Introduction
Before actually printing with colour, it is necessary to understand some of the factors that influence the appearance of colours on a printed document. Listed below are some of the important points to consider:
Although colour guides are a good tool for determining the colour that should be used, (such as spot colours), they should be used only as a guide. There is no guarantee that the final printed colour will look exactly like the colour in the guide. The colours in the guides tend to fade, so the guides are usually valid for about a year. The colour swatches in a guide are usually printed with a saturation that may be hard to achieve on some applications.
The type of paper on which colour is printed has a huge affect on the way the colour appears. The same colour will appear to be quite different when printed on coated and uncoated papers. The ink absorption rate, along with the brightness and the colour of different papers can result in significant changes in the way colour appears on different papers.
There can be differences in ink pigments between different ink manufacturers, which is another reason why it is difficult to perfectly match the colour in a guide. Most printers use ink from one vendor, so they can usually expect consistency in the ink they use.
Lighting conditions affect the appearance of the ink colour. Differences in daylight during different times of the day and differences in artificial illumination, such as fluorescent and incandescent, can cause a wide shift in the appearance of a colour.
Precise colour matching can only occur when the preceding factors are considered. A colour viewing booth can be used in order to accurately match the printed colour with the proof. Lighting conditions, materials, and the techniques used, must be controlled in order to achieve the best results when printing with colour.
Spot Colour
Spot colour is usually considered to be any colour used on a document that is not a process colour. It can be used simply as a secondary colour or as a colour to provide emphasis to an area of a printed piece. Spot colours can be used as an area of emphasis on a four-colour process job with the spot colour generally printed as a colour on a separate plate and not as part of the four-colour process.
Specialty inks can also be used as a spot colour to provide even greater emphasis to an area. Specialty inks can range from fluorescent, fade resistant, opaque, and metallic inks.
Many of the specialty inks demand unique considerations for their proper use at the press. Metallic inks may require that the press run at a slower rate than usual and the preparation and clean up time may be longer. Fluorescent inks are very transparent and may require that the printed document be run a second time through the press to achieve the desired coating of ink. The challenges created by the specialty inks in terms of printing may increase the price of a printed document to a degree that it may not be cost effective to use the specialty ink for a particular application unless a large quantity is produced.
Four-Colour Process Printing
Printing an image in full-colour is a complicated process involving a number of steps. A basic knowledge of these steps is not only essential for printers, but it is also important for designers in planning and creating the best layouts and for print buyers in understanding some of the issues that are raised when sending colour work to our production hubs.
COLOUR THEORY
Red, Green and Blue (RGB), are the primary colours of white light and are called the Additive Primary Colours. We are able to see colour because different objects reflect and absorb, or subtract, the primary colours of light differently. For example, we see an object as yellow because it absorbs (or subtracts) blue light from white light. Since the blue light has been absorbed, the red and green light is leftover and is reflected back to our eyes. The red and green light combine to make yellow and so we see the colour of the object as yellow. This is known as the Subtractive Colour Process because portions of the visible light spectrum are subtracted from white light to reveal colour. If 100% of the red light is subtracted from white light the resulting colour is cyan. (The green light and blue light are the remaining primary light colours after the red is removed and the green and blue combine to form cyan). If 100% of the green light is subtracted from white light the result is magenta (red and blue light form magenta) and when 100% of the blue light is subtracted from white light the result is yellow (red and green light form yellow). Cyan, Magenta, and Yellow (CMY) are the Subtractive Primary Colours, and are used for colour reproduction in process colour printing.
Pigments
The combination of the primary colours in varying degrees is what creates the illusion of a full colour printed image. In the lightest and whitest areas of the printed image, very little of the primary colours are present so these areas appear very light. In the darkest areas of the printed image, much more of each colour is present making the areas appear darker. The combination of the three subtractive primaries, each at 100% strength, should create black, but since cyan, magenta, and yellow colour pigments are not perfect, there is no way of creating a true black colour when they are combined. If full strength cyan, magenta, and yellow, are combined, the resulting colour is actually a dark, muddy brown colour. Even if it were possible to combine all three primaries at full strength to create black, printing 100% of any or all of the subtractive primaries can cause a saturation of colour on the printed surface which can result in very poor print quality. Because of this problem, the addition of black is necessary to assist the cyan, magenta, and yellow in creating the darkest hues, to deepen shadows, and to produce a true black text. Cyan, Magenta, Yellow, and Black, (CMYK), form the basis of the four colour printing process. The 'K' is used to signify black to avoid confusion with blue.
Colour Separations
In order to reproduce or prepare a full colour image for printing using four process colours, the image must be divided into the individual subtractive primary colour components. The separation process can be accomplished photographically or electronically.
Photographic Separations: Using a large process camera, a full colour image is converted into halftone negatives which, contain a series of dots of various sizes to represent shades of gray. The dots are achieved with the use of special screens placed over the negative material during exposure. When printed, the smaller dots create the lighter areas of the image, with the lightest appearing white, and the larger dots make up the darker areas of the image, with the darkest appearing black. This system of using the halftone screens directly with the film is known as the Direct Screen Photographic Colour Separation method. A halftone negative is made for each of the separate subtractive colour components of the image (cyan, magenta, yellow and black). The four halftone negatives are records, in varying shades of gray dots, for each of the separate colour components and are known as 'Colour Separations'.
Another photographic technique known as the Indirect Photographic Colour Separation method is also used. The original image is separated into the individual primary colour components and continuous tone films are made for each of the separations. From the separated films, the halftone negatives or positives are created. The advantage in first creating continuous tone separations before creating the halftones is that the continuous tone separations can be reduced or enlarged and corrected before the actual halftone films are produced.
Colour separations created from photographic methods are used to prepare analog plates for traditional printing methods. Each plate corresponds to one of the four colours, cyan, magenta, yellow, or black. The plates are coated with the corresponding ink colours during the printing process and the inked image from the plates is transferred to the substrate. The varying sizes of the halftone dots on the plates, determines the amount of ink that will be printed on the substrate. The dots from each of the different coloured images overlap to produce the full colour image.
Electronic Separations: The electronic separation of an image into its individual colour components has become the most popular and labour saving method of producing colour separations. The separations can be produced from an image produced with analog methods, such as photography, or from a digitally generated image.
When producing separations from an image produced with analog methods, the image is first placed in a scanner, which converts the image into a digital record. The digital image can then be imported into an image editing software program where the colour separations can be produced with the click of a mouse. The scanned image and/or the colour separations can be quickly manipulated and edited in an infinite number of ways with the tools provided with the software.
Colour separations can also be produced from digitally produced images, such as from a digital camera or images composed entirely in design or illustration software on a computer. Digitally produced images eliminate the scanning step from the workflow.
Colour separations created electronically can be output to plates by Computer-to-Plate applications and digital printing.
Screen Angles - The rows of dots on the screens used for each colour separation must be positioned at a specific angle so that the printed dots do not form a distracting pattern. The angles are: 45 degrees for black, 75 degrees for magenta, 90 degrees for yellow, and 105 degrees for cyan. The angles form a rosette pattern, which is merged into one continuous tone by the human eye.
If the screens are positioned at the wrong angles and then printed, the rosette pattern will not be correct and a moire pattern appears which results in an image that no longer has a smooth gradation of colour. An example of a moire pattern is shown here.
Dot Gain - Dot gain occurs when the size of the halftone dot increases due to several variables. If the size of the dot increases too much, then the image and colours print darker than what was originally intended. Some of the variables that affect dot gain are:
Prepress - if the negatives from different sources are duplicated to produce the final film, the dots may increase in size.
Printing plates produced from the duplicated negatives may be incorrect which may result in a change in the dot size.
Printing Equipment - the type of press, the age and condition of the equipment, and press settings such as plate and blanket packing, printing cylinder settings, and ink and water balance, may determine the degree of dot gain on a printed sheet.
Ink - ink is not the same from all manufacturers. Inexpensive or low-grade ink is more likely to cause dot gain problems than a higher priced, high-quality ink.
Paper - it is normal for printed dots to increase in size slightly as wet ink spreads and is absorbed by the paper. Different papers cause ink to absorb and spread differently at different rates. An uncoated paper stock may not be as suitable for colour printing as a coated stock, because it absorbs ink at a faster rate. It is always best to consult the print provider when determining which paper is best suited for a particular application.
Registration- Several factors can cause the individual colours of a four-colour image to become misaligned, which results in gaps and colour shifts between objects. Some of these factors include:
Prepress - If printing plates are produced incorrectly, the image on the plate may become crooked, resulting in poor registration of the individual colours when printed.
Plates - improper plate and/or blanket packing on the press can affect the image on the printing plates. The plates can also be improperly installed on the press resulting in poorly registered colours.
Paper - occasionally, paper stretches and distorts when it absorbs moisture as it passes through a printing press. The stretching and distortion of the paper may result in poor registration of the individual colours.