolation programs.

3.3 3.4

SNAP DECISIONSWhenever you are asked to incorporate scanned photographs in a print job, check thefollowing:

Does the image contain a wide range of tones?

Some scanners compress the tonal range,heightening contrast. An original image with awide range of tones helps to minimise thisproblem.

Is a limited range of tones intentional?

For instance, the client may have chosen to usehigh key or low key images to accentuate mood.Automatic scanning software may attempt tocompensate by redistributing the tones. Manualadjustment using scanner or image editingsoftware may be necessary.

Has the original been screened before?

If the answer is yes, the original must bedescreened at the scanning stage, or duringprocessing using image editing software.Descreening will reduce the risk of moir causedby interference between the halftone screen onthe original image and the halftone screenapplied to the scan.

Is the image sharply focused?

Scanning or post scanning software can help tocompensate for a small amount of blur, but outof focus images will not reproduce well. Use apowerful eyeglass to check for sharply focuseddetails. (Even the sharpest images may benefitfrom a small amount of software sharpening.)

Were individual photographs taken on thesame film stock?

Different films have different colour balanceswhich can lead to some images appearingwarmer or cooler than others. This isespecially noticeable when photographs ondifferent film stocks are used next to each other.If mixed film stocks are batch scanned (severaldifferent images scanned at once) the images willgenerally require some editing at the processingstage.

Are you scanning from a duplicatetransparency?

Duplicating an image tends to boost contrast(sometimes unacceptably), alter colours andreduce overall sharpness. Avoid using dupes ifpossible.

How big is the original image?

The larger the original, the larger it can beprinted without sacrificing sharpness or smoothtonal transitions. Enlarging a photograph tends tolower its colour contrast, so ensure that picturesfor large scale output have rich, saturatedcolours to begin with.

Are the originals damaged?

Fingerprint marks can usually be removed bycareful cleaning prior to scanning and minorscratches can be removed in an image editingprogram.

Size = = 5

Size = = 2

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Bit depth

Each pixel in a digital image contains colour andtonal information. However, due to the inherentlimitations of the RGB colour model, the pixelshave a smaller colour gamut (a smaller range ofreproducible colours) than continuous tone colouroriginals, such as colour transparencies and artistsillustrations. When the original image is convertedto a bitmap during the scanning process, certaincolours in the original cannot be matched exactlyusing RGB colours, so pixels are assignedwhichever RGB colour is the closest match to thecolour found in the original image.

Bit depth is a measurement of how many tones orcolours each pixel can reproduce. It follows, then,that the greater the bit depth, the more accuratelythe colours of the original image can be mappedto the digital image.

At its simplest level, a pixel can be turned on(white) or off (black). This level, or depth, ofinformation is known as 1-bit data (2 to the power1).

By increasing the bit depth, the pixel can showintermediate levels of gray (levels of brightnessbetween light and dark). For example, pixelscontaining 2-bit data (2 to the power 2) can showfour levels of information: black, white, light grayor dark gray. 8-bit data (2 to the power 8) allowsthe pixel to show 256 levels of gray includingblack and white.

3.5 3.6

SCAN RESOLUTION AND SIZING

CHAPTER 3

Down-sampling

Down-sampling is the opposite to resampling itdescribes the process whereby a computerprogram is used to remove pixels from an image,to reduce its dimensions. This method has fewerdrawbacks compared with resampling, but it canlead to staircasing if it is used excessively.

Scaling percentages

Some scanning programs require that you set theimage size by specifying a scaling percentage (orgive you the option to do so). This is calculated asfollows:

required sizeoriginal size

Determining the correct scan resolution is important for high quality results. It is calculatedusing a combination of the following:

THE HALFTONE SCREEN RULING THE SCALING FACTOR THE QUALITY FACTOR

The quality factor describes the fact that a scan resolution of approximately twice the screenruling gives the best quality printed image. Thus, if an image is to be printed at 55 lpi, the scanresolution should be 110 ppi. This is not a hard and fast rule, but rather a general guide whichhas been agreed upon across the graphic arts world.

Increasing the quality factor makes no difference to the appearance of the final print. However,at halftone screen rulings of 133 lpi and above, it is possible to reduce the quality factor to 1.5,to keep file sizes manageable, provided the image contains no obvious geometrical patterns orstraight diagonal lines.

output sizeactual size

36036

The scaling factor is a measurementof the amount the original imagemust be enlarged or reduced to fitthe dimensions of the final printedimage. This is calculated as follows:

Suppose that a transparency measuring36 x 24 mm is to be printed at a finalsize of 360 x 240 mm. Measuring thelonger dimension the scaling factorwould equal 10. Thus:

The scan resolution isdetermined by a simplecalculation:

halftone screen ruling x scaling factor x quality factor

Suppose that the imagementioned above were to bereproduced using an 85 lpihalftone screen. The correct scanresolution would be:

85 x 10 x 2 = 1700(lpi) x (scaling factor) x (quality factor) = (ppi)

Suppose, then, that your clientsupplied a large print measuring720 x 480 mm instead of a 35mm transparency. The size of theprint would need to be halved tomatch the size of the final print,so the new scan resolutionwould be:

85 x 0.5 x 2 = 85(lpi) x (scaling factor) x (quality factor) = (ppi)

You can see from the above examples that the greater the scaling factor (magnification), thehigher the scan resolution all other factors being equal. This is why repro houses use drumscanners with very high optical resolutions the higher the optical resolution of the scanner,the higher the possible scan resolution. This allows them to work with small originals whichneed to be greatly enlarged for output. On flatbed scanners with lower optical resolutions youwould have to resort to using an interpolation program to add extra pixels, which would meancompromising the sharpness of the image (see, Resampling).

BRING IT DOWN

If the output size of the digital image isnot known at the scanning stage, make alarge scan (by setting a high scalingpercentage) at a high scan resolution itis better to down-sample than resample.

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4

5 6

FIG 3.3. It is very simple to resample and down-sampleimages using Adobe Photoshop (though try to avoidresampling whenever possible). First check that BicubicInterpolation is selected in the General Preferences box(File/Preferences/General Preferences). Once selected,there is no need to alter it on subsequent occasions.Next, open the Image Size dialogue box (Image/ImageSize) and make sure that resample is selected (1).Select Constrain Proportions (2) if you want to maintainthe same height/width proportions and enter newvalues for the width and height in the Pixel Dimensionsboxes (3). (If you have clicked Constrain Proportions,the width will automatically be changed when you entera value into the height box and vice versa). If you wishto enter values as a percentage of the currentdimensions, select Percent as the unit of measurement(4). The new file size for the image appears at the top ofthe Image Size box with the original file size shown inparentheses (5). Click OK (6) or press the return keyon your keyboard to resample the image or down-sample it if you have entered smaller image dimensions.Finally, apply the Unsharp Mask filter(Filter/Sharpen/Unsharp Mask).

x 100= 10

1-bit 2 shades(levels of brightness)

2-bit 4 (levels ofbrightness)

8-bit 256shades (levelsof brightness)

Fig 3.4

Black and white 2 shades(1-bit)

Grayscale 256 shades (8-bit)

RGB colour 16.7 millionshades (24-bit)

Fig 3.5

Which colour model?

A scanner can record a full-colour original imagein RGB or CMYK colour.

Specialist repro houses use powerful colourseparation software for converting the RGB datacaptured by the scanner to a CMYK file and havethe necessary experience to make high qualityseparations. If you are using a repro house, thegeneral advice is to leave the separations to them,rather than handling them yourself in an imageediting program.

However, you should bear in mind that reprohouses are generally set up to service lithoprinters, so the scans they supply may be far fromideal for the screen printing process. Unless youare scanning the images yourself, or the reprohouse is willing to change the settings on itsscanner to accommodate your particular screenprinting parameters, some image editing will benecessary. In this case, it may pay to have scanssupplied as RGB files and handle the colourseparation yourself. The bonus of working withRGB files is that they are smaller than CMYKequivalents, so processing times are faster andcomputer memory overheads lower.

THE PROCESSING STAGEOnce the original image has been scanned, thedigital image file is transferred to a desktopcomputer or workstation (see, File formats andtransfer) for processing.

Computer processing may involve the use of animage editing program to alter, say, colourbalance or repair scratches. An image editingprogram may also be used to separate the imageinto CMYK colours and set relevant parameters for example, dot gain and black generation (howto control these variables is explained later.)

The amount of image editing already carried outat the scanning stage will determine what isnecessary at the processing stage. Experiencedscanner operators using high end equipment andsoftware can handle most of the essential colourseparation work, provided they are supplied withall the relevant information regarding theseparation parameters required for the printingpress and substrate in question.

FILE FORMATS AND TRANSFER

Digital image files can be saved innumerous different formats. Someformats are more open than others they can be read by a wide rangeof computer programs on differentcomputer platforms (such asMacintosh, Windows, Windows NTand so on). Suitable formats include:

TIFF (Tagged Image File Format)

EPS (Encapsulated PostScript)

DCS (Desktop Colour Separation)

Avoid the use of lossy compressionformats, which discard digital data inorder to reduce file size. Non-lossycompression formats, such as TIFF LZWand non-lossy JPEG (Joint PhotographicExperts Group) files are fine, as they donot degrade image quality noticeablyand help to keep file size small.

If you are using a specialist reprohouse, ask which file formats theyprefer to work with usually, eitherEPS or DCS. Similarly, if you arehandling any of the repro process in-house, you will need to specify whichfile formats you wish your customers touse. This will tend to be dictated by thesoftware you are using, though thethree file formats mentioned above arecompatible with all of the mostcommonly used programs.

3.7 3.8

8-bit data is needed to produce smooth tonaltransitions in digital images. This is because theeye can differentiate between at least 150 levelsof gray (brightness levels). 7-bit data enables apixel to show only 128 levels of gray, meaningthat 8-bit data contains the least amount ofinformation necessary to reproduce all the graylevels the eye can see.

With an RGB colour image, 8-bit data (256 graylevels) is required for each colour channel. Hencethe term, 24-bit colour (3 x 8-bits). When adigital file is separated into the CMYK colourmodel, extra colour and tone information isrequired, as there are now four colour channels,each requiring 8-bits of data. Hence, the CMYKcolour model is often referred to in terms of 32-bit colour (4 x 8-bits).

Some scanners are able to capture more than 8-bits of data. The latest desktop flatbed scannersare capable of capturing 12-bit data and top of therange repro flatbed and drum scanners cancapture 16-bit data.

The primary reason for capturing this extra data isto allow pixels to show a wider range of subtleshadow details an important consideration whenworking with high density transparency films, suchas Fuji Velvia. Image editing software is now alsocapable of handling 16-bit data, which means thatthere is more scope for colour correction of digitalimages and greater control when it comes toseparating RGB images into the CMYK colourmodel.

FILE TRANSFER

At some point in the repro workflow you will need to transfer files between different systems for instance, from the scanner to the computer, or the computer to the imagesetter. You will alsoneed to have a facility for your clients to transfer their layout files to your system and you willneed to transfer files to and from a repro house if you are sending out part of the work.

The most commonly used means of transferring files is via removable disk drives. The disk is used tostore the data. They are relatively cheap to buy and the removable disks mean that storage capacityis, effectively, unlimited. Typical drives include:

Computer processing equipment can be roughlysplit into two types: desktop computers and highend graphics workstations. Desktop computers aremuch cheaper than workstations, but workstationstend to process data faster. The right choice foryour operation will depend on numerous factors far too many to be covered in the space availablehere. Sericols Imaging Team or a specialistcomputer consultant will be able to advise on theoptions.

Colour monitors Again, there are a number ofkey factors to consider when you are choosing amonitor for repro work. The most important are:

screen size (the bigger the better, as it reducesthe amount of scrolling and screen redraws, whichincrease editing times)

dot pitch the size of the individual screenpixels (smaller is better)

display resolution the number of pixels whichcan be shown on the screen (the higher thebetter)

bit depth how much colour information amonitor can display (a 24-bit colour display canshow more than 16 million colours)

refresh rate a high refresh rate helps toprevent the display image from flickering

calibration profiles some monitors can becalibrated so that on-screen colours more closelymatch both the colours captured by the scannerand the ultimate colour of the printed image

Add-on graphics cards can increase the rate atwhich the screen redraws. Similarly, graphicscards or extra video memory can allow you todisplay higher resolutions (more pixels) or agreater number of colours (higher bit depth).

The monitors surroundings must also beconsidered carefully, as reflections and glare canaffect on-screen colours and contrast, as well asthe comfort of the operator. SericolsEnvironmental Services team or a computerconsultant can advise on these and otherconsiderations.

Software Most important of all is your choice ofsoftware, as this will determine the level ofcontrol you have over the final printed image.

Adobe Photoshop has established itself as theindustry standard image editing program. Itcontains an extremely powerful set of digitaltools, which afford a remarkable amount ofcontrol over the image editing and separationprocesses. (Photoshops most useful tools areexplained in the following section.)

The major page layout programs are currentlyQuark Xpress and Adobe Pagemaker. Bothprograms afford precise control over thepositioning of elements on a page. They areinvaluable for arranging text, graphics and digitalimages within a design and can also be used toset separation parameters, halftone screen rulingsand so on. They also allow different scanresolutions to be brought together on the samepage. All images are represented on the page bylow resolution screen images which are linked tothe high resolution file used for output. Thisresults in faster processing times.

Page layout files often contain low resolutionpositionals to show where the scanned imagesshould be placed and how they should becropped. Once the scans have been made andedited, they can be substituted for the lowresolution positional. Clients may even scanoriginals themselves and supply the files at the

correct size and resolution for output. In this case,all that is required is to open the scans in animage editing program to check that the correctseparation parameters for your printing presseshave been specified.

Digital printers Proof prints of the completedlayout file can be output from a digital colourprinter (such as a 4-colour dye sublimation printer)for client approval. However, this type of proofcannot be used to check dot gain, exact colourbalance and so on, so the proof is only useful forchecking that the design layout is correct.

PostScript files Once the image editing has beencompleted and the page layout has beenapproved, the page layout file must be convertedfrom so-called application code (the code used bythe page layout program) to a PostScript file. Theindustry-standard PostScript page descriptionlanguage can be read by nearly all output devicesand contains all the information needed for animagesetter to output a file as 4-colourseparations.

THE OUTPUT STAGERIPs The PostScript file is transferred to what isknown as a raster image processor (RIP). This maybe custom-designed hardware or, more likely, asoftware emulator which can be run on anysuitable computer. Its job is to rasterise thePostScript file (convert the data into a matrix ofhalftone dots the raster which the imagesettercan subsequently output to film). The RIP can alsohandle the colour separation process if required.

Imagesetters The rasterised file is transferred to animagesetter for output to the four film positives one each for the CMYK inks. The imagesetter usesa laser beam to record opaque, fixed-size dots,known as device pixels, on to the films lightsensitive emulsion. The minute dots (measuringmere hundredths of a millimetre in diameter) areoutput at very high resolution a high endPostScript imagesetter can achieve an outputresolution in excess of 3,200 dots per inch (dpi).

Once the film has been exposed, it is processedchemically using a carefully controlled processingunit, which maintains the correct chemistry,temperature and processing speed for correct

3.9 3.10

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CHAPTER 3

How much RAM?Image editing programs require a largeamount of random access memory (RAM) towork quickly and efficiently. When youopen a file, the computer attempts to loadthe entire file into RAM. If insufficient RAMis available, the computer can load only partof the file being worked on at any giventime. This means that it has to continuallyswap data between RAM and the hard disk,which slows down processing speedsconsiderably.

As a rule, you need to install at least threetimes the amount of RAM as the largest fileyou are likely to be working on. This allowsthe computer to hold the entire file in RAM,plus the last unsaved version (allowing youto undo a manipulation if you dont like aneffect), and still have enough availablememory to perform editing operations.

With any new computer, check for themaximum amount of RAM you can install, asthis will dictate the largest file size you canwork on comfortably.

Jaz drives very widelyused, offer largeamounts of storage(1 or 2 gigabytes)and are currentlythe best option forthe transfer ofdigital image files.

Zip drives cheap, widely used,but individual disks have limited storagecapacity (100 megabytes a tenth of thesize of the smallest Jaz disk).

Optical disks widely used, very robustand offer a large storage capacity, but areslow to read and write to.

Rewritable CD-ROMs widely used, robust,inexpensive and have a reasonablestorage capacity. However, writing data to acompact disc is more complicated andprone to errors, compared with other transfersystems.

results. The film can then be checked for problemssuch as unwanted fringing (halos) around thehalftone dots, low dot density and excessive dotgain.

A high end imagesetter is an expensiveinvestment. There are, however, alternatives,including the most recent innovation thethermal imagesetter. This type of imagesetter usesheat to record dots on a special heat-sensitivefilm. It is considerably cheaper than high end,photo-based versions and is very quick and simpleto use. However, it cannot match the very highoutput resolution found on more expensiveimagesetters.

Fig 3.6. High end imagesetters can output film positives atvery high resolution, but are very expensive to buy andoperate. The Aspect 600 thermal imagesetter has a lowermaximum resolution, but represents a far more cost-effectiveoption for many in-house repro studios.

Output resolution

An imagesetters output resolution has animportant role to play in determining the qualityof the final print. As explained previously, a digitalimage requires 256 levels of gray per colourchannel to reproduce smooth tonal gradations andaccurate colours. The total number of gray levelsthat can be reproduced on the final print isdependent upon the halftone screen ruling andthe output resolution of the imagesetter. It iscalculated using the following formula:

output resolution2

halftone screen ruling2

Looking at this formula, it is clear that, providedthe halftone screen ruling remains the same,higher output resolutions will provide a largernumber of gray levels and, as a result, a smoother,more accurate print. Similarly, higher outputresolutions will be required when working withfine halftone screen rulings such as 133 lpi or150 lpi if you are to guarantee the reproductionof 256 levels of gray.

The following examples illustrate the relationshipbetween screen ruling, output resolution andlevels of gray:

1. Suppose you were to try outputting your fine150 lpi film positives using a 600 dpi laser printer.The total number of gray levels would be equal to:

6002 (dpi)1502 (lpi)

It is not possible to reproduce a continuous toneimage faithfully using just 17 levels of gray theprint will have a posterised appearance.

2. Having obtained an unsatisfactory result usingyour laser printer, suppose you decide to send thejob to a repro house for output on a high endimagesetter with an output resolution of 2540 lpi.The total number of gray levels would now beequal to:

25402 (dpi)1502 (lpi)

288 levels of gray is ample to produce a smoothimage which exhibits accurate colours.

3. Finally, suppose you decide to print the job onthe laser printer once again, but using a muchlower halftone screen ruling for instance, 37 lpi.

The total number of gray levels will now be equalto:

6002 (dpi)372 (lpi)

As the above examples show, output devices witha low output resolution are capable of producingquality results, provided the halftone screen rulingis not too high. Indeed, the use of a high endimagesetter for this type of work would besuperfluous. To produce smooth tonal gradationsand accurate colour with higher halftone screenrulings, however, requires the use of an outputdevice with a high output resolution in theregion of 2,500 if you are attempting to print atvery fine screen rulings, such as 150 lpi.

Fig 3.7 The relation of halftone screen angles toscreen mesh stretched at 5 from base right.

Consider using a standard specificationsheet (see sample at back of this manual)and use a symbol for the orientation of thehalftone angles you require. On an ellipticaldot halftone screen, the angles should bespecified in relation to the direction that thedots join.

Fig 3.8 Use a simple symbol to indicate that all theangles are measured from base right.

3.11 3.12

HARD AND SOFT

An important point to look out for is thetype of dot the imagesetter records. Oldermodels tend to record soft dots, whichhave a halo around their edge. This cancause colours to be reproducedinaccurately at the printing stage. Newerversions record hard dots, whichreproduce colour more accurately on thefinal print.

OTHER OUTPUT CONSIDERATIONS

A commonly overlooked differencebetween offset litho printing and screenprinting is the orientation of the emulsionon the film positive (the emulsion carriesthe halftone dots). Offset requires theemulsion to be downward when looking ata correct reading image. Screen printingrequires the film emulsion to beuppermost, so that this side is in contactwith the stencil emulsion when exposingthe screen. If the film emulsion was on theunderside you would be exposing throughthe thickness of the film base and lightwould scatter under the image. This wouldresult in a loss of fine detail and, in thecase of halftones, a complete loss of dotsin the highlight areas.

So, remember to specify: emulsion up right reading.

Also, you must be consistent in how youspecify the halftone screen angles yourequire when you are using angled mesh.Many imagesetter software programsmeasure the angles from a base right. Ifyou have established a set of anglessuitable for the mesh you print through maybe, one screen has to have the meshangled to avoid a clash between it and thepositive measuring angles from a baseleft would produce a totally differentrelationship to the mesh.

Differential betweenhalftone and meshangles = 70

Halftone angle 75 frombase right

Differential betweenhalftone and meshangles = 80

Halftone angle 75from base left

Gray levels = + 1 = 17

Total gray levels = + 1.

Gray levels = + 1 = 288

Gray levels = + 1 = 264

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Printing inks are not perfectly transparent, so the order in which they are printed can have a marked effect on the appearance of the image.

The gamut of reproducible colours using CMYKinks is limited compared with that of the RGB colour model. This means that there may be noaccurate CMYK match for some colours contained in an RGB scan. (A Gamut Warning control allows you to preview which colours cannot be matched precisely, before you proceed with the separation process.)

Increases in dot percentage values do not produce a uniform increase in colour density - that is, colour change is not uniform across the tonal range.

Fig 3.9 This RGB colour specification cannot be mapped toCMYK

In fact, the mathematical equations used toaccurately map RGB colours to CMYK ink valuesare extremely complex and require a great deal ofprocessing power to calculate. To speed up theseparation process, therefore, separationprograms store the equations for individual colourconversions in a colour look up table (CLUT).During separation, the colours of each pixel in theimage are compared with the colours in the table:if an exact match is found, the pixel is given theappropriate CMYK value; if no exact match isfound, the nearest CMYK value in the CLUT isused.

Off-press proofs (or contract-quality proofs),such as Cromalin, Matchprint and Agfaproof, areused for client approval before a job goes into fullproduction. However, they have been developedto meet the needs of the litho printing industry, sothey give a less accurate representation of howthe image will appear when it is screen printed.(The possibility of tailoring dot gain on a screenprinting press so that it matches the dot gainfound in off-press proofs is discussed in Chapter6.)

For absolute accuracy, you can run a press proof.This is the only way to determine exactly how theparticular substrate and ink system will affect dotgain under the specific production conditions usedto print the job. By running a series of tests onvarious substrates using the appropriate inksystems, you will have the information necessaryto predict fairly accurately the amount of dot gain,without having to proof each job.

Any problems that might arise during proofingcan be corrected by further editing of the imagefiles. For example, suppose a models faceappears slightly too red. A densitometer testshows that the problem is caused by a higher thanexpected dot gain reading for the magenta screenin the fleshtones. Instead of experimenting withink formulations to try to correct the colourbalance on-press, it is a simple matter to return tothe processing stage, alter the image file tocompensate for the dot gain and re-run the films.Provided the press set-up remains unchanged,you can be certain of an accurate result. In thisway, colour can be controlled without the need toconstantly modify the inks. Careful control andmodification of the halftone film positives allowyou to reproduce any image on a wide variety ofdifferent substrates, using standard 4-colourprocess ink sets. It goes without saying that thisreduces the number of different halftone inks theprinter needs to carry, introduces a strongelement of standardisation and ultimately speedsthe print process considerably by removing thetrial and error approach to getting colours righton press.

The next step, then, is to look at how computersoftware can be used to achieve accurate colourson the finished print. (Adobe Photoshop 5.0 willbe used throughout to demonstrate the varioustechniques involved.)

CONTROLLING THE KEYHALFTONE PRINTING VARIABLESThe most powerful, and important, feature ofcustom scanning software and image editingprograms is that they allow fast and accuratecontrol over the all-important colour separationprocess.

Colour separation

In order for an RGB image to be printed, it mustfirst be converted to the correct CMYK values, sothat film positives can be produced and screensexposed. This process is known as colourseparation and is carried out automatically byscanning or image editing software all you needdo is select the correct colour model from a pulldown menu and click the mouse. However, theuser-friendly manner in which colour separation isselected belies the very sophisticated processresponsible for the actual conversion from onecolour model to the other. While it is not essentialto master the maths, it is useful to have anunderstanding of how the separation works,especially if you wish, at some stage, to bring thedigital repro process in-house.

Mapping colours During the separation process,the computer maps the RGB values for each pixelin the image to the appropriate CMY values andgenerates a value for the black ink. (Blackgeneration is covered later.)

Mapping RGB values directly to CMY values,however, does not produce an accurate result.There are a number of reasons for this:

Even the best printing inks have inherent impurities, which mean that their spectral properties are not perfect. In effect, this means

that the different coloured inks absorb some of the light they are supposed to reflect and reflect some of the light they are meant to absorb. Cyan ink is especially problematic as it tends to absorb a high amount of the blue and green light falling on it. This reduces the amount of blue/green light reaching the eye, which leads to a warmer image on the final print. (For this reason, separation software will always reduce the level of magenta and yellow

ink when they are printed together with cyan.)

COLOUR LOOK UP TABLES

Some colour look up tables (CLUTs)contain more information than others.High end scanning software works witha very detailed CLUT containing a largenumber of RGB/CMYK colour matches,so the separation process is veryaccurate. Image editing programs tend towork with scaled down CLUTs (forreasons of processing speed), which canlead to less accurate separations.However, the sophistication of the CLUTused with high end scanning softwaremeans that it takes a long time tocalculate, so repro houses tend tostandardise on a table which gives thebest results for printing SWOP(Specifications for Web OffsetPublication) inks on to coated paper. Theadvantage of using an image editingprogram is that the smaller size of thetables means that the image editingsoftware is able to contain multipleCLUTs. This means that it is possible tocreate and store a variety of tablesgeared for screen printing with differenttypes of inks.

3.13 3.14

CHAPTER 3

Black generation Theoretically, printing cyan,magenta and yellow inks together should produceblack. However, this is not the case: the threesubtractive primaries alone cannot produce a highenough density to render a true, rich black - theyproduce a muddy brown colour. A vital part of theseparation process, then, is the generation of ablack ink value.

Most colours in a scanned image will containvarious amounts of each of the subtractiveprimaries. One or two of them will be dominantand will dictate the hue. The remainingcomplementary colour(s) has no effect on the hue,but simply makes the colour lighter or darker. Thismakes sense when you recall that equalproportions of cyan, magenta and yellow combineto create gray. The equal amounts of the threesubtractive primaries contained within any colouris known as the gray component

Gamut Warning

Fig 3.10. Equal proportions of three subtractive primariescombine to make gray a colours gray component.Removing equal amounts of the three colours and replacingthem with black ink reduces the total ink weight printedwithout noticeably altering the hue.

If you remove equal levels of cyan, magenta andyellow (the amount is dictated by the level of thecomplementary colour) and replace them withblack, therefore, the hue will remain the same(only the gray component will be removed), butthe total amount of ink printed will be reducedwhile the maximum density will be increased(thanks to the inclusion of the black).

More importantly, it is possible to reduce thetotal ink limit (the maximum weight of inkdeposited on the substrate). Controlling the totalink limit is especially important as a high ink buildup can cause a number of problems:

A heavy deposit of ink is more likely to cause smearing of the halftone dots, reducing the definition of the image and affecting colours and tones.

A high build up of ink over successive colours may cause the height of the ink deposit to impair the lay down of the final colour. Simply put, all of the ink printed through the final screen may not be able to reach the substrate.

The slightest misregister in areas where there is a large deposit of each of the three subtractive primaries can reveal unwanted gray tones within darker colours.

Higher ink weights can lead to increased dot gain. (By limiting the maximum dot sizein shadow areas, there is less filling in, so alower total ink weight will help to preserve shadow details.)

Heavy ink deposits can lead to drying problems and prolonged drying times.

For the reasons outlined above, the generation ofa black value involves replacing a certain amountof each of the subtractive primaries with black.

There are two methods by which black values canbe generated: under colour removal (UCR) andgray component replacement (GCR).

UCR reduces the amount of cyan, magenta andyellow in the darkest neutral and near neutralcolours of an image and increases the amount ofblack accordingly. UCR becomes active in any areaof the image where the total ink percentage ofcyan, magenta, yellow and black exceeds thespecified total ink limit.

To select the appropriate UCR settings for yourpress and printing parameters, first open theSeparation Setup dialogue box(File/Preferences/Separation Setup) and click onthe UCR radio button. Then set the total ink limitby entering the percentage value in theappropriate box. (If it were possible to print a100% dot in all four colours, the total ink weightwould be 400%. In practice, it is usually setbetween 240% and 340% depending on theprinting conditions. You need to determine theoptimum total ink limit for your press and thetype of ink you are using.)

An area where UCR has been shown to beadvantageous is printing with UV curing inks.These inks have a low volatile component, so inkbuild is high, which can cause problems, asdescribed above.

UCR improves ink transfer in the darkest shadowareas of the image because the amount of inkpresent does not exceed the optimum level thatthe screen printing press can produce. Thestability of the gray balance is also improved,becaus