Repro and film adjustment

i = ARCTIC PAPER RECOMMENDS Once the image has been processed and adjusted, the next stage is to adapt it to the printing technique and the paper grade being used. This is true whether printing on coated or uncoated paper. There are however a number of conditions which are particularly important to consider when dealing with uncoated paper. The most important issue to address is dot gain, which is greater on uncoated than coated. This is because uncoated paper has a coarser surface structure, and more ink is needed to fill the irregularities. Adding extra ink makes each dot swell, and this has to be compensated for. Moreover, in four-colour printing the ink accumulates with every added layer, so the total percentage of ink must be decreased. Now, of course, it may sound difficult to correctly adjust repro and films for uncoated paper. This is not the case, but you still need to know about the causes and effects. We would therefore like to highlight the factors that should be considered and handled with special care, and out-line how to adapt repro and films accordingly. At the end of the day though, it is always the skill of the individual professional that determines the final result.

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	Raster/Screening

Raster dots and raster density are traditionally used terms in the graphic design and printing industry, although these days we tend to refer to the ‘screen’, ‘screening’, ‘screen ruling’ and ‘screen density’ of a digitally processed image. The word raster comes from the Latin rastum, to divide. A normal photograph consist of continuos tones or gradients of colour. However, these tones cannot be reproduced in print – quite simply, a printing press cannot handle the task. In order to make the images printable they are therefore screened, i.e. divided into a multitude of microscopically small dots. The dots in a screened image therefore give the eye the illusion of a single continuous image, with tone values of differing strengths just like a ‘normal’ image. Screen dots are created in a Raster Image Processor (RIP), and then exposed onto film in an image setter or direct to the plate (CTP). The size of screen dots ranges from 1 to 100 per cent, creating half-tones. Tones of colour can be varied by varying the size of the dots, and thereby increasing the range in the image. When the dots are at 100 per cent, for instance, this means that they have completely run over into each other to produce a fully covered surface, which is called a tint plate. The opposite, i.e. ?per cent, would therefore be a totally white surface. On a conventional screen, the screen dots are arranged in lines /rows. The density of these lines is called the screen ruling or screen density, and is normally measured in LPI (lines per inch) or l /cm (lines per centimetre). The denser the screen ruling, the harder it is to distinguish the dots, the image improves and the tint compass – the range between the lightest and darkest parts of the image – increases in terms of the number of lines. When talking about screen dots, resolution must also be mentioned. This is because screen dots are in turn divided into small pixel dots which define the printout resolution. This is usually measured in DPI (dots per inch), or PPI (pixels per inch), and producing a high screen density also requires a high printout resolution. When the image is finally printed an ink dot is created for each colour, and they jointly make up the final image. In order to avoid unwanted patterning – called a moiré effect – the dots have to be placed at special varying screen angles. Screen angels A screen rose. The lines are placed at different screen angles, one for each process colour. The tone is varied by varying the size of the dots. When the dots are laid overlapping one another, a new colour is created. The illusion of a different colour is also created when the screen dots are close to each other. i It is essential to adapt the screen ruling to the paper and printing method, especially when dealing with uncoated paper. Too high a screen ruling, for example, may make the inks run together and detract from the image’s contrast or detail, or the dark areas may become completely black. Too low a screen ruling on the other hand may compromise the quality and tint compass of the image, making it dull and lacklustre. 133 or 150 lpi usually produces excellent results on uncoated paper. A slightly lower screen ruling should be used when working with very rough uncoated grades. On a smooth uncoated paper, where the repro is otherwise optimal, up to 175 lpi can normally be used without problem. Another type of screen is the stochastic screen (stochastic = random). Unlike a conventional screen, all the dots on a stochastic screen are the same size. Colour tones are instead altered by varying the number of dots in a particular area: light areas contain few dots, and dark areas many. In other words, the higher the frequency of dots, the darker the area. Another – and in fact a more correct – name for this would be a frequency modulated (FM) screen. i A stochastic screen is ideally suited to uncoated paper, but it also demands close control at all stages of the process, especially film output and plate production. Positive film or Computer to Plate (CTP) are therefore the preferred methods. One advantage of a stochastic screen is that decorative elements and high-detail images are not broken up by ink dots. Another advantage is the fact that moiré effects can be avoided. It is also worth mentioning that 21-35 micrometres is a suitable dot size on uncoated paper.

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	Dot gain

Dot gain, or dot expansion, is a necessary concept to know about in printing on any paper, but especially uncoated. Dot gain occurs in all printing, but is greater on uncoated than coated grades, partly due to the rougher surface on uncoated paper. The phenomenon occurs in the various stages of the printing process. First of all the dots are transferred from digital form onto the film, and then onto the printing plate. The plate picks up ink which is transferred onto a rubber blanket, and finally onto paper. At each stage, the diameter of the dots increases slightly, and as a result the dots – and therefore the colours – bleed out and the images lose both coverage, particularly in the dark areas, and detail definition. In practice the images often come out too dark and drab, and this must be taken into consideration during repro and film preparation. There are a number of important ground rules that should be followed to reduce dot gain on uncoated papers. In dark areas, repro must be adapted to ensure that the screen is not ‘full’, which will reduce coverage in dark segments and give them the appearance of a totally black tint plate. i The aim is to compensate so that the dots are smaller from the start, and end up exactly the intended size on the paper after dot gain to ensure the right colours are produced. If you have made the right repro and film adjustments – and ideally over-compensated slightly – it is easy to increase the ink coverage and the ‘back pres-sure’ in the printing press to achieve the right size of dot and thereby a good image result. However, if you have under-compensated in repro the ink cover-age will have to be reduced instead to ensure the colours do not bleed together, thus risking a dull or lacklustre image. To produce a successful result, you should open up the repro and make the darkest dot on the plate around 85-90 per cent – dot gain will make the dot the right size in print in any case. Dot gain is greatest in the middle tones. In a 50 per cent dot, gain can be between 15 and 40 per cent depending on the paper, press, ink coverage and back pressure. This has to be compensated for. On the other hand, light areas could be kept totally white and support points removed, as these can become too large and prominent after dot gain. In this way, the clear paper surface will help create good contrasts in the image. And this is what we want, because at the end of the day all printing is about contrasts, about the difference between the reflecting light of the unprinted surface, and the absorbing light beams of the printed surface. DOT GAIN 1 The illustration above shows how a screen dot expands as it goes from one medium to the next in the repro and printing process: from digital dot, to film, to plate, and finally to paper. The greatest gain occurs in the transfer from plate to paper (the third and fourth dots above). On the screen Three original dots: one in the dark part, one in the medium part and one in the light part. On plate or film The same three dots with and without dot gain compensation. Blue = compensation Yellow = no compensation On paper The result: The compensated dots (blue) match the original. Without compensation (yellow), the dots are larger than they should be. DOT GAIN 2 The illustration above shows three different dots in a halftone area ranging from 0 to 100 per cent (digital). For the image to look right in print, the final dots on the paper also have to be the same size. If the dot in the darkest area is set at a maximum of 85 – 90 per cent, it will re-assume the correct size in print. Otherwise the dots will completely overlap into a full tint plate and spoil the detail definition. The middle tone area experiences the most dot gain, and this has to be compensated for. Otherwise the dot can expand by15 – 40 per cent depending on the paper, press, ink coverage and back pressure. This makes the images dark and drab. To ensure the lightest white areas come out as light and white as they should, the smallest dots can be set at 0 per cent. Otherwise the smallest dots can expand a few per cent and create an unwanted shade.

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	Under colour removal and total ink coverage

So far in the repro stage, we have chosen a suitable screen ruling for the paper, and compensated for the paper’s dot gain. The next step is to separate the image. For a scanned digital RGB image to be printed, it has to be transformed into four process colours – i.e. four-colour – which when mixed correctly can recreate all printable colours. The four colours are called CMYK (Cyan, Magenta, Yellow and Key-colour, i.e. Black), and these days you can generally choose at the scanning stage to separate a digital image into CMYK straight away, or keep it in RGB throughout the process and separate it in the final phase. When the image is separated it is divided into the four constituent colours, and reproduced again as an image on the printed paper, with all the colours and shades of the original. Each of the CMYK colours is transparent, and the various shades are blended to form a colour image as the dots are printed directly on top of each other, ink on ink. In screen-dot printing, dots of different colours side by side also create an illusion of a new colour. However, this stage demands thought and care. Because the four colours are being printed onto one another, in extreme cases this may mean you are using 100 per cent ink in each layer, giving a total ink coverage of 400 per cent. As the quantity of ink is also increased when printing on uncoated paper, the result will be far too much ink to produce a satisfactory printing result. Had the material been printed anyway, the result would very likely have been a worse image with bleeding, reducing coverage in dark segments. Also, the ink would have smeared and the drying time would be far too long. Picture These images show each process colour (C, M, Y and K), and how they combine to form the final four-colour image.  i Therefore, in order not to over-ink the paper, a maximum limit for total ink coverage in the image has to be set when separating the image. Assuming the printing equipment is precisely calibrated and repro and films have been prepared to perfection, the ink coverage on uncoated paper can be a maximum of 280 per cent. However, a general guide limit is 250 per cent, as the results are rarely improved if the quantity is increased. Instead there will be problems with ink drying, set-off and smearing, and the detail definition in the image will certainly be compromised. The values given above refer to the total ink coverage percentage obtained on the film /plate and transferred to the paper. This is important, as it can affect the final total ink coverage percentage, depending on how dot gain is compensated for and how the image is processed. So what can you do to reduce the total ink coverage without detracting from the image’s quality? A common solution is Under Colour Removal (UCR) or Grey Component Replacement (GCR). And how do you do that? Tradition has taught us that equal parts of the first three primary colours, i.e. cyan, magenta and yellow, always have a common grey component in all shades. Let’s look at an example: If you have a shade that requires 70 per cent cyan, 50 per cent magenta and 50 per cent yellow, the three 50 per cent proportions actually only create the grey shade in the colour. You can therefore replace 3 x 50 per cent with 50 per cent black/grey. Add the remaining 20 per cent cyan and you obtain the desired colour. The difference is that the overall colour saturation decreases by 100 percentage points. Of course all parts of an image never require high ink quantities, but in most cases there are one or more dark shaded areas. UCR/GCR therefore allows you to keep the total ink coverage below the recommended 250 per cent limit for uncoated paper, while at the same time you achieve a more even degree of inking and avoid problems with long drying times, set-off and smearing.

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	Ink Density and Inking Degree

Ink coverage in the press is a vital parameter when it comes to printing an image. Too little ink makes for a flat, dull, washed-out image as some of the ink is absorbed into the paper. Too much ink on the other hand can cause dark areas to run together, and again there are the problems of set-off, smearing and extended drying times. So the right amount of ink is essential. The measure of how much ink is laid on the paper in the printing press is called ink density, and is measured with a densitometer. On uncoated paper grades, which usually have a rougher surface than coated papers, more ink is recommended to ensure the image has the right lustre and impact. If you have also taken dot gain into account when preparing repro and films, optimised the total ink coverage percentage, and do not print with too dense a screen ruling, you can use plenty of ink. In order to press the ink down into the paper, you should also increase back pressure by a minimum of 20-40 micrometres. However, it is important to remember always to use the same back pressure and density values throughout the entire printing process, and in all future print runs of the same product. Otherwise the appearance may vary. Finally, here are some good starting ink values when printing on uncoated paper, mesured on wet inks: Cyan 1.3-1.45; Magenta 1.3-1.45; Yellow 1.2-1.4; Black 1.6-1.8. NB! Remember to check that the densitometer is properly calibrated.

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	Black and White, Duotone and Tritone

As we know, beautiful, well-reproduced black and white images are aesthetically effective. However, it is essential that they are reproduced and printed in the right way. This can be a problem, as it is sometimes difficult to reproduce fine details, blackness and depth by using only various shades of black ink. In order to produce a softer result, with a broader spectrum of fine details, one option is to use duotone or tritone – i.e. tinted grey-scale images. Indeed, the fact is that four-colour printing is often used even for black and white images – as we explained earlier, the same amount of each colour gives a grey or black shade. As the name suggests, duotone involves using two printing inks rather than one, often black in combination with another colour. With tritone a further ink is used, and usually one or two blacks are mixed with one or two indexed or process colours.  Both duotone and tritone are suitable for uncoated paper, as it can otherwise be difficult to achieve enough black in the dark areas. They also improve detail definition, and the image can be given that black feel which is probably the aim. One tip: add 40-60 per cent blue to increase the blackness in text headers.

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	Indexed Colours

Indexed colours are specially pre-blended colours available in a host of different shades.The most common colour system is the Pantone Matching System (PMS), but silver, gold and other metallic colours are also indexed colours. Indexed colours are used for example when only one or two colours are to be used, or when you want to print in a colour other than black.They are also suitable when the colour reproduction has to be exact, such as in logos. It is also important to check against the right sample charts – for uncoated paper – to ensure the correct colour and shade reproduction. When printing gold or silver on uncoated paper, apply a suitable base colour first.

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	Process colours

The process colours are the four CMYK colours (cyan, magenta, yellow and black), and the system is also called the subtractive colour system. The system is used for printing colour images. The process colours are transparent and are laid on top of each other in layers, so that each layer shows through the others to jointly form the different shades in the image. i Process colours can also be printed in combination with one or more indexed colours. The indexed colours are usually added last.

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