The repro process will ensure that the reproduction of the original label design, including the correct color meets client expectations, within the tolerances of the chosen print process.
The traditional method of origination was historically very labour intensive, requiring highly developed skills and was very expensive to undertake.
Today almost all repro is carried out using digital procedures which shorten the production chain and permits greater creativity, with a shorter lead-time.
TYPICAL REPRO WORKFLOW
Typical components of a modern repro workflow are illustrated in Figure 3.1.
Figure 3.1 - Components of a typical repro workflow
At the core of a typical repro system is a file storage system usually in the form of a server.
A server is a computer which has been set up specifically to store and 'serve' files to other computer users. In a design studio or pre-press environment, this is essential to prevent the duplication of files and to ensure that an effective backup policy can be implemented.
All the components of a repro system are typically linked by a PDF workflow.
Adobe's Portable Document Format (PDF) was developed as a data format for the paperless office, but has established a place in the pre-press environment. Unlike other digital document formats, there is no need for separate text files, fonts, image files or vector art. Acrobat can incorporate them all into the file during PDF generation so that the PDF can be used as a "virtual job bag" to which all necessary pre-press ingredients can be added. A key feature of pre-press workflow systems is the initiation of all processing steps without significant operator involvement.
Most repro workflows feature a high resolution pre-press scanner that uses a high-speed rotating glass drum (known as a drum scanner). These scanners are used to scan transparencies and photographic images.
Drum scanners give a much more detailed reproduction of an original than flatbed desktop scanners and can capture a much greater range of tones. The performance gap between some of the top flatbed scanners and drum scanners is however narrowing.
An image setter is a high resolution output device that can transfer electronic text and graphics directly to film, plates, or photo-sensitive paper. An image setter uses a laser and a dedicated raster image processor (RIP) and is usually PostScript-compatible, to create the film used in computer-based pre-production work. These films are used to create the plates that go on the printing press.
Image setters are rapidly being replaced by CtP (computer to plate) systems.
RASTER IMAGE PROCESSOR (RIP)
A RIP is a hardware or software tool that processes a digital PostScript file and then converts it (i.e. rasterises it) to a printable format.
CtP (Computer to Plate) refers to the manufacture of a printing plate from a computer using laser imaging. The CtP process uses a photopolymer plate coated with a black (ablation) layer that is sensitive in the infra-red range. After removal of its protective film, a laser images the information onto the black mask. In the flexo process the black layer evaporates where the laser beam hits this layer and the actual photopolymer is laid bare in these areas, for the main exposure to follow. After the main exposure, the plate is conventionally washed out, dried, post-exposed and after-treated.
The use of CtP introduces digital pre-press into the pressroom, thereby replacing conventional step-and-repeat technology. It requires adequate digital color proofing and tighter process control. Front-end software and workflow tools must also be adapted to the CtP environment.
A platesetter is a machine that generates plates for a printing press. It is similar in function to an image setter, except that instead of producing film from which the plates are made, the plates are imaged and processed in the platesetter.
Most repro workflows rely on services that can distribute digital content securely (e.g. WAM!NET, ISDN or WWW. * WAM!NET is a leading provider of secure managed data file delivery and data management.)
Using a dedicated service such as WAM!NET the content may be kept completely secure, the status of the delivery can be tracked and both senders and receivers can be notified by email or SMS when packages have been delivered to their destinations.
All repro systems will have a proofing capability. These systems are capable of producing either hardcopy or electronic facsimiles suitable for early, intermediate or final approval.
The digital production route as illustrated opposite (see Figure 3.2) has eliminated many of the process steps required with conventional production. A comparison of conventional versus digital production, highlighting typical devices used is illustrated in Figure 3.3.
All the elements which make up a design can be presented on a computer monitor.
Figure 3.2 - Possible routes to press - conventional versus digital
Figure 3.3 - Conventional versus digital production routes highlighting typical devices used
Alterations in sizes, layout, and content can be programed with a minimum of delay and without leaving the desk top. Once the designer is happy with what is on the screen, a printout can be made for submission to the customer. This proof can be dispatched direct to the customer, via a web based system, and any changes communicated back to the repro house, almost immediately. Where existing photography, such as slides, transparencies or photographs are to be incorporated these can be scanned, changes incorporated, colors manipulated, etc. and the final result downloaded to the screen for incorporation within the overall design.
Computers and computer peripherals handle graphics in two fundamental ways. They either create a list of drawing instructions, or define a two dimensional grid of individual picture elements, or pixels.
Most input and output devices build up graphics in a series of raster scans. Scanners, display monitors, printers, and image-setters are all raster devices and they sample or output graphic objects as a series of separate pixels.
Each pixel is assigned a color value defining the intensities of its primary color components.
Scanned images are therefore referred to as sampled images and their resolution corresponds to the sampling frequency, or distance between individual samples.
If an image needs to be changed in size and it is not practical to re-scan it, pixels must be added or Conventional production route removed in order to maintain the same resolution. This activity is known as re-sampling.
The removal of pixels is a relatively straightforward manipulation, provided that not too many pixels are removed, which would create a 'stepped' effect. This will result in a loss of some detail and parts of the image which will appear out of focus. When the requirement is to enlarge the image the procedure is not quite so straightforward.
Pixels have to be added in order to maintain image quality. This can entail adding just one or more pixels beside existing ones. Once an image has been separated into pixels the color range and the tonal dot size is fixed. The color, or grey scale range, is fixed by the bits allowed per pixel. One bit per pixel will provide either black or white with no grey tones, whilst 8 bits will provide some 256 grades of grey between the black and the white.
The first image will have a posterised* appearance whilst the second will have a photographic look.
The same relationship applies when expressing colors. By increasing the bit value the graduation of colors can be taken into many millions. Far above what is in fact reproducible by conventional four color process printing.
The relationship between halftone, and process color screening should relate to the number of pixels in the final size of the image.
Two pixels per dot is the acceptable ratio which will provide even color graduation. If it becomes necessary to adjust the size of an illustration after it has been bitmapped and toned it is better to follow through the procedure of removing the tonal dots, adjusting the pixels and then reapplying the tonal dots in the correct ratio. This will ensure the best possible reproduction.
If the requirement is to re-size the image out of proportion, it is then necessary to add pixels in one direction and remove them in the other. If this is the ultimate intention it might prove beneficial to re-scan the image, oversize, and crop to the desired dimensions prior to creating the pixelised image.
Image processing which involves calculating new values for each pixel can be performed at high speed regardless of the apparent complexity of the image. Any operation which requires the writing of new values for every pixel, such as scaling and rotating, is performed much slower than it would be using vector based methods.
*Posterisation- occurs when an image's apparent bit depth has been decreased so much that it has a visual impact (resulting in banding). Posterisation occurs more easily in regions of gradual color transitions.
GRAPHIC FILE FORMATS
Numerous file formats have evolved to describe images for different purposes. They are primarily identified by the degree of resolution, the number of bits that are used to describe each pixel, and the information that can be included in the file header or tag. EPS, JPEG, BMP, GIF and TIFF are the main file formats used for color images. Other formats are also available, but in some instances they do not support four color process requirements.
The PostScript page description language (EPS) allows objects to be incorporated into page files. They can also include a low resolution preview of a bitmap image for display on screen, together with a reference to a high resolution PICT or TIFF file. EPS files can also incorporate the halftone parameters for when the image is output, including screen ruling, angle, dot shape, and any transfer functions that may have been applied. Some EPS files incorporate a four color image either as a composite, or as a color separated image, in which each color is saved in a separate file.
Tagged Image File Format (TIFF) can be read by most graphics applications. Adobe Photoshop is capable of reading all the extensions that are relevant to graphic arts applications and can convert images from one format to another.
REPRO SOFTWARE AND GUIDANCE ON FILE PREPARATION
Adobe® Illustrator®, Adobe® InDesign® and Quark XPress® files are all excellent programes for typesetting and building multiple page documents, however, they are not always ideal for packaging design. Placed images within these files often have a low-resolution preview, making exact placement of elements a challenge. Support files therefore must be supplied for all placed images, including those that are embedded.
Photoshop® files should be 300 dpi at the size at which they are placed into the final file. It should be recognised that the resolution of a raster file decreases proportionally when enlarged.
When supplying Photoshop® files it is often preferable not to compress the layers in the file. Equally any fonts that are not compatible with the platform being used must be converted to outlines. (Be aware, however, that converting text to outlines, limits the ability to make any content or layout adjustments).
Before sending fonts, it is important to make sure that the licence allows the fonts to be used by both the designer and those outputting the files. It is therefore recommended that one version of the file with the fonts converted to outlines, and one with live text is supplied.
Always include either a PDF or a hard copy proof of the final file so that when the file has been received it can be verified and that there are no issues with fonts or special effects.
If there is a color target that needs to be matched for a CMYK illustration, these also need to be supplied along with swatches of any special match colors.
Using a scanner, it is possible to incorporate existing illustrations taken with conventional cameras or from a printed document into a file, although this is now rarely the case. The widespread use of digital cameras shortens the chain of procedures.
Digital cameras use a Charge-Coupled Device (CCD), which is a micro-electronic device, to capture the image in pixelised form. The quality of the image varies with the number of pixels that the image is divided into.
This will vary with the quality of the camera from as few as 75,000 and as many as 18 mega pixels or more. The images are downloaded to a computer and relayed onto the screen. When a hard copy (print) is required this may be obtained from a printer linked to the computer.
The imaging method of the printer will dictate the quality of the image produced. Once used the CCD device may be cleared and used again and unused images compressed and stored for future reference. It is necessary to check the amount of memory required with that available, as storage, even in compressed form, can use a considerable amount of space.
One of the many advantages of digital origination rests with the ability to transmit illustrations over the internet with no loss of quality. This may be in the form of bespoke Asset Management Systems of FTP (File Transfer Protocol) methods. Once the illustrations are received they may be manipulated and color adjusted to suit the method of reproduction.
A TYPICAL REPRO CHECKLIST
Some of the aspects which must be considered, irrespective of the repro techniques, are shown in Figure 3.4.
Figure 3.4 - A typical repro checklist
All label and pack dimensions should be checked thoroughly before the repro process commences.
Once dimensions are correct then all bleeds should be checked. Bleed is the area to be trimmed off and is a term that refers to printing that goes beyond the edge of the design outline.
Artwork and background colors can extend into the bleed area and this gives the printer a small amount of space to account for movement of the substrate, and any design inconsistencies.
CAD/Dimensions are separated onto a layer of its own. It is important to set CAD dimensions to a color of their own and ensure it is overprinting.
COLOR SPLIT/ SEPARATION
When required for multi-color process printing a full color picture or illustration is normally first divided into four separate subtractive color components by either electronic/laser scanning or through the use of color filters, in a process camera prior to the production of the printing plate.
The term ‘color separations’ usually refers to the set of four films, one each for the yellow, magenta, cyan and black (CMYK) used for plate production. (Figure 3.5)
Figure 3.5 - Color separations
Each color on the film is represented as lines of dots set at specific angles which, when overlaid, will combine as layers of dots forming tiny repeat rosette patterns that simulate shades of color when seen at a distance.
If an image is required for line printing only, the separated films still represent each of the colors, (which in some printing processes may be more than four colors), but there is no dot formation or rosette pattern involved.
The print-ready data is streamed to a storage medium for incorporation into a finished assembly at a later stage or exposed directly to film or plate.
There may be circumstances where for cost or other reasons an image has to be created out of 3 rather than 4 colors. The visual impact of this is illustrated in Figure 3.6.
Figure 3.6 - Illustrates the effect of creating an image from 3 colors instead of 4 - the image on the right has had black replaced with blue. It can create a similar effect to that of the black if these is not much black in the image
COLOR SPLIT CHECK LIST
Below is a check list of factors to be considered when preparing the color elements of a job for repro:
Determine the number of colors and ensure the palette only contains these colors and that tints are specified correctly.
Minimise the amount of colors where possible without compromising the design/branding
Detect and redefine colors
Spot colors in text - There are times for example, in which color text is supposed to be obtained from 4 color process, however this may not be possible due to register problems. If this is the case then the text has to be redefined as a spot color and approved by the customer
Spot colors in images - if images are required in spot colors, channels need to be generated.
When printing an image, large areas of flat tone and gradients are often difficult to reproduce. In order to achieve correct color density therefore, it can be beneficial to run a screen tint underneath the solid color.
The effect on color density when using an additional screen tint is illustrated in Figure 3.7.
Figure 3.7 - The effect on color density when using an additional screen tint
A typical example when running CMYK would be to introduce a 40% screen tint of cyan underneath a black to increase its density.
An important aspect of origination is to make sure that it is economically practical for the intended printing process and press to print the image repeatedly, to an agreed quality. The term quality relating to the printed image is wide ranging, but register is a key element and takes the leading position in quality assessment.
With any printing process, misregister or poor print registration is unacceptable.
The output from label printing presses, however superior the engineering, may not be absolutely or consistently precise. Misregister can be caused by mechanical variation within the printing press and also due to temperature changes or movement of the substrate as it progresses through the printing units.
As the printing process is a mechanical process, quality limits should be imposed relative to the complexity of the final result, which would include the economic running speed for the type of label, substrate stability and final application method.
If a label design calls for line colors to butt up to each other (edge to edge) then any movement of the web will cause gaps or unsightly overprinting where the colors meet. The press will not be able to run economically without considering the effect of trapping.
The problem can easily be managed at the repro stage. The basic intent of trapping is to provide an overlay between adjoining colors as a part of the overall design (Figure 3.8).
Figure 3.8 - The extra tolerance added is called ‘trapping’ - Trapping is used to minimise the white halo effects of misregister
Trapping usually involves expanding the lighter of the two colors to overlap into the darker one.
Computer generated artwork can be trapped by applying a colored stroke or outline to each of the colors affected.
Half the width of the stroke will fall inside and half will fall outside the element it is placed against. When a stroke is specified to overprint, the adjoining color will be trapped under the outside half (Figure 3.9).
This technique however needs to be used with caution when text is included. Such trapping used with small or fine type could ruin the image by making the type unreadable.
A number of software programes include automatic trapping functions (often referred to as ‘spreads’ and ‘chokes’) that can be applied to any area where two colors butt up.
A “choke” is a specific adjustment or distortion of an image whereby the perimeter, in total or in part, is slightly pulled in (choked) towards the center. Choking of an element is normally used in conjunction with the “spreading” of a neighbouring element to guarantee that there are no color fringes or white borders around the image due to misregister.
Another technique for negating the effects of misregister involves the creation of a black key-line to cover the abutting edges and also form an integral part of the line printed elements of the label.
SUITABILITY FOR THE PRINTING PROCESS
In terms of overall printing quality the various processes are all capable of producing printed images on a variety of substrates to a high standard. Each process requires specific considerations when preparing for plate making. One of the greatest benefits to be derived from computer prepared origination is that adjustments can be made in the software programs and applied automatically during preparation of the color separated film. It is even possible to take this a stage further by incorporating the requirements of an individual press should this be desired.
Automatic controls may include such things as minimum line thickness or dot size, dot gain between what appears on the film and what will appear on the substrate. Variations of ink viscosity (ie the thickness of ink being deposited) will affect the color density of the printed image. Such variations may be allowed for by building the necessary information into the program, to be automatically implemented in the origination.
Typically registration marks are included in the design file of a new label job.
Registration marks used in the label industry are positioned on the edges of the web, in the area that is part of the waste matrix. Each color or embellishment (hot foiling and cold foiling) being printed will have its own register mark which is usually in the form of a cross hair positioned within a circle in the form of a target. The register mark image consists of very fine line work (Fig 3.10).
Figure 3.10 - Typical “target” registration mark
Registration marks are a method of monitoring the “print to print” register whilst the press is running. This facility allows the press operator to make manual adjustments to the print register and is particularly useful during the make-ready phase of the job.
Print register is achieved by moving each plate cylinder circumferentially and also sideways until the register mark of each color falls exactly on top of one another. This is called “in-register" and any movement in an individual color will indicate that the print is in “mis-register” (Fig 3.11).
figure 3.11 Shows the magenta color out of register
Once the correct print registration has been established and the press speed is increased a register monitoring system will display the register marks as a fixed image on a monitor. This gives the press operator an instant view of the “print to print” register, thereby allowing manual adjustments to be made to the register during the print run.
Another type of registration mark is the one used when the press is fitted with an automatic print registration system.This mark is usually rectangular in shape approximately 5mm x10mm and is printed in the “key” color.
This is the color to which the other colors are to print in close register. When the auto system is being used each print unit reads the key color and the system moves the circumferential register into the correct position and then holds it “in-register”.
More commonly known as “dispro”, disproportioning is the calculation that ensures that a printing plate produced in the flat will print an image of the correct size once it is wrapped around a printing cylinder. It involves reducing a plate-ready film in overall image size to compensate for the known distance that the photopolymer plate will stretch or distort on the cylinder.
The amount of distortion will depend upon which print process is being used. The thickness of the printing plate will vary according to the print process and in the case of the flexo and letterpress processes, the thickness of the tape used to mount it on the plate cylinder will need to be factored in as well. Generally, thicker plates and shorter repeat lengths will increase elongation.
Direct-to-plate imaging on to a curved plate, or to a plate already mounted on the cylinder, avoids the need for disproportioning. As the image is applied to a curved surface, no stretching occurs (Figure 3.12).
Figure 3.12 - "The dispro" effect
It is important to note that in order to achieve a good printed result, a high quality original is a necessity. Generating a high resolution image in Photoshop from a poor quality original will result in poor print quality (Figure 3.13).
When resizing original images a key factor to remember is that the more dots per square inch the higher the resolution. The fewer dots per square inch the lower the resolution. The optimum image resolution for printing is 150 lpi screen = 300 dpi*.
*dpi - Dots per square inch expresses the number of lines of halftone dots per square inch. Maybe expressed in-lines (lpi, lines per square inch) or dots per square centimeter.
Effective resolution is the resolution of the images with the effect of scaling taken into account. A 300 dpi image at 400% scaling would be 300dpi x 100 /400 = 75dpi dpi effective resolution (Figs 3.13 and 3.14).
Figure 3.13 - These two illustrations show the impact on resolution on resizing. Increasing the size of the image drastically reduces definition
Figure 3.14 - Image resolution – scaling up reduces the effective resolution of an image
IMAGE RESOLUTION CHECKLIST
Ensure an adequate resolution of original images so that the pixels do not show on print
Generating a high resolution image in Photoshop from a poor quality original will result in poor print quality
Optimum image resolution for 150 lpi screen = 300 dpi*
The more dots per square inch the higher the resolution
The less dots per square inch the lower the resolution
Dot gain is a characteristic of most printing processes where the size of the half tone dot changes, as a result of plate to substrate pressure, during the printing process (Figure 3.15 and 3.16). Because a conventional printing press is mechanical by nature, the dot gain can vary dependent on the age and condition of the press. Dot gain can make a considerable difference to the final printed result and may result in a complete change in the tonal values of color printing.
Dot gain is most prevalent and visible in the highlight and shadow parts of the tonal reproduction scale, giving unacceptable results especially in four color process printing.
Figure 3.15 - Dot gain - half tone dot changes under pressure
Figure 3.16 - Effects of pressure on dot structure
The problem with dot gain is that it is only part of the picture. Each press runs at its own color densities. In order to understand the color effect it is important to understand the effect of these densities. (Figure 3.17 and 3.18).
The printing characteristics of an individual press can be measured (via finger printing*) and the results built into the preparation of films and plates to give a predictable printed result.
The established characteristics are only valid if none of the printing parameters change. Any change, be it different plates, inks or substrates requires a new fingerprinting exercise to be undertaken. The same rule applies if the press is modified in any way, or the overall running speed is altered or the anilox changed (in flexo).
Typically a printer will carry out a press fingerprinting exercise in order to obtain, amongst other things, dot gain information. The printed effects of dot gain are illustrated in Figure 3.18 and 3.19.
Figure 3.17 - The diagram shows 3 identical dot gains but with 3 different ink colors and densities. They all look slightly different
Figure 3.18 - The effects of dot gain
Figure 3.19 - Comparison of images produced with 50% half tone (left image) versus the same image produced with 15% dot gain (right image). To overcome the effects of dot gain on the printed image the repro file has to be digitally adjusted to reduce
*The printing of a special test run on a press so as to determine the registration, dot gain, distortion and other characteristics of the press.
Once known, these can be compensated for at the design, film or platemaking stages.
DOT GAIN – A SUMMARY OF KEY POINTS
Dot gain is critical to color management
Dot gain is a method of measuring the dot size variation between digital data and press caused by the mechanical process of printing
Dot gain information is required at the repro stage to compensate for a measured gain
Each press has a unique dot gain percentage
Fingerprint trials must be individual to each press
SCREENING - SOLIDS VERSUS SCREENING
Tint (screen) – is a solid color which has been reduced in shade by applying a screen during the origination process. The resulting tint is specified as a percentage screen of the original solid color.
Tint (solid) – a dense area of color without screening, usually defined as a percentage of a defined solid color, as opposed to a screened tint of a solid color. A solid tint color is obtained by ink mixing, rather than as a pre-press operation.
Halftone screening is a way of splitting tones into separations that can overprint each other to make different colors (see Figure 3.20 and 3.21).
Typically the screens need to be on different angles (Figure 3.22) and a screen’s coarseness can vary dependent upon the quality of the material and print process.
Figure 3.20 - Cyan separation
Figure 3.21 - 4-color separations
Figure 3.22 - Screen angles
A rosette pattern that is created when all four color halftone screens are placed at the traditional prescribed angles to each other (see Figure 3.23)
Figure 3.23 - Rosette pattern
A Moiré pattern, found in both black and white or color halftone printing, describes the irregular unwanted interference pattern of screen dots caused by combining one, regular, halftone pattern with another similar one, so causing disturbing patterns or patches - either over the whole image or in certain color combinations (see Figure 3.24). It results from incorrect screen angles being used when overprinting colors or when reproducing from an already printed subject.
Figure 3.24 - Examples of moiré patterns caused by incorrect halftone screen angles. Right-hand illustration shows correct screen angles
The use of stochastic screening can avoid the problems of moiré and are ideal for use with Opaltone processes (the Opaltone process is explained below).
Stochastic screening is an alternative to conventional halftone screening in which the image is separated into very fine, randomly-placed microdots (measured in microns), rather than the more conventional grid of geometrically aligned halftone cells. Dots are the same shape and size in most versions, but there is varying spacing between the dots (see Figure 3.25)
Figure 3.25 - Stochastic screening - Random placing of dots in stochastic screening compared to AM (or conventional) screening
Sometimes called frequency modulated or FM screening, stochastic screening eliminates screen angles and the possibility of moiré patterns and provides greater image detail due to the lack of screen rulings and screen angles.
There are no rosette patterns with the process and results are impressive on both coated and uncoated paper and film.
The stochastic screening process involves imaging dots on film using special randomizing software. The software uses mathematical expressions to statistically evaluate and randomly distribute pixels under a fixed set of parameters.
Stochastic screening produces smoother gradations when vignettes, blends and degrades are involved. It enables almost any combination of colors to be used in the creation of subtle or dramatic effects.
The dot size used in stochastic screening is extremely small when compared to the size of the highlight dot in conventional screening. It is recommended that stochastic screening is used for flexography only after the printer and color separator have undertaken press fingerprinting to determine the ideal dot size and accurate compensation for dot gain.
Opaltone® is a patented imaging technology that digitally mixes CMY+RGB process inks.
The CMYK system can only reproduce a limited color gamut. For example oranges, reds, bright greens and blues are common colors that cannot be faithfully reproduced in CMYK.
The “traditional” way to overcome the CMYK gamut limitations has always been to print expensive spot colors. The Opaltone system is a way of digitally simulating spot colors.
At the latter stages of repro the aim is to help the customer visualise their job as accurately as possible in the form of a proof (Figure 3.26).
Figure 3.26 - Proofs help customer visualise their job as accurately as possible
Printing proofs are used for checking that all text and graphics and colors come out as expected before going to press. Aside from the finished piece the proof is often the only part of the production process that the client will see.
ADDITIVE AND SUBTRACTED COLOR EXPLAINED
Color makes up the visible light spectrum, which is made up of red, green and blue (known as the additive colors). Red, green and blue are called additive because when you add them together the result is white light (see Figure 3.27).
Figure 3.27 - The primary colors of light—red, green and blue—as they relate to the additive colors—yellow, cyan and magenta
Figure 3.28 - The principles of subtractive color
Cameras take images and scanners scan in RGB. Computer monitors and web based applications are classed as RGB media. RGB images have much more range of brightness than CMYK images.
Printing on the other hand uses ink and uses the subtractive colors (cyan, magenta and yellow or CMY), plus black (abbreviated to K). CMYK color is also called “process color” or “full color.”
The primary colors (hues) used for process color printing subtract light and when overlapped produce other colors and black images. See Figure 3.28.
COLOR CONTROL AND MANAGEMENT
Quite naturally end users expect the color they have seen and approved (via a proof) to be the same as the color they get on the final printed result. All printers will have processes in place (color management systems) to ensure that this is achieved.
Color control and management processes are set out to achieve the following objectives;
Ensuring color consistency amongst different input and output devices so that printed results meet expectations.
To ensure that the colors on the monitor are matched by the finished results on the press
Color matching – the exact matching of a given color sample in terms of hue, value and intensity
Some of the key methods used to ensure good color matching are detailed overleaf;
INK COLOR MATCHING
It is critical in color matching to duplicate the exact hue, value and intensity of the color in the ink blend. Each color should be verified under the correct lighting conditions and the use of appropriate color measurement instruments adopted to ensure an acceptable match.
An instrument known as a colorimeter can be used to measure the spectral reflectance of a color and compute numeric values for color intensity, hue and purity.
Color calibration is the process whereby a series of graphic input and output devices are calibrated, using color profiles, in order to ensure color consistency across the design, pre-press and printing operations.
Input devices, such as scanners and digital cameras and output devices, such as monitors, printers, proofing devices and image-setters are calibrated.
The range of colors available to a specific output device, are typically known as the color gamut. The RGB color range is much broader than the CMYK color gamut (which is what most pre-press output devices use). Colors specified using the RGB gamut will often fall out of the gamut range when output on a CMYK device.
ICC FILE STANDARD
The file standard for describing spaces is called the ICC (International Color Consortium) profile. The ICC framework allows pre-press processes to convert color scientifically.
Most artwork packages and operating systems now offer some sort of color management, but the profiles available are generic printer and monitor profiles.
To be completely accurate individual devices used need to be profiled.
ICC profiles can be used to convert between color spaces maintaining color as close as possible
ICC profile can be used at operating system level or on a document by document basis in Photoshop. See Figure 3.29.
Figure 3.29 - Same data viewed in Photoshop using 3 differet ICC profiles - The impact of ICC profiles demonstrated
New tools for fine tuning CMYK separations are developing which complete the transformation RGB/CMYK to CMYK in a single step.
A series of colored shapes printed outside of the finished print area. These bars are often used to verify the accuracy of the printing job and allow the press operator to calibrate the print job and adjust the press if necessary. (see Figure 3.30)
Figure 3.30 - Color control bars help the printer control quality on press
Color correction is the process of adjusting an image so as to correct color errors.
Overall color correction is a basic function of color reproduction. This may be required for any of the following reasons:
Poor film/print processing
All repro houses perform overall color correction and simple aesthetic retouching to varying standards in order to optimise the printed image (see Figs 3.31 and 3.32 below).
Figure 3.31 - When the whites are darker and the blacks are lighter (left image) the image may lack detail. With the correct repro the image appears more vibrant (right image)
Figure 3.32 - Example of color corrected image (before and after color correction)
Spot colors printed by CMYK can be a tricky business as there are limitations on the densities and color that can be achieved. Figure 3.33 illustrates the difficulties in reproducing a spot color using 4 color process and variability that can occur if the same job is produced by two different printers.
The left column of each is the spot color which you can see are identical. The right column of each is the process conversions.
Figure 3.33 - The two panels in the graphic above show two identical spot to process conversions printed by two different printers
These can vary dependent upon the printer their relevant dot gains and densities.
If colors cannot be successfully and consistently reproduced using CMYK then single ‘spot’ colors can be used. This will of course require an extra printing head on the press.
SUBSTRATES CHARACTERISTICS AND EVALUATION
The substrate used will have a distinct influence on the printed result and final color of the job.
There are a range of substrate characteristics that need to be evaluated as part of the specification process.
The image and reproduction will be affected by the following characteristics;
Materials act like ink pigments and reflect color to varying degrees. Inks are not 100% opaque (they have an element of transparency) so some of the characteristics of the underlying substrate are transmitted through the inks.
The appearance of an image printed on a white substrate for example will differ considerably from one printed on a brown material. The impact of the underlying substrate on the final result must be carefully considered and anticipated at the repro stage (see Figure 3.34).
Figure 3.34 - The impact of substrate color on the visual appearance of a printed design
Other optical properties may have an impact on the final printed result and must therefore be carefully considered. These factors include;
Color – Absorption within the object or product
Gloss – Specular reflection at the front surface of the object
Translucency – Transmission through the object
Texture – Spatial variation in reflection / transmission / absorption caused by surface texture
It is important to note that inks are not 100% opaque - they have an element of transparency and some of the characteristics of the substrate can often be transmitted to the inks.
If a semi transparent substrate or ink is being used on a pack it is important that the impact of container color or product color is anticipated.
For example a dark or strong colored container can have a significant influence on the appearance of the label design. This influence is demonstrated in Figure 3.35 below.
Figure 3.35 - Background influence on color of Containers_Products_Liquids
The translucency of an ink or material can also have a dramatic impact on appearance.
The type and characteristics of the inks to be used on a job must be considered and their impact evaluated. Surface coatings to provide additional gloss, surface or product resistance for example, must be evaluated and factored into the specification.
Other factors relating to inks and varnishes to be considered include the following;
Toy Regulations – the labetling of toys and children’s products must comply with the requirements of the relevant national or international safety standards.
All of the above ink considerations will be explained in the Inks, Coatings and Varnishes training module.
Once the repro process is completed the job is ready to go to print. There is however a final process called pre-flighting that is often used in order to confirm that the digital files required for the printing process are all present, valid, correctly formatted, and of the desired type.
The files to be used for the printing of the job are checked to make sure they are in a format that can be interpreted by the RIP (raster image processor). Once the incoming files have passed the pre-flight check, they are ready to be put into production. Without this pre-flight check significant and expensive production delays could result.
PDF is the industry standard file type for submitting data to a RIP. Printers will make sure the pre-flight settings match their specific production requirements.
Files are verified by a pre-flight operator for completeness and to confirm the incoming materials meet the production requirements.
The pre-flight process typically checks for:
images and graphics embedded by the client have been provided and are accessible
image files are of formats that the application can process
image files are not corrupt and are of the correct resolution and color format
required color profiles are included
fonts are accessible and compatible to the system and are not corrupt
confirm that the correct separations are being output
Advanced pre-flight steps might also involve the following;
converting fonts to paths
removing hidden objects (i.e. objects outside the printable area and objects on layers below)
flattening transparent objects into a single opaque object
gathering embedded images and graphic files to one location accessible to the system
compressing files into an archive format
ON PRESS CHECK
The on press check takes place after a printing press is set up, but before the print run commences.
While errors should have been corrected during the approval and proofing stages, the main purpose of a press check is to make sure that the color on press comes as close as possible to the agreed color proof.
There can be inherent differences between some color proofing methods (apart from wet proofs) and the printing process itself.
Areas that are commonly evaluated at a press check are as follows;
Flesh tones or corporate logo match colors
Overall color balance across the web
Substrate (checking for correct color, weight or texture).
Content (looking for missing elements and confirming copy changes).
Registration (checking sharpness, color overlapping, edges of images and screened type)
Physical defects (checking for broken type, hickeys*, spots etc)
*Hickey - The effect that occurs when a spec of dust or debris (frequently dried ink) adheres to the printing plate and creates a spot or imperfection in the printing.
In many cases the client will inspect and sign off a job on the printing press and indicate acceptable color variances (if they have not already been specified).
POST PRESS CHECKS
Most printing is usually not complete until it is converted into a "finished" product. Post press activities include various types of finishing work such as slitting, embossing, foiling, die-cutting.
Post press checking therefore can include:
Embossing – Checking for defects such as un-sharp edges, pinholes, ruptures and "halos" (shadows around the emboss).
Foil stamping – Defects to be avoided are feathering, color changes, scuffing, peeling and un-sharp edges.
Die-cutting – Defects that are commonly checked for are clean cutting and correct register.