nce a digital design has been completed on screen, many output ‘routes’ are available to the designer. If the design is one of a number of alternatives being prepared for review by a client, then the file containing the design may simply be copied to a recordable medium such as a diskette, a Zip cartridge or a CD and mailed to the client for viewing. Alternatively, the file may be attached to an e-mail message and be sent to the client via the Internet. If the design is to be shown to an audience at another location, the file may be output to an image recorder, creating a 35 mm colour slide which can be viewed using a conventional slide projector or it can be copied to a laptop linked to an LCD overhead projector and projected on to a simple projection screen or a convenient office wall.

By far the most common output method is via one of a number of different types of desktop printer, either to paper or to transparency for use on a conventional overhead projector. Desktop printers come in a wide variety of technologies and prices, producing results ranging from crude to photorealistic.

A removable Zip cartridge stores up to 100 Mb of data

Desktop Printing

As discussed earlier, printers use the subtractive colour model to reproduce colour, mixing the subtractive primaries, cyan, yellow and magenta, to produce other colours. Even when a printer has been carefully calibrated and a colour management system is used to optimise the match between screen colours and printed colours, the results obtained depend on the printer’s colour gamut – the range of colours which it can reproduce, as defined in its device profile.

An overhead transparency projector

The ideal printing technology would be one able to emulate the traditional mixing of liquid paints in the proportions necessary to produce the desired colour at every point on the paper. Most available technologies are unable to do this. Certainly not the colour dot matrix printer, as the ‘ink’ is not in a liquid form and is deposited on the paper by firing pins to impress coloured ribbons against the paper. Similarly, colour laser printers, which use dry coloured toners, have no means of mixing the toners. Even inkjet printers, which use real liquid inks, can either expel a minute droplet of ink at each position on the paper, or not, have no capability of mixing the individual droplets. The same limitation holds true for solid ink and thermal wax technologies, although recent advances have produced a thermal wax printer which will produce over 4000 colours by varying the amount of ink delivered to a given location, hence altering the size of the dot produced. Only dye sublimation can genuinely produce a full spectrum of ‘real’ colours by applying differing amounts of each ink to the same point on the paper.

A slide projector

Most desktop printer technologies produce colours within their gamuts by interspersing dots of cyan, yellow, magenta and black in one of a variety of dither patterns. Aneven mix of cyan and magenta dots, for example, will be per-ceived (if the dots are small enough and the viewing distanceis great enough) as the colour blue in the eye of the viewer. Reproducing subtle differences in tone using only a limited number of colours can be achieved by using groups of dots to represent different shades, but the limited number of dots made available by the printer’s resolution (dots per inch or dpi) makes this process imperfect. Low resolution results in visible ‘banding’ as the printer driver switches from one whole number of dots to the next, fractional numbers of dots being impossible to produce.

Low resolution priting produces a visible banding effect

In conventional colour printing, it is important to understand that, in the process of simulating a particularcolour, the final resolution of the printed image is not the nominal resolution of the printer (e.g. 360 360 dpi). This resolution must be divided by the size of the dot groups used;even a 3 3 grouping reduces the resolution down to a coarse and ugly looking 120 dpi. Recent developments, however, in this fast-growing market have pushed resolutions up to 1440 720 dpi. When combined with the development of new printer drivers capable of more sophisticated configuration of dot positioning, the result has been a significant increase in quality.

Higher resolution produces a smoother result

Printer Driver Software

Printer driver software is becoming increasingly ‘intelligent’, as vendors strive to make it easier for users to optimise colour output. Intelligent driver technology is marketed under a variety of names – Xerox’s Intelligent Color, Hewlett Packard’s ColorSmart, Tektronix’s TekColor and QMS’s QColor, for example. The basic principle behind these technologies is that the contents of a page are analysed by the driver as it is being rasterised and the halftoning method and colour mode are customised to the contents of the page. The most advanced solutions can resolve individual text, graphics and bitmap objects within the page and apply different optimised settings to each. For example, a page containing text, a coloured pie chart and a scanned photograph would have the text rendered in solid black, the pie chart using amplitude modulated screening and ‘vivid’ colour mode, and the photograph using frequency modulated screening and ‘photographic’ colour mode. Fully featured driver software such as that provided with the Epson Stylus Colour inkjet printer also provides direct access to basic image editing controls and allows selection from different colour management options.

Epson’s printer driver provides the user with a high degree of control over how an image will be printed

Page Description Languages

Desktop printers print by depositing dots on paper, but those dots can be configured in different ways within the computer/printer subsystem. This is where page description languages (PDLs) play an important role. The minimal type of printer architecture is a nonintelligent device which simply outputs a bitmap that has been created by the host computer. Such are the so-called GDI (Graphics Device Interface) printers which are designed to work with Windows; the GDI commands used to display the screen image are directly converted to bitmap form by the host PC, stored in the PC’s memory and then sent to the printer for output. GDI printers are relatively cheap, since they do not require processing power or large amounts of internal memory, instead relying on the power and memory of the host PC.

Page description languages (PDLs) such as Hewlett-Packard’s PCL and Adobe’s PostScript use a completely different printing architecture. These high-level PDLs require special drivers installed on the host PC to convert GDI commands into PCL or PostScript code, which is then sent to the printer, where an on-board processor decodes the data, rasterises it (turns it into a bitmap which the printer can output) and stores it in memory until the print engine is ready to print the dots. This need for processing power and memory makes PCL and PostScript printers more expensive, the trade-off being their ability to render complex pages containing graphics and multiple fonts more efficiently and – in the case of PostScript – have much greater control over how the output device renders pages. As ever, the best choice of printing architecture depends on the kind of output being processed. If output is mainly text and simple charts, then a GDI printer is an acceptable choice, as it shouldn’t impose excessive demands on the host PC. For general business documents which contain some graphics, a PCL printer is perfectly adequate, while for complex graphics and desktop publishing work, PostScript is the preferred solution. PCL, the native language of Hewlett Packard’s LaserJet family, has become the industry standard for general office printers and is supported by virtually all PC and printer vendors, application vendors and operating system vendors. PostScript is often available as a firmware upgrade to mid-range PCL printers.

The mode of operation of the main desktop printer classes is described below.

Inkjet

An inkjet’s printing head holds a central reservoir of liquid ink connected via a tube to a matrix of microscopic nozzles set in a square or rectangular array. These draw ink from the reservoir by capillary action. Each nozzle is equipped with an electric element which is controlled by the printer’s central circuitry. When a current passes through the element, it heats up, causing tiny bubbles to form in the surrounding liquid. As the bubbles merge, a droplet of ink is expelled on to the paper’s surface. Expulsion of the droplet causes the bubble to contract, drawing more ink from the nozzle. Graphics or text characters are constructed by selectively activating the nozzles as the head moves horizontally back and forth across a forward-moving printing surface. Budget colour printers use three such heads for cyan, yellow and magenta, while more expensive products use an extra head for black. The type of paper used makes a significant difference to the quality of inkjet output. If the paper is too absorbent or fibrous, the ink will be absorbed and will spread out. Most manufacturers offer a coated paper which prevents this.

Thermal Wax

The two principal components of a thermal printer are its printing head, which stretches the entire width of the page and its paper transport mechanism. Although employing an application technique which is similar in principle to that of a typewriter using a single strike typewriter ribbon, the thermal wax printer is much more expensive; each wax-coated ribbon can be used only once, regardless of how much of each colour is applied. The ribbon, which has an area equal to that of the printed page, is coated with alternate panels of cyan, magenta, yellow and black, running parallel to the paper, and passes from an input to an output cassette. The result is good-quality dithered output, producing vivid, slightly glossy colours.

Thermal wax printing requires a bright white, clay-coated paper to reflect the maximum amount of light back through the translucent dyes. As the paper passes through the printer four times – once for each colour – registration problems can occur over time as the transport mechanism wears. In a thermal wax printer, the printing head, which stretches the entire width of the paper, is made up of hundreds of tiny heating elements. As an element heats up, it melts an identically sized dot of wax from the film backing on to the moving paper below. Wax beneath the unheated elements stays in place, leaving the underlying paper surface clear. Once a page-sized colour layer of, say, cyan pigment dots has been applied, the ribbon moves to the next colour while the paper returns to its starting position ready for the second coloured layer of dots to be added, and so on.

Wax Phase Change

Another version of thermal wax technology is used by the ‘phase change’ printer. Sticks of wax are heated to 140°;C and melted wax is fired at the paper from a head scanning across the page, solidifying on contact with the paper, with little spread. This is caused by the wax’s abrupt phase change curve, the wax melting sharply above 140°;C and solidifying almost instantly below it. Colour density is good as the colour is dye based, rather than a pigment based. Printing takes place in a single pass, minimising registration problems, and running cost is relatively low as only the wax deposited on paper is consumed.

Dye Sublimation

Dye sublimation printers can vary the volume of dye transferred to paper in 256 steps as well as the intensity of the individual colour printed. The amount of dye released from the film substrate is temperature dependent. The higher the temperature of the head, the more dye is deposited. With three or more dyes, the result is a true continuous tone image of up to 16.7 million possible colours for each CMYK dot deposited on a photographic paper which contains a quantity of chemical fixer to complete the print process. The near-photographic result is achieved in spite of a relatively low resolution – usually 300 dpi. The down side to the excellent results produced is that the dye sublimation printing process is slow compared with other technologies and is also the most expensive. It also costs the most to run – up to £4 per page.

Dye sublimation printing

Colour Laser

Laser printing – a form of electrostatic printing – uses the same imaging technology as the original photocopier, although the optics involved in colour work are more complex. Colour laser printing multiplies the original black and white electrostatic process by a factor of four, with different manufacturers using different techniques to implement the imaging process. Canon’s photosensitive drum, for example, is imaged four times, while Xerox exposes a long photosensitive belt with all four colours exposed on the belt end to end. The latent images then pass under the corresponding toner hoppers attracting toner on to the paper. Laser printer resolutions are typically 300 or 600 dpi. As the developmentof toner technology continues, higher resolutions can be expected.

Colour laser printing

Desktop Proofing of Colour Separations

Proofs of colour separations can be printed on a black-and-white desktop printer to verify that objects appear on the correct separations and that colours overprint or knock out as expected (see explanation of overprinting and knock-out later). Colour separations should be proofed on a PostScript printer, as non-PostScript printers cannot accurately show how separations will image on a PostScript output device.

Photoshop’s layers show thumbnails of the four CMYK images of which the composite image (below) is composed

To proof separations from DTP applications such as Pagemaker on a PostScript desktop printer, the PPD for the printer is first selected in the printer dialog box, Colour/Separations is clicked and the inks to be used in the final separations are selected. Clicking Print causes the printer to output a page for each colour selected.

Commercial Printing

Photographic Colour Separation

As we have seen earlier, the subtractive primaries cyan, yellow and magenta can be combined to recreate all the colours of the spectrum. Therefore, in theory, it should be possible to print a full colour image just using cyan, yellow and magenta inks. To do this, it is first necessary to separate the original image into its cyan, yellow and magenta components. This can be done by photographing the image – e.g. a colour photograph – three times, through filters which are the same colour as the additive primaries – red, green and blue. When the image is photographed through the red filter, green and blue are absorbed and the red passes through, producing a negative with a record of the red. By making a positive of this negative we will obtain a record of everything that is not red, or more specifically, a record of thegreen and blue. The green and blue, as we have seen earlier, combine to produce cyan; therefore, we have a record of cyan. The same process is repeated for magenta, using a green filter, and for yellow, using a blue filter. As each filter covers one-third of the spectrum we now have a record, on three sheets of film, of all the colours in the original image. When the sheet of film containing the cyan content of the image is placed in contact with a printing plate and then exposed, it transfers the cyan content of the image to the plate; printing on to paper with the plate, using cyan ink, then produces a print of just the cyan content of the original image. Repeating the process with the yellow and magenta film and printing two further passes using yellow and then magenta inks should, in theory, reproduce the original full colour image. Unfortunately printing inks are not pure, absorbing colours that they would not absorb if they were pure. For this reason, the printed image will appear ‘muddy’ unless colour corrections are made on the separations to compensate for these ink deficiencies. Another problem with using just the three separations is a lack of density in the shadow areas. To overcome these problems, a fourth, black, separation is made by using a yellow filter or a combination of all three filters. The addition of black improves shadow density and overall contrast.

The cyan, magenta, yellow and black separations of the apples image (opposite) used to create plates for CMYK printing

When the printing plates are made, the four separations are screened at different angles so that the halftone dots for each ink print in a symmetrical rosette pattern. Traditionally, the cyan screen is printed at 105°, the magenta screen at 75°, the yellow screen at 90° and the black screen at 45°. If one or more of the process inks are set to print at different angles, or if the paper rotates slightly as it moves through the press, then the rosette pattern does notprint correctly and a moire pattern appears, disrupting the smoothnessof the colour gradation.

The four colour separations are screened at pre-defined angles

When printed, the image is reproduced as thousands of tiny dots laid down in thin layers of colour. The colour perceived by the eye is determined by the size of the dots, the manner in which they overlap, and their relation to one another, i.e. the colours are produced not in the physical mixing of the inks, but in the optical mixing of individual colours by the viewer’s eye.

Most photographic colour separations are now made using high precision colour scanners. The original image, or a positive transparency of it, is placed on a drum, and a laser light beam scans rapidly back and forth over it. The reflected (or, in the case of a transparency, transmitted) light is divided into three separate beams which pass through red, green and blue filters, activating extremely sensitive photocells. Depending on how much light the photocells detect, signals of varying strength are sent to laser light generators, which automatically expose a set of separation negatives by emitting precisely controlled bursts of light.

Howtek Scanmaster drum scanner

Digital Colour Separation

Many desktop applications now provide facilities for colour separation. Using PageMaker, for example, spot and process colour separations of a publication can be imaged directly to a PostScript imagesetter using paper or film. When Separations are selected in the Pagemaker ‘s Print Colour printing dialog box, the information found in the PPD file for the optimised screen option, and the angle and frequency fields are displayed. One sheet of paper or film is produced for each spot or process ink to be printed. A commercial printer thenuses these separations to prepare plates for the printing press. Spot colours normally print at the angle specified in the PPD (PostScript Printer Description file) for Custom Color, which is usually 45°. Process colours normally print at the same angles as those evolved for the traditional separation process as described above – cyan 105°, magenta 75°, yellow 90° and black 45°

Linotype imagesetter

Due to the fact that imagesetters simulate halftone dots by grouping printer dots together in halftone cells, producing consistent angles at 75° and 105° can pose problems. A number of vendors offer screening solutions to address this problem, notable among them being Agfa’s Balanced Screen Technology and Linotype-Hell’s HQS Screening and Rational Tangent Screening systems. While these systems offer improved colour results, they are still based on the traditional screen ruling and angle combinations.

Creating shades of grey by means of grouping dots in halftone cells

Even better results may be offered by recent developments in the use of frequency modulated (FM) or stochastic screening, also from Agfa and Linotype-Hell. While the traditional halftone screening uses the size of the halftone dots to convey shading, FM screening does not arrange dots into halftone cells, but simulates the different shades of an image by controlling the number of dots in each area – more dots producing a darker shade and fewer dots producing a lighter shade. Because there is no regular dot pattern in FM screening, the problem of Moire patterns is avoided; also, since FM screening uses smaller dots, more detail and subtle changes in colour may be reproduced.

Printer’s Marks

Part of the process of preparing separations for printing is ensuring that the prepress bureau and/or print shop are provided with all the information necessary to produce film and plates and to monitor the consistency of print quality during the print run. Many digital applications provide the means of adding information to the individual separations covering the following requirements.

  Crop marks – marks indicating where the printed pages will be trimmed

  Register marks – normally in the form of cross hairs or star targets. After a file is separated and printed, the print shop uses the register marks whichappear on the negatives to align the separations to create proofs and plates. Star targets are harder to align than cross hair register marks but they are extremely accurate. Each image should have at least four registration marks

Bleed area – area which falls outside the cropping area. Bleed is included in an artwork to compensate for shifts of the image on the printing press, or to allow for a slight margin of error for images which will be stripped into a keyline in a document. A press bleed – one which bleeds off the edge of the printed page –should be at least 18 points

  Text labels – specifying, for example, file name, page number, line screen used and screen angle and colour of each plate

  Colour calibration bar – used by the print shop to check colour consistency during the print run. There are two types of colour bar, called progressive and black overprint. The progressive colour bar consists of a solid colour square of cyan, magenta and yellow as well as various combinations of these three colours. The black overprint colour bar prints the various combinations of cyan, magenta and yellow with a solid swatch of black over the colour combinations to check for show-through of underlying inks

  Gradient tint bar – used to check for consistent tint values in separations. Tints usually range from 10 % to 100 % in 10 % intervals

Corel DRAW’ Print Preview dialog box

Offset Printing

Still used today to reproduce full colour output, halftone colour printing was introduced in the 1890s, although many years passed before its full potential was realised. Although colour reproduction theory was fairly well understood, the lack of colour film restricted colour work to studios where the necessary separation negatives had to be made directly from the subject, under the most exacting conditions. As reliable colour film became available in the 1930s, colour reproduction became both more common and more accurate. The offset plate is made of a base material – such as aluminium, stainless steel, or, for very short runs, paper – coated with a photoreactive substance. After exposure, the plate is developed and then treated to enhance its ink-attracting or water-repel-ling properties. For very long print runs, bimetal plates are sometimes used; typically, copper forms the image area, while aluminium or chromium is used for non-image areas. Recent developments which have seen dramatic increases in the light sensitivity of photopolymer coatings offer the possibility of producing plates in future which will no longer require film exposure, but will instead be digitally imaged by a scanning laser.

The offset lithographic printing process

When printing process colours, two factors must be controlled to ensure the quality of the finished work, namely the number of halftone dots which print per inch (called the screen frequency or screen ruling) measured in lines per inch (lpi) and the angle at which they print (called the screen angle). If these factors are not correctly specified, the process inks may not print correctly in relation to one another, and distracting moire patterns may appear in the final printed colours. The default screen settings in the selected printer’s PPD are based on specifications from the printer manufacturer and are optimised for the printer. A prepress service provider may, however, suggest different settings in some circumstances.

Trapping

When printing overlapping coloured objects in a composition on an offset press, conventionally the top object is printed and the equivalent area of the bottom object is not printed or is ‘knocked out’ in printing terminology. As mentioned earlier, colour misregistration can occur if the paper rotates slightly as it travels through the press. The same effect can occur if a plate is misaligned or if the plate or paper stretches slightly during printing. Because of the knocking-out convention, such misregistrations can cause unsightly white slivers between adjoining colours. To compensate for this problem inthe traditional separation process, the platemaker used photographic techniques to ‘spread’ or ‘choke’ adjoining objects on the separate plates to allow for misregistration, using a process called trapping. Spreading involved enlarging the size of an overlapping object, while choking involves reducing the size of an underlying object.

Effect of misregistration when the bottom object is knocked out

Using digital separation methods, trapping is applied to a publication, either manually or automatically, before film separations are created. Trapping is required mainly for overlapping objects created in a vector drawing application and printed using distinct spot colours applied from separate plates. Trapping is less important if the objects use process colours which share a sufficient quantity of common inks and normally no trapping is necessary for artwork consisting of continuous tone images, as the colours blend naturally together. Trapping of complex objects such as those involving blends or graduated fills is a skilled and exacting process as the trap colour and shape must change as the colour on the perimeter of the object changes. Assistance is available in the form of software such as Adobe’s Trapwise, which provides more sophisticated trapping than that found in illustration or page makeup applications.

Spread–overlapping object enlarged

Choke–underlying objectg reduced

Trapping in a Drawing Application

CorelDRAW’s Trapping dialog box

Trapping by overprinting selected colour separations. Using Corel’s Advanced Separations Settings, one or more of the CMYK separations can be set to overprint graphics, text, or both.

To trap by overprinting selected objects. Overprint Fill causes the top object to print over the underlying object (instead of the underlying object being knocked out), which makes ‘white gaps’ impossible. This option is best used when the top colour is much darker than the underlying colour, otherwise an undesirable third colour might result (e.g. red over yellow would result in an orange object). Overprint Outline causes the top object’s outline to print over the underlying object. The safest choice is to assign the colour of the top object’s fill to the outline. When setting the outline thickness, it has to be remembered that the outline straddles the path which defines the object’s shape. Therefore, an outline of, for example, 0.20 points actually creates a trap of 0.10 points.

CorelDRAW allows saving of both artwork and colour separation instructions in a .PRN file for sending directly to an output device by a service bureau, where the file will be processed through a Raster Image Processor (RIP) in order to rasterise its PostScript instructions. The rasterised file will then be loaded to an imagesetter to produce the film separations which in turn will be developed in a film processor.

Creating a PostScript PRN file using the Print to file option

Trapping in a Painting or Photo Editing Application

Adobe Photoshop traps by spreading, according to the following guidelines:

All colours spread under black

Lighter colours spread under darker colours

Yellow spreads under cyan, magenta and black

Pure cyan and pure magenta spread under each other equally

Generally speaking, four colour images need only be trapped when solid tints are being used in CMYK mode. Excessive trapping may generate a keyline effect (crosshairlines) in the C, M and Y plates. This problem is not visible in the composite channel, showing up only when output is made to film.

The procedure for creating a trap is as follows:

1. The image is first converted to CMYK mode
2. Trap is then selected from the Image menu, causing the Trap dialog box to appear
3. A unit of measurement is selected from the Size Units menu
4. In the Width box, the required trapping value, as agreed with printer, is entered

Specifying a trapping value

Trapping in a Page Makeup Application

PageMaker traps text to underlying PageMaker-drawn objects (rectangles, polygons, lines and ellipses), and traps PageMaker-drawn objects to each other, but it ignores imported graphics. Imported graphics must first be trapped in the illustration or image-editing program used to create them. PageMaker applies the correct trapping techniques on different parts of the object even if text or a PageMaker-drawn object overlaps several different background colours. The trapping adjustments are made automatically throughout the publication, although the application allows the user to vary settings from the default in particular situations.

PageMaker decides whether to trap based on ink density values, and places the traps based on the neutral densities (relative lightness or darkness) of adjoining colours, in most cases spreading lighter colours into adjacent darker colours. In all cases, the overprint trapping technique is used – the trap colour prints over the darker of two adjoining colours. The trap colour used depends on the component inks of the two adjoining colours. For adjacent process colours which require a trap, PageMaker creates the trap colour using only the CMYK values in the lighter colour which are higher than those in the adjoining colour. For aprocess or spot colour next to a spot colour, the lighter colour is used as the trap colour.

PageMaker Trapping Options dialog box

Dot Gain

Many variables – from the photomechanical processes used to produce separations, to the paper type and press used – affect the size of printed dots. Typically, dots increase in size as wet ink spreads, under pressure from the offset press rubber blanket, as it is absorbed by the paper. A 50% halftone screen, for example, may show an actual density of 55% on the printed image when read with a densitometer. Dots may also increase in size as negatives from different sources are duplicated to produce the final film, or can result from miscalibration of an imagesetter during the imaging process. If too much dot gain occurs, images plug up and colours print darker than specified.

Some applications, such as Photoshop, provide the means for compensating for dot gain. When a Dot Gain value is entered in Photoshop’s Printing Inks Setup dialog box, the program uses this percentage as the midtone dot gain value to generate a dot gain curve. Changing the dot gain makes the image appear lighter (if a lower percentage is entered) or darker (if a higher percentage is entered) on the screen. It does not affect the actual data in the image until Adobe Photoshop uses the setting to adjust the CMYK percentages for dot gain during the conversion process.

Undercolour Removal and Addition and Grey Component Replacement

In theory, equal parts of cyan, magenta and yellow combine to subtract all light and create black. As explained earlier, due to impurities present in all printing inks, a mix of these colours instead produces a muddy brown. To compensate for this deficiency in the colour separation process, prepress operators remove equal amounts of cyan, magenta and yellow from the C, M and Y plates in areas where the three colours overlap, and add black ink instead via the K plate. Called undercolour removal or UCR, the process adds depth to shadow areas and to neutral colours, reduces the amount of ink required and helps prevent ink trapping.

UCA or undercolour addition – the converse of UCR – is a way of compensating for the colour thinning which can occur with GCR or UCR, by adding back colour. UCA produces rich, dark shadows in areas that might have appeared flat if they were printed with only black ink. UCA can also prevent the posterisation which can occur if there is a lot of subtle shadow detail.

Adjusting UCR via Photoshop’ Separation Setup dialog box

GCR or grey component replacement is the process of substituting black for the grey component which would have been created in an area of a printed image, where all three colours combine. In GCR, more black ink is used over a wider range of colours. GCR separations tend to reproduce dark, saturated colours better than UCR separations do and GCR separations maintain grey balance better on an offset press.

Adjusting GCR via Photoshop’s Separation Setup dialog box

A number of applications provide the user with the means of manipulating UCR, UCA and GCR values via a Separations Dialog Box. The type of separation adjustment required is determined by the paper stock being used and the requirements of the print shop.

High Fidelity Colour

High fidelity colour printing uses additional process inks to increase the gamut of printed colours by as much as 20%. For example, Pantone Hexachrome colours are reproduced using cyan, magenta, yellow, black, orange and green inks. PageMaker can create separations for up to eight inks, including both process and spot colours, varnishes and high fidelity colours.

High fidelity colour extends the gamut of colours which can be printed

Ink and Paper

The paper used has a major influence on the quality of the colour printed on it. Because process inks are transparent, it is the light reflected from the paper’s surface which supplies the colour to the ink. For example, when light passesthrough cyan ink printed on paper, the ink acts like a filter, absorbing the colour it is not (red) and allowing the colour it is (blue and green) to pass through. These two colours then reflect off the paper and back up through the ink. What the viewer sees is a blend of the blue and green colours which constitute cyan. It is the quality and quantity of the reflected light which dictate the quality of the reflected colour. For this reason, the paper must be bright and neutral in colour if it is to reflect maximum light without introducing any colour change. Also, the paper should be smooth and flat as a rough surface will scatter the light and distort the colour.

Proofing a Publication

Digital Proofs

This category of proofs includes those generated from inkjet, laser, thermal wax, phase-change, or dye sublimation printers. Data is imaged directly from the original file on to paper. This method is quick and economical, and is useful to give a first pass representation of how a page will print, but it is not usually accepted by print shops as being a good enough representation of what they are expected to produce, as the proof is not produced from the film which will be used to make the printing plates. In particular, digital proofs cannot reproduce press conditions such as screen frequencies and angles, dot gain, etc.

Off–Press Proofs

These are made from the film separations which will ultimately be used to make the printing plates. This category includes blueprints, overlay proofs (e.g. Color Key) and laminate proofs (e.g. Cromalin, Matchprint or Agfaproof).

Press Proofs

Produced using the very plates, inks and paper which will be used for the final print, press proofs provide the most accurate but also the most expensive proofing method. They are generally reserved for high-end projects